Troubleshooters.Com Presents

Troubleshooting Professional Magazine

Volume 6 Issue 8, Fall, 2002
Roadside Overheating Diagnosis
Copyright (C) 2002 by Steve Litt. All rights reserved. Materials from guest authors copyrighted by them and licensed for perpetual use to Troubleshooting Professional Magazine. All rights reserved to the copyright holder, except for items specifically marked otherwise (certain free software source code, GNU/GPL, etc.). All material herein provided "As-Is". User assumes all risk and responsibility for any outcome.

See also Troubleshooting Techniques of the Successful Technologist
and Rapid Learning: Secret Weapon of the Successful Technologist
by Steve Litt

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Editor's Desk

By Steve Litt
Holy @@@@! A quick glance at my temperature gauge showed one tiny notch before the red. I was 450 miles from home!

Cutting off several cars, I did a quick exit, pulled into a rest area, shut it down, and organized my mind.

This began one of the most worrysome eight hour periods of my life. If you're curious, this entire mess of a day is documented in my book Twenty Eight Tales of Troubleshooting, in a story called "Trouble Trouble, Miles from home."

Repair by an unknown shop isn't a pleasant thought when your trunk is filled with your business's main desktop computer, backups, clothes, personal items, and your best skateboard. Certainly, here in central Georgia my Florida plates told the shop owner that I was out of options and in no position to negociate price, repair time or quality. If at all possible, I had to get home, and fix the problem there. I nursed the car home, and learned from my misfortune.

So if you ever find yourself overheated, far from home, with no reliable shop, no tools, no bargaining postiton, and in fact nothing but your wits and your knowledge, remember what you learn in this month's magazine. As you know, anyone can productively troubleshoot under easy conditions. What separates the Ninja from the Wimp is their effectiveness when the pressure is really on. And when you're miles from home, with your valuables in the car, far from the shops you know -- that's pressure!

So kick back, relax, and enjoy the first issue of the new Quarterly Troubleshooting Professional Magazine. Now more than ever, if you're a Troubleshooter, this is your magazine.

Steve Litt is the author of "Troubleshooting Techniques of the Successful Technologist".  Steve can be reached at Steve Litt's email address .

What Happened to the August and September Issues?

By Steve Litt
You might have noticed there were no August and September issues of Troubleshooting Professional Magazine. What happened was that Troubleshooting Professional Magazine split in two. All the Linux/Open Source/Free Software content was moved to a monthly magazine called Linux Productivity Magazine.

The Troubleshooting material remained in Troubleshooting Professional Magazine. Troubleshooting Professional became a quarterly mag because I just didn't have the resources to publish two monthly magazines, and my statistics showed that the Linux issues drew far more readers than the Troubleshooting issues.


Automotive-related magazine issues also drew very large audiences, so we'll try increasingly to accommodate our automotive readership.

If you'd prefer that Troubleshooting Professional be the monthly magazine (and therefore Linux Productivity become quarterly), vote with your browser. Troubleshooters.Com has a home-grown web statistics program that we view daily to discern the readership of each and every page. If Troubleshooting Professional readership outstrips that of Linux Productivity on a regular basis, Troubleshooting Professional will once again become a monthly magazine.

And once again, thanks for your many years of faithful readership.

Steve Litt is the creator of the Universal Troubleshooting Process.  Steve can be reached at Steve Litt's email address.

Disclaimer and Disclosure

By Steve Litt


The information on in this document is information is presented "as is",  without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose. the entire risk as to the quality and performance of the information is with you. Should this information prove defective, you assume the cost of all necessary servicing, repair, correction or medical care.

In no event unless required by applicable law or agreed to in writing will the copyright holder, authors, or any other party who may modify and/or redistribute the information, be liable to you for damages, including any general, special, incidental or consequential damages or personal injury arising out of the use or inability to use the information, even if such holder or other party has been advised of the possibility of such damages.


I am not a mechanic, nor even a particularly mechanically inclined car owner. My primary place in automotive Troubleshooting is reporting of symptoms and conducting tests. The limits of my automotive mechanical work are replacement of batteries, and rare minor tuneups infrequent thermostat replacement. My knowledge of specific cars extends only to those I've owned, and my wife's car.

The information you see in this document is my opinion, based on personal experience, experiences reported by Troubleshooters.Com visitors, and research. It wouldn't surprise me to find that some of the information in this document is erroneous. If you choose to use this document's information, you should use it in addition to information from other sources.


There was a time this type of disclaimer would have seemed absurd, but today lawyers rule the land. Now with UCITA rearing its ugly head, failure of an electronic author to absolve himself of all responsibility can be disastrous. To the 99.9% of you who take responsibilities for your own actions and earn your money through hard work rather than lawsuits, I apologize for the necessity of this disclaimer.

