By Steve LittRage filled my soul as steam billowed from the engine compartment of my stalled 82 Buick. Only two years before I had paid $1150.00 to repair the results of an overheat. Now my car sat crippled less than 3 miles from the top of the Grapevine. Had I broken another head gasket?
Beside me stood my new bride. Sylvia held my hand and told me everything would be all right. We were less than 40 miles from home, returning from a wonderful honeymoon along the entire coast of California. The Grapevine, that eight mile stretch of 6% grade, had claimed another in its long line of victims.
The Auto Club towed us to the nearest garage, which sat perched on the peak of the Grapevine. Once there, I started giving the mechanic orders on how to diagnose and fix the problem. After all, I was Mr. Troubleshooter, and he was just some shop unlucky to be near the site of my car's breakdown. In my state of extreme attitude violation, it never occurred to me to realize that this mechanic sees more overheating problems in a week than the average mechanic sees in a year. The mechanic smiled at me, explained that he was a professional, and went to work. Somewhere deep inside of me, the last vestige of a rational person watched the mechanic's diagnostic procedures, and learned.
After letting the car cool, the mechanic turned it on and felt the hoses for evidence of thermostat opening and closing. The thermostat was OK. Then he did a pressure test. He explained that coolant loss causes overheating, and overheating causes coolant loss, and a pressure test is how you determine which is the root cause and which is the symptom. The pressure test showed no leak. The mechanic saw thick deposits in the radiator and explained it needed replacement, and that would most likely cure the problem. He didn't have a radiator in stock. He filled up the coolant and sent us on our way. We coasted down the Grapevine and arrived home OK. A few days later I had the radiator replaced by a heavy duty 3 row, and the overheating problems abated.
That was ten years ago. My new bride is now the Mother of our 7 year old triplets. The 82 Buick was junked 6 years ago after the tranny gave out. We moved to Florida, so now the Grapevine is 2500 miles away. But I still carry with me the lessons of that long-ago mountaintop mechanic. And thanks to correspondence from hundreds of Troubleshooters.Com visitors, I've added to his knowledge.
This Troubleshooting Professional Magazine issue is devoted to troubleshooting automotive overheating. It's an anguishing and potentially expensive affliction often compounded by erroneous diagnosis. Those bad diagnoses cost our society many millions. Worse, my research tells me that many cars on the road are among the walking dead -- assumed healthy but on the verge of catastrophic overheat.
I'm a computer guy, not a mechanic. But by the nature of Troubleshooters.Com, I'm the guy asked for advice when auto shops fail. As a Troubleshooter, I have no choice but to get to the bottom of it. And given enough correspondence to check and correlate, I've drawn what I feel are some pretty sound conclusions, and propose what I believe to be an overheating diagnostic strategy that minimizes expense by minimizing inaccurate diagnosis.
It may seem strange to see an issue of Troubleshooting Professional Magazine devoted to cars. Yet look a little deeper and it's evident that automotive cooling systems present a simple yet dramatic lesson in use of the Universal Troubleshooting Process.
Disclaimer and Disclosure
By Steve Litt
DisclaimerThe 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.
DisclosureI 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. I have placed it here after a substantial number of Troubleshooters.Com visitors reported mechanics coming to logically inconsistent conclusions and troubleshooting by random replacement during cooling system troubleshooting. I'm no more perfect than the average mechanic, so 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.
ApologyThere 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.
Mental Model of the Cooling System
By Steve LittA car converts about a third of its fuel's energy to mechanical energy to move the car. About a third goes out the tailpipe unused. Most of the remaining third is released as heat. That heat must be conducted away from the car's engine, or the engine will reach temperatures fatal to the engine. At the simplest conceptual level, here's what happens:
The water pump pushes cool coolant from the radiator into the engine, where heat from the engine is transferred into the coolant, thereby cooling the engine. Assuming the thermostat is open or partially open, the hot coolant leaves the engine through the thermostat and is transported to the radiator, where its heat is transferred to the air blowing through the radiator, thereby cooling the coolant. The cooled water is then ready to once again go to the water pump.
Note that the areas in the engine through which the coolant flows are called the "water jacket". Note that some of these areas include intentional holes in the head gasket. Those holes do not constitute a head gasket breach, because there's coolant on both sides. It's only when a part of the head gasket separating coolant from combustion gasses from oil that the head gasket is considered breached.
Note that a parallel coolant flow path goes through the passenger compartment heater. This is why it's advised to turn on your heater full blast in the case of an overheat (but remember never to jeopardize personal safety doing this, and remember this is a last ditch workaround and the car should not continue to be driven under these circumstances).
The airflow through the radiator depends on the fan at low driving speeds, but at high speeds most cars force air through from the grille. Note also that the air conditioning coil assembly is in front of the radiator, so if the air conditioning is on, the air flowing through the radiator will have been warmed by the air conditioner, thereby reducing cooling efficiency. That's why huge inclines like the Grapevine usually post signs to turn off your air conditioner before beginning ascent.
In summary, the engine passes its heat to the coolant, which flows to the radiator and then passes its heat to the radiator, which passes its heat to the air being blown through it. Any interference with this heat flow dangerously reduces cooling capacity. If cooling capacity falls below the amount of heat generated by the engine, overheating results. The answers to the following questions help determine the cooling capacity of the cooling system:
Later in this Troubleshooting Professional issue these simple heat transfer questions will be transformed into troubleshooting tactics.
- Is the water jacket full of coolant?
- Are deposits in the water jacket reducing heat flow away from the engine? (hard to determine)
- Is coolant flowing swiftly and continuously while the thermostat is open?