Safety Notice

CAUTION! Safety first. Be extremely careful when looking into or  putting your hands or any other body part into your engine compartment. Use safety glasses. Keep all body parts away from open carburetor or air intake -- assume it will backfire. Backfires can produce blindness and/or third degree burns. Keep long hair VERY securely pinned up, remove all jewelry, do not wear loose clothing while working on a car. Heed these warnings even when the car is not running. Remember, modern cars have electric fans that can turn on at any time.

Be careful not to short the battery -- battery explosions can throw potentially burning/blinding acid. Don't open the radiator cap of an overheated car. When it's cool enough to open, use a rag to block any spray, and open very slowly. Wear hard shoes capable of shielding impact from a dropped part.

Take proper precautions when jacking up a car so the jack doesn't "kick out". A "kicked out" jack can be every bit as destructive as a kick from a large horse. Block the wheels, put on the brakes, seat the jack properly. Never crawl under a jacked up car unless it has been properly blocked  up and completely secured. Even then, remember that earthquakes, sudden wind and drunk driver impacts happen. A professional lift is always best.

Be careful of overheated engines. Even at normal operating temperature, removing a radiator cap can cause a geyser of scalding coolant. It's worse with an overheated engine. Be careful observing an overheated engine, because extreme pressure can cause a hose to break and spray scalding hot water, causing severe burns or even blindness.

The preceding warnings are by no means an exhaustive list of the risks encountered when working on a car. Always use common sense. You assume full and complete responsibility for the use of the information on this page.

This isn't just theory. An old skating buddy lost a finger when his wedding ring was grabbed by a fan belt. I met a guy at MacDonalds who spent 2 weeks in the hospital when an open-carb  backfire caused third degree burns over most of his forarm. Skin graft city. Imagine if his face had been by that carb. We've all read about people who crawled under a jacked up car and got crushed.

Be very careful, or else do not work on cars.

Steve Litt is the author of Rapid Learning: Secret Weapon of the Successful Technologist. He can be reached at Steve Litt's email address.

Establishing a Baseline

By Steve Litt
Roadway diagnosis takes place with few tools, supplies or alternatives. You must take your opportunities where you find them. Specificially, you can take steps before the diagnosis becomes necessary. In your regular driving, observe the following:

You might want to actually write down the answers to these questions, because the changes over time can be slow. But changes to these answers usually indicates upcoming cooling system problems. This is ESPECIALLY true if variations in normal operating temperature increase. Increasing variations indicate a system less under feedback control by the thermostat, meaning either a worsening of the thermostat or reduced excess cooling capacity.

Increased need to fill the reservoir indicate a coolant loss problem. A pressure test is indicated, and if that produces no explanation, possibly a block test to test for combustion gas in the coolant.

Increased uphill operating temperatures indicate reduced cooling capacity that will probably cause a problem soon. While driving uphill, open all the windows and turn the heater on full blast for a few minutes. Does the temperature go down? If so, you've probably ruled out the water pump. If not, either cooling capacity is so compromised that the heater can't help, or else the problem involves the water pump. Try the heater test on a lesser grade and see if it makes a difference. If not, don't be surprised if a technician tells you to replace the water pump.

If these numbers have increased, be very careful taking a long trip in the car, especially in extreme heat or hills. BE SURE to bring lots of antifreeze and water with you. BE SURE to look at the temperature gauge every 5 minutes, because long trips are where most overheating takes place. Better yet, diagnose and fix the problem before taking your trip.

Your first opportunity to fix a cooling system problem is before you hit the road, and that requires detection. The best way method of early detection is to establish a baseline.

Your baseline measurements are no good unless they're available on the road. Write them all down, and keep a copy in the glove compartment, so when you're on the road and things get hairy, you can compare your situation to the baseline, and draw valid conclusions.

Temperature Gauge

Some cars don't come with temperature gauges. Instead they come with a light that lights when the car overheats. Such lights are called "idiot lights", because when it lights it's telling you "hey idiot, you just broke your head gasket". Idiot lights tell you nothing about normal operating temperature, nor just how hot you are, or how long it took to get that way. Idiot lights are lightbulbs, so they burn out. If it's burned out, the first you'll know of an overheat is when the car stalls and steam pours out the hood.

Have a shop install an aftermarket temperature gauge. There's absolutely no substitute for a temperature gauge. It's the only way you can establish a baseline, and the only way to discern between a potential problem and a clear and present danger.

Steve Litt is the author of Rapid Learning: Secret Weapon of the Successful Technologist . He can be reached at Steve Litt's email address .

Preventive Maintenance

By Steve Litt
As mentioned in the  April 2002 Troubleshooting Professional Magazine, preventive maintenance is essential for preventing catastrophic cooling system repairs. Preventive maintenance should happen early and regularly, but if in the past you've been lax, start now. As the old saying goes, the best time to plant a tree was 20 years ago, the second best time is today. Unless you practice preventive maintenance, you WILL experience many expensive cooling system repairs later in the vehicle's life.