- Is the radiator free of deposits that reduce coolant flow and heat transfer?
- Is the radiator filled to the top with coolant so that its entire surface transfers heat to the air?
- Is the radiator designed to transfer the amount of heat being generated, in the weather encountered?
- Does sufficient cool air flow through the radiator at driving speed ( >30mph)?
- Does the fan supply sufficient cool air flow through the radiator at low speeds and stops?
- Is the coolant free of combustion gasses and other gasses and contaminants?
Excess Cooling CapacityAutomotive cooling systems must have HUGE levels of excess cooling capacity. Next time you drive 60 mph on a flat deserted road, notice how far you push on the gas pedal. Probably a millimeter to a centimeter. Now see how much you need to push the gas pedal to ascend a 6% grade at 45mph. Probably an inch or 2. Go up to 65 and on some cars you'll be near flooring it. 1/3 of all that gasoline is converted into heat.
Your cooling system must be able to get rid of all that heat. Difficult enough, it becomes even more of a challenge if the air temperature is warm (less heat transfer from radiator to air), and brutal if your car is heavily loaded or towing something. If the heat generated by combustion significantly exceeds the cooling capacity, you'll severely overheat quickly (typically after a mile or two of climbing).
A well functioning cooling system has the capacity to maintain the engine at under 100 degrees temperature during continuous 50mph level drives on cool days. But of course the temperature needs to be 160-230 Fahrenheit, depending on the car (consult your owners manual). That means in most driving situations the cooling capacity must be partially defeated. This is accomplished by the thermostat, which acts as a deliberate bottleneck, regulating the amount of cooling to keep the temperature at a proper level. A somewhat typical thermostat would be closed until 180 Fahrenheit, after which it would open further as the temperature increases, until at 195 it's completely open. This means that in the 15 degrees between 180 and 195, the cooling capacity would go from 0 to the full capacity of the system (enough to scoot up the Grapevine at 65 mph carrying 5 people in a well designed, maintained machine). Below is a graph showing how temperature increases with increased engine heat production (i.e., more gas):
The portion in blue represents a level of heat production so small that it can be disbursed by the direct contact of the engine with ambient air. In practice this might be achieved in the case of a 40mph wind blowing into the open hood of a car idling in the deep of a northern Minnesota winter's night, but otherwise this condition is never seen in real life. An idling engine, and certainly driving, at anything resembling normal conditions requires radiator cooling.
|NOTE: Don't make the mistake of thinking the preceding diagram represents temperature versus time. While that graph would look similar at the leftmost part of the graph, that's not what's being represented. You can think of the preceding diagram as a graph of various driving conditions, each maintained for 10 minutes or more.|
The violet portion represents heat production levels within the regulated cooling capacity of the cooling system. The slight temperature gain across this range is due to the fact that the thermostat opens slowly and steadily over a range of about 15 degrees Fahrenheit. This is the normal operating range of the vehicle. All driving should be done in this range.
The red portion represents a heat production level beyond the cooling capacity of the cooling system. The temperature goes sky high. On a well maintained vehicle, you would expect the red portion only when the car is used beyond its design capabilities, like using a compact car to pull a trailer up the Grapevine.
The bottom line is that on a well maintained vehicle, the bottleneck, by a huge margin, is the thermostat. Contrast that with a vehicle with a compromised cooling system not capable of cooling a hard worked engine, or in extreme cases even a lightly worked engine:
Here there's no regulation. The entire graph is basically a straight line. Moderate hard usage sends the car into the red. The operating temperature of a vehicle in this state of repair would vary widely with ambient temperature and length of time driving. Typically no "typical operating temperature" can be identified for a vehicle in this state. Such a vehicle will almost certainly experience a catastrophic overheat the first time the driver takes a lengthy drive, or drives in hot weather, or drives up a moderate hill.
The controlling bottleneck of this vehicle is not the thermostat -- it's something else in the cooling system. The automotive technician's task is to find that bottleneck.
Examining the Two Cooling Chicken-and-Eggs
By Steve LittWhich came first, the chicken or the egg? Did the chicken cause the egg, or did the egg cause the chicken? Here are two more chicken and egg questions:
Meaningful overheat solutions are elusive until these two questions are answered. Except when the cause of the overheat is completely obvious, these two questions must be answered. Even in the most obvious cases, remember than even if the root cause wasn't a broken head gasket, the overheat could have caused consequential failure of the head gasket.
- Did the overheat cause the busted head gasket, or did the busted head gasket cause the overheat?
- Did the overheat cause coolant loss, or did the coolant loss cause the overheat?
Did the overheat cause the busted head gasket, or did the busted head gasket cause the overheat?Overheated engines often break their head gaskets (and sometimes even bend or break the heads themselves). This is especially likely on a bimetal engine, where the heads are aluminum and the block is iron, or vice versa. Two metals expanding at different rates are an excellent way to stress both the metals and the gasket between them.
But did you know that a broken head gasket can CAUSE an overheat? Many mechanics don't know this, but an otherwise symptomless broken head gasket (no yellow gunk on the oil cap, no steam out the exhaust) can allow combustion gasses from the cylinders to leak into the water jacket. Such leakage has several possible effects, all of which make overheating more likely:
Sometimes the pressure created by the combustion gasses do not force coolant out, but instead build up until the combustion gasses are "burped" out the radiator cap and into the reservoir. Such a situation can create "cyclic overheating", in which the temperature raises and lowers at regular intervals (20 minutes has been often reported to Troubleshooters.Com). Another possible mechanism for cyclic overheating is where the elevated temperature "seals" the hole in the head gasket