Preventive maintenance was described in detail in the April 2002 Troubleshooting Professional Magazine, but one additional piece is that you should measure the voltages on the radiator to determine the potential for electrolysis corrosion. Metal radiators are often not grounded, but from what I hear, they should never be more than 0.7 volts from ground in either direction.

Steve Litt is the creator of the Universal Troubleshooting Process.  Steve can be reached at Steve Litt's email address .

Trip Preparation

By Steve Litt
Your next opportunity to fix a cooling system is before you start your trip. The more challenging the trip in terms of temperature, hills/mountains, and stop and go driving (especially high speeds mixed with stop and go), the more important the preparation becomes.

The Coolant

Prior to a long trip is an excellent time to change your coolant and do some serious flushing. A challenging trip could knock loose some scales, possibly clogging lower radiator tubes. I think that's what happened to my car during my round trip to Chicago. By flushing the scale out of the system you prevent that, postponing the day you need to replace the radiator, heater or water pump. Make sure when you leave for the trip your coolant is clean and full, and your reservoir is filled to specification.
Be aware that flushing may "break loose" deposits that could conceivably clog and disable the thermostat. Always flush BEFORE you replace the thermostat, so if there is a problem, you don't gum up a brand new thermostat.

Any time there's a thorough flush, it's important to test the thermostat before considering the job complete.

Belts and Hoses

If your belts and hoses are getting old, you might want to replace them before the trip. If you drop a fanbelt or blow a hose, you have about 30 seconds to pull over and shut down before doing permanent damage. Once sidelined, you're at the mercy of whatever shop is within towing distance.

The Heater

Make sure your car's heater works, and it supplies hot air. Make sure it's capable of being cranked up to the maximum temperature. Some electronic controls go bad or intermittent and cannot turn the heat on. However, many have overrides such that if you set a temperature of 90 degrees, they'll be strongarmed to full on.

If you don't have a heater, you can't do the easy water pump test, and you can't attempt emergency cooling via the heater. During overheats, your car's heater is a VITAL component of the cooling system.


Unless your cooling system is ULTRA reliable, bring enough antifreeze and distilled water to fill your tank. If your thermostat is easy to get to, you might want to bring a spare thermostat, gasket, gasket paste, gasket scraper, and the necessary socket wrenches to remove the thermostat. That way if you suspect your thermostat has gone bad you can replace it or run for awhile without it, thereby ruling out both the thermostat and gas buildup around the thermostat. Bring rags to open the radiator cap and wipe up any spills. Bring funnels to install the liquids without spilling, and something for measuring equal amounts of antifreeze and distilled water. Bring safety glasses to protect your eyes when working on the car.

You might consider spare fanbelts and possibly even spare hoses if your fanbelts or hoses are old and you choose not to replace them before the trip.

Bring a couple bright flashlights. You might need to perform diagnostic tests in the dark (I hate that!).

Bring a voltmeter. It's essential for diagnosing electric fan problems.

Another helpful "supply" is a cell phone capable of making calls in any area. If you break down on the road, you can call around and talk to several shops, so when you get towed in it's not to a third rate garage preying on broken down motorists a thousand miles from home. If you call around you might even be able to locate a good radiator shop. Nothing's sadder than being forced to replace your radiator with a low capacity model when the shop you went to had only one radiator choice.

Starting Each Day

Before starting a day's drive, check the reservoir level and then unscrew the radiator cap (the car should be cold after a nights rest) and check the coolant level. It should be up to the top. If not, top it off with the proper distilled water/antifreeze mixture.

Before Major Mountain Passes

If your confidence level is less than perfect, or if your car has a history of losing coolant, you might want to pull over and take lunch just before starting your ascent. Take an hour for lunch, and hopefully in that hour the car will have cooled enough to take off the radiator cap. If you can comfortably place your hand on the radiator tank for 20 seconds, the car is probably cool enough to open the radiator cap. Place rags over the cap, and twist it off. If it starts spraying, push and twist it back on or jump out of the way -- whichever seems safer.

Make sure the coolant is up to the top. If not, fill it with the proper mixture.


Many overheat related roadside breakdowns can be avoided by preparation. Preparation includes possibly a coolant change, evaluation of the belts and hoses, bringing supplies, checking at the start of each day, and checking before major mountain passes.
Steve Litt is the author of " Troubleshooting Techniques of the Successful Technologist".  Steve can be reached at Steve Litt's email address .

Diligence on the Road

Diligence, awareness and alertness on the road can make the difference between a minor mishap and a head gasket crunching disaster. Glance at your temperature gauge at least once every 5 minutes. It's not hard -- you glance at your speedometer more often than that.

If you see the gauge going above normal operating temperature, be VERY alert. Turn off your air conditioner and note the effect. If you're still higher than normal after a few minutes of driving without air conditioning, open all the windows and turn the heater on full blast. If the temperature drops after a few minutes, you've ruled out the water pump.

The time to perform the heater test is long before the temperature approaches redline. Many cars have interlocks that shut off the water to the heater in the case of overheats. I have a feeling the purpose of the interlock is to prevent passenger injury in the event that temperature and pressure rise to the bursting point. In a car with such an interlock, once you redline, all you can do is pull over and shut down. The trouble is, for the first few minutes after shutdown the temperature continues rising. Or, you could try pulling to the side and idling fast in the hope that a reduced gasoline flow combined with fan action will lower the temperature. But if it continues to rise you're in REAL trouble.

If you can lower the temperature using the heater, you might decide to complete your trip that way. However, in hot weather doing so could be very dangerous to the health of the driver and passengers and is not recommended. But at least with the heater technique, you should be able to make it to the next exit, so you can find a shop or pull over and start making calls to find the best repair shop or radiator shop.


If you never look at your temperature gauge, the first you'll know of an overheat is when steam comes out the hood and the car stalls. By that time it's likely your head gasket is cracked, a thousand miles from home, and you're faced with a $2000 repair by an incompetant and dishonest shop who knows full well that you have no other choice. Stay operative by being diligent.
Steve Litt is the author of Rapid Learning: Secret Weapon of the Successful Technologist . He can be reached at Steve Litt's email address .

Diagnosis on the Road

By Steve Litt
So it happens. In spite of your preparation and preventive maintenance, your operating temperature goes significantly above baseline. It's time for diagnosis.

On-the-Road Tests

Assuming you're still well below redline, the first diagnostic step can be taken while driving. Turn off your air conditioner, open all windows, and turn the heater on full blast. Does the temperature go down? Good. Now leave the air conditioner off, but turn off the heater. Does the temperature rise? Great -- you've just ruled out the water pump. A weak water pump overheats by virtue of inadequate water flow. That water flow is inadequate whether it's all through a radiator, or half through a radiator and half through a heater. By toggling the symptom with the heater, you've shown the immediate cause to be an inadequacy in the radiator's ability to cool, as opposed to the water pumps ability to supply water.

If the radiator does not effect the symptom, either you have a water pump problem, or your overheating is so severe that even the combination of radiator and heater cannot shed the engine's heat.

Offroad Testing, Idling, Hot

Do not put your hands or any other body part in a car that's running!!! Your fingers, hair, or clothes could get caught in a belt, causing severe injury or death. A carburetor backfire could give you 3rd degree burns.

All your roadside overheating diagnostic tests involving touching things can be done when the engine is stopped. Don't take unnecessary chances.

The next step is to do some offroad testing. Depending on the seriousness of the situation, and whether the heater can control the temperature without endangering the health of the driver and passengers, and depending on the urgency of arriving on time, you might choose to postpone this testing until you  arrive at your destination. Or you might do some of it right away, and the rest at the destination.

Because much of a full on the road diagnosis involves waiting an hour for the car to cool, it's often best coupled with a meal in a fast food restaraunt with a big parking lot. If your car isn't yet near redline, and your temperature is under control, you might want to proceed to the next exit with a restaraunt and gas stations. 

However, if you're flirting with redline, pull over as soon as it's safe -- an overheat can crack a head gasket and cost you over $2000.00.

Pull to the side. When pulling over, make it a point to observe the area where you will be parking. Try to pick a spot with no stains on the pavement. That way, if your car leaks, you can more easily detect it, and know for sure that it was your car that leaked.

Open the hood, and if you have an electric fan verify that it's spinning. If the fan is not spinning, shut down instantly before you experience a catastrophic overheat. An overheated car whose fan doesn't turn on has either a bad fan, bad fan electronics, or a loose or missing electrical connection to the fan or fan electronics. If the overheating happened primarily in stop and go traffic, the fan problem might be the entire root cause of the overheat. If the overheat occurred at a speed over 30mph, there's probably more wrong than just the fan.

In a safe manner, check whether the lower radiator hose is collapsed. Try to observe it from above, because with an overheated car it could be dangerous to look from below. DO NOT feel it with your hands when the engine is running.

Assuming a working fan, rev the engine (1200rpm for mechanical fans, idle to 800rpm for electrical fan), with the heater on full blast, and see if the temperature comes down. Give it about 3 minutes, but if the temperature continues rising, shut it down long before redline. If the temperature comes down, turn off the heater and see if it goes up. Once again, if you can toggle the engine temperature with the heater, you've ruled out the water pump. If you cannot, even at fast idle, it points a string finger of suspicion at the water pump, or possibly a low coolant level.

Do not fast-idle a car with an electric fan!

If you have an electric fan, make sure it's spinning before attempting to reduce temperature by idling. If the fan isn't on, idling will increase the temperature. 

Be aware that most electric fans, especially if there's only one, cannot cool a radiator whose engine is being revved fast. And know that as long as the alternator is providing enough current to run the fan and heater, the speed of your electric fan(s) does not vary with engine speed.

If you have an electric fan, cool the engine by running at normal idle, or if at normal idle the alternator doesn't provide enough current to properly run the fan and heater, rev slightly (max 800rpm).


Obviously, the heater test proves nothing if no hot air comes out of the heater. With a hot car, the heater should be blowing some genuinely hot air. If it doesn't, you cannot draw any conclusions from the heater test.

If a hot car's heater doesn't blow hot, either your overheat has triggered the car's heater shutoff, or the heater isn't working.

Anyway, pull over, and assuming you're not yet redlined, open the hood and observe. Is the coolant level in the reservoir high, low, correct or empty? Is the reservoir bubbling? Is there steam? Does the engine seem really hot? Anything else seem wierd?

Offroad Testing, Shut Down, Hot

Now shut off the engine. Wait for a minute or two to observe whether boiling occurs in your reservoir.
The boiling actually takes place in your radiator, but the bubbles and steam come out the reservoir.

In a safe manner, check whether the lower radiator hose is collapsed. Try to observe it from above, because with an overheated car it could be dangerous to look from below. Now that the engine is not running, you can probably safely feel the lower radiator hose. But do not get your arm caught where you could get burned, and keep your hand away from an electric fan that could turn on.

For safety's sake, wait until the coolant inside the reservoir quits bubbling before touching anything in the engine compartment. Once the boiling has subsided, feel the top and bottom radiator hoses. If the top hose is cool, you probably have a bad thermostat, or possibly the water pump is bad (but of course if the heater made the car cool down then you already almost certainly ruled out the water pump). If the bottom hose is cool to the touch when the top hose is very hot, you might have a completely clogged radiator.

Now feel the radiator. A fully functioning radiator should be hot on the side with the top radiator hose, and as you follow the tubes to the other side, it should get cooler. But in an overheated car the entire radiator should be at least fairly hot. If you feel cool spots, especially if entire groups of tubes appear cool, you probably have a clogged radiator. Generally speaking, if the temperature gets cooler as you move along the tubes, that's normal. If the temperature gets cooler as you move to different tubes, that indicates a clogged radiator. See the following diagrams, where red represents hot and blue represents cool:

Temperature gradient of a properly functioning radiator Temperature gradient of my malfunctioning radiator

Last but not least, check the ground for leaked coolant (you did observe the pavement before parking there, didn't you), and observe the engine compartment for dripping coolant.

Skipping the Cooling Period

I recommend you take an hour to let the car cool down. That's the way to get the most thorough and accurate on the road diagnosis, and also to minimize the chance of head gasket destruction. But sometimes other factors are involved. You might have an expensive hotel reservation, and can't stay out in the boondocks. You might be late for a vital meeting.

These factors should be balanced against the increased possibility of breaking your head gasket, necessitating a $800.00 to $2500.00 repair. Do you really think you can drive to the destination without redlining? How will you do it?

If possible, go for is the full diagnosis. Let the car cool for an hour while you take a leisurely lunch and collect your thoughts. Drink plenty of water so you can work in the sun. If you have sunscreen, put it on.

You have a decision to make. Are you going to wait an hour for the car to cool and then do a full diagnosis, or do you want to hit the road quickly. Unless you're in a terrible rush, go for the full diagnosis. But if not, while the car's still hot, try to raise your coolant level by filling the reservoir. Fill to the "full hot" mark with 50/50 antifreeze/distilled water mixture. Wait 5 minutes and check again, filling to the "full hot" level again if necessary. Repeat every 5 minutes until the reservoir level remains constant several times. Keep track of how much fluid you added, because that's the amount you were running low. If the car takes all the fluid, it's possible your car was just low on coolant, and might make it. Naturally, as soon as possible you need to diagnose the root cause of the coolant loss -- probably with a pressure test.

If you choose to do the full diagnosis, don't add coolant at this time, but instead read on.

Offroad Testing, Shut Down, Cool

It takes about an hour for an overheated engine to cool to the point of safely removing the radiator cap. Before removing that cap, you should verify that it's cooled enough. The tank where the top hose attaches should be cool enough that you can comfortably hold it in your hand for at least 20 seconds without getting burned. If not, wait some more.

Removing the cap on a hot radiator causes a spray of boiling water that can cause severe burns or blindness. Be VERY careful. Also, make sure there are no spectators and nothing on the ground to prevent you from jumping back if it starts spraying.

Always use a large rag to open the radiator cap, to prevent spray. Open the radiator cap very slowly. If it starts to hiss or spray, try to push the radiator cap back on, and turn it clockwise to lock it. If you cannot safely put it back on, let go of it and back away as fast as possible.

I recommend using safety glasses when working with a car.


The electric fans in many cars are hooked up in such a way that they can turn on, even while the engine is off and the keys are out of the ignition. Such fans are connected straight to the electronics, without benefit of the ignition switch.

It is therefore prudent not to place your hand anywhere near the fan. However, if you absolutely need to place your hand near the fan (to feel radiator tube temperatures, for instance), make sure to have your hand or wrist placed in such a way that they will prevent the fan from spinning. A plastic fan's danger comes primarily from speed, not force. 

Before opening the reservoir or radiator cap, check the ground for leaked coolant (you did observe the pavement before parking there, didn't you), and observe the engine compartment for dripping coolant.

Now open the reservoir cap and observe the coolant level therein. Is it above, at or below the "full cold"? If it's significantly below, it's likely you've been operating a low coolant condition.

Cover the cap with a bunch of rags and slowly twist it off. If it starts to sputter, twist it back on, or if you can't safely do that, jump back and let it spray. With the cap off, check the coolant level in the radiator. If it's low, inspect the inside. You'll see one or more rows of little oval holes. These holes are the ends of the tubes comprising the radiator core. Look for corrosion or buildup, especially what colored lime buildup. If you see a lot of corrosion or buildup, the radiator has seen better days, although that might not be the root cause.

Now fill the radiator with a proper coolant solution (50/50 to 70/30 antifreeze/water). Find a way to measure how much you put in, because that's important. If you put in a quart of coolant, low coolant probably wasn't the problem. If you put in a gallon or more, low coolant very well may have played a part in the overheat. A pressure test will tell you whether low coolant caused the overheat, or whether the overheat caused the low coolant. But of course you don't have a pressure tester right now, so just make a note, and look for evidence of coolant leakage. If it's all around the reservoir, it's likely the overheat drained the coolant. If it's elsewhere, suspect a leak.

Inspect the radiator for undue impediments to air flow (bugs, etc).

While the car's turned off and cool, inspect the mechanical fan. Turn it by hand, and see if it's easy to turn more than 3/4 of a turn. If so, you probably have a loose fan clutch. If it's hard or impossible to turn more than 1/2 revolution, the fan clutch is probably fine.

On the subject of mechanical fans, carefully observe your fan shroud to verify that all sucked air comes through the radiator.

Last but not least, remove the oil cap and look for white or yellow gunk on its bottom. Such white or yellow gunk is a strong indicator of a broken head gasket.

Offroad Testing, Idling, Cool

Now that the car is cool, you can start it up and observe its behavior in the absense of excessive temperature.

Make sure the radiator cap is removed before starting the car. Part of the diagnostic process is to view the coolant flow in the filler pipe area.

Start the car with your eye in the rear view mirror. Notice whether a lot of white smoke comes out the exhaust. Voluminous white smoke can indicate a broken head gasket.

As soon as possible, feel the top hose and observe the filler pipe area. When the car warms up you should feel an increased pressure in the top hose, and you should feel it start to get hot, and you should be able to observe water turbulance in the radiator fill pipe. If those things happen, the thermostat has begun to open. Make a note of the temperature gauge reading at that point.

Observe the electric fan. Is it on yet? If so, once again observe the temperature gauge.

Once the thermostat has opened, carefully look at the fill pipe. You should see quite a bit of water flow and turbulance. If not, there could be a flow problem.

What you hope not to see in the fill pipe is lots of bubbles. Those bubbles could indicate combustion gas in the coolant -- a result of a broken head gasket. However, not all bubbles are combustion gas, do don't assume the worst. If you see bubbles, get a block test or smog sniffer test, when convenient, to test for combustion gas.

Look for dripping coolant leaks. If you have reason to believe there's a leak, buy extra antifreeze and distilled water, and plan to shut down, cool and fill often.

The offroad idling tests should last only a few minutes, and certainly you should terminate them long before the car goes redline.

Summary and Conclusion

The list of possible root causes for an overheat is actually pretty short: And here's how you evaluated each...
Clogged Radiator You got a pretty good handle on the radiator by observing the temperature gradient, and, assuming the coolant was a little low, but observing corrosion in the fill pipe.
Fan problems Your driving tests indicate give a good indication of fan problems. If the overheat occurs exclusively below 30MPH, it's likely you have fan problems. If the overheats occur above 35MPH, it's likely the problem is elsewhere, although you could have a fan problem in addition.

With the car shut off, you manually spun the mechanical fan to test the fan clutch, and you inspected the tightness of the fanbelt.

If you have an electric fan, you verified whether it spins in an overheated condition (if it does not, there's a fan problem).

Impediments to air flow
(bugs, etc)
You inspected for this, to the extent that you got a clear view.
Collapsed lower radiator hose You inspected for a collapsed radiator hose while the car was still running, so if you got a good view you've probably either verified a collapsed bottom hose or ruled it out.

Water pump
If you can lower your temperature with your car's heater, either while driving or idling, it's very likely that your water pump is pumping sufficient water. Note that because temperature can vary with time, the best test is to alternately turn the heater on and off, and verify that within a few minutes engine temperature follows.
Thermostat You felt the top hose after you shut down. If it was hot, it's likely the thermostat works. You also felt the hose after starting up from cooldown and felt it get hot. Also, you visually verified moving coolant at the fillpipe. A hot top hose and moving coolant mean a working thermostat -- if the top hose is cool while the engine is fully warmed up, and if there's no coolant movement when the engine's warmed up, that points an accusing finger at the thermostat.
Coolant leak You looked for dripping coolant, and you also parked and looked on the ground for lost coolant. Further, you've been checking and if necessary filling your coolant every morning of your trip. None of these tests is as accurate as a pressure test, but they certainly yield good information.

Note that a component could leak even though other tests showed if functional. For instance, a water pump could leak, even though it pushes enough coolant, as proven by the heater test. Naturally, the leaky component needs to be replaced, even if in other ways it's functional.

Combustion gas in the coolant You looked for bubbles in the coolant immediately upon the thermostat opening after starting the cooled engine. An absense of bubbles indicates that it's unlikely your overheat is caused by a blown head gasket. You also looked for the classic head gasket signs, white or yellow gunk on the oil cap and white smoke (really steam) out the exhaust.
Overly high idle speed
in stop and go traffic
If your idle speed has been set high to avoid stalling at idle, you probably know it. If you have an electric fan(s), such a high idle speed could overwhelm the cooling capacity of a the fan, even in a properly designed and maintained cooling system.

Cranking up the idle speed to avoid stalling is a coathanger solution, and like all coathanger solutions it has side effects. One such side effect is overheating at idle. As soon as possible, have the real root cause of the stalling fixed. Until then, avoid traffic jams, especially those involving freeway construction.

EXTREMELY de-tuned car It's theoretically possible for a car to be de-tuned to the extent that you need to push way down on the gas, thereby giving off more heat than a properly designed cooling system can disburse.

If you have such a car, you know it. If your car bucks and sputters, and you need to push half way down on the gas pedal to get the car to go 35MPH, and if you get half the gas mileage you're supposed to, then this might be the cause of your overheating. Otherwise, it's unlikely that a problem with the engine could overheat a properly designed and maintained cooling system.

Inadequate radiator Occasionally cars are manufactured with inadequate radiators. Other times a car is designed to be used in a flat place (Chicago, for instance) and then driven to the mountains (Colorado, for instance), and overheats. Still other times, you buy a used car and it overheats from day one, because, unknown to you, the last owner replaced your Dodge Coronet radiator with something he pulled out of a 6 cylinder Dart at the junkyard (this happened to me).

An inadequate radiator is not clogged, so the gradient tests would not reveal the problem. An inadequate radiator is best revealed by baseline tests. If the car has always had the tendency to overheat, it's possible or even likely that the radiator is inadequate. If the car has never been driven in the mountains before, and now that it is, it overheats, an inadequate radiator is possible (though I'd rule out other causes before replacing the radiator).

If your baseline testing shows the overheating problem is newer than your radiator, then assuming you're not experiencing a brand new environment (5 miles of 6% grade, for instance), then you can safely rule out inadequate radiator.

So in about an hour, with absolutely no tools, you were able to get a pretty good handle on the root cause. Now, when you limp into town, and the guy at Ricky Ripemoff's Garage tries to sell you a new water pump when your heater brought the temperature down, you know to look for another garage. It was an hour well spent.

Steve Litt is the creator of the Universal Troubleshooting Process.  Steve can be reached at Steve Litt's email address .

The Fan System

By Steve Litt
No diagnostic session is complete without an understanding of the fan system. There are two basic types of fan systems: Mechanical and electrical. Mechanical fan systems are the old style systems, where a fan belt turns a fan clutch which is mounted on the engine block via bearings. The fan clutch is coupled to the fan. The fan clutch is intended to slip more when it's cold. To a substantial extent, the fan speed varies with the engine speed. That's why on an overheated engine with a mechanical fan, you can often cool the engine by pulling over, putting the transmission in neutral, and spinning the engine at 2000 RPM.

Contrast this with an electrical fan. Electrical fans are mounted directly to the radiator, so there's no need for a fan shroud, and there's no risk of a fan blade going through the radiator if a motor mount fails. Electric fans are controlled by electronics, and the electronics are controlled by a heat sensor. Most fans are either off, typically if the engine temperature is low, or full on if the engine temperature is high. To save power, the switch-on temperature of the fan is typically set higher than the thermostat temperature. That way, while driving normally, the fan does not spin. But in stop and go traffic, where not enough air is forced through the radiator, the engine temperature rises until it exceeds the fan turnon temperature, at which time the fan pulls air to cool the radiator. Once the engine cools, either due to the fan's action or due to  faster driving, the fan turns off. But the fan turns off at a lower temperature than the turn-on temperature. That way the fan doesn't constantly switch on and off.

So if you see a system where the fan is always on, there's something wrong. Either the fan electronics failed in the always-on mode or, more likely, the cooling system's cooling capacity has been compromised and the engine is running hot. When a cooling system is so compromised, a catastrophic overheat is just a long steep hill away.

Do NOT make the mistake of thinking the fan is sufficient to push large amounts of heat away from the radiator. In many cars it is not. For instance, the radiator for my 1988 Buick Park Avenue (3800 engine) has one fan that covers less than half the surface area of radiator. This is sufficient to cool the engine at idle, but not if the engine is getting much gas. If I rev the engine at 2500 RPM in neutral, the temperature will rise until I stop stepping on the gas. The fan is not sufficient to remove the heat generated by the gasoline used to run at 2500 RPM.

This has some hill climbing ramifications. On a car with a mechanical fan, it might be a good mountain climbing strategy to put the car in first gear and crawl up at 15 MPH. Because of the low gear ratio, the fan might be spinning fast enough to compensate for the fact that the car's velocity is insufficient to push enough air. Contrast that with my car, where the fan spins at one speed that's insufficient to cool a car under load. In my car, climbing a hill at less than 30 MPH is a recipe for disaster. Only velocity can send through enough air to remove the heat generated by a hill climb.

It also has overheat ramifications. Upon overheating, you should pull over and put it in neutral, but with an electric fan you should NOT rev the engine, as doing so would cause more overheating. Instead, you should either leave the car idling, or if doing so lowers the battery voltage to the extent that the fan slows down, you should rev the engine just enough to charge the battery (800-900 RPM).

There's one more ramification of an electric fan. If you've adjusted your idle speed high to compensate for engine problems, the greater fuel intake causes greater heat generation, and the fan may not be able to blow off all that heat, in which case your car will overheat in stop and go traffic, even though the cooling system itself is operating as designed.

Low Speed Overheating is Serious

Always fix low speed overheating as soon as humanly possible. Fan system repair is pretty cheap. Don't be tempted to let it go because most of your driving is on unbusy freeways. I've seen a 1 hour, 5MPH traffic jam at 10:30 at night caused by road construction. If you get stuck in those, it could be 20 minutes before you get to an exit, there's no shoulder to pull off onto, and you could literally risk assault by other motorists if you shut down in a traffic lane.

You can usually get to the next exit with most cooling problems. But if you have a low speed cooling problem, a serious traffic jam could break your head gasket long before you reach a place to pull off.

The Fan System's Role in On the Road Diagnosis

First and foremost, if the car overheats at speeds over 40MPH, you almost certainly have a non-fan problem (although it's possible that you could have a secondary problem with the fan). If the car overheats in stop and go traffic, but not at a steady 40MPH or above, suspect the fan system. If the car also overheats at a steady 40MPH or above, the fan system is probably not the problem. This determination is your main fan-related on-the-road test. Other tests require you to pull over.

Electric fan systems require some baseline observations. While the car is functioning and idling in neutral in the driveway, give it enough gas to raise the temperature. If possible watch the fan and the temperature gauge at once, and record the turn-on temperature (either in degrees or as a temperature gauge position). If you can't watch both at once because the hood is in the way and you can't see through the crack or in a mirror around the hood, have another person tell you when it turns on. Once it turns on, back off the gas and wait until it turns back off, and record that temperature. These temperatures should be recorded along with your other baseline measures.

One other baseline activity is to ask an automotive technician exactly how to run the fan directly off the battery without damaging the fan electronics, the fan, or anything else. Knowing how to do this (and having the necessary tools/wire/connectors) is vital both as a diagnostic technique, and a way to limp to the next exit should your fan stop operating.

Mechanical fans are typically tested by checking the fanbelt tension and surface, checking the fan clutch (can you easily spin the fan more than 3/4 revolution on a car whose engine is shut off, and is there too much or too little play in the fan clutch bearings). Visually inspect the fan shroud to make sure that all the air pulled by the fan goes through the radiator.

The first test for your electric fan is to pull over while the car is still hot. Is the fan spinning? If not, you probably have a flaw in the fan or fan electronics. Check the temperature gauge, and compare that temperature to your baseline turnon temperature. If it exceeds the baseline, it's probably either a bad fan, bad fan electronics, a fuse or fusable link, or a disconnected wire. Investigate. Sometimes you might be able to connect the fan directly to the battery voltage so it spins all the time. Be careful as not to harm your electronics, fan or anything else.

If the fan won't operate while connected directly to 12 volts (as measured by the voltmeter), you almost certainly have a bad fan. Replace it. If there's a likelihood of traffic, have the car towed rather than risking overheating in a traffic jam.

If it runs constantly when connected directly to 12 volts, either the fan electronics are bad or the electricity supply to the fan electronics is bad, either because of a blown fuse/fusable link or because of a loose connection. Use your voltmeter to try to determine which, so you can replace the proper part when you get to town. Before driving, verify that when the fan is strongarmed on, and that there's no overheating problem at idle. If there isn't an overheating problem at idle, drive to a place likely to be able to fix your problem. Remember, your fan will likely be on even when the ignition is off, so it can drain your battery if parked for long.

Steve Litt is the author of Rapid Learning: Secret Weapon of the Successful Technologist . He can be reached at Steve Litt's email address .

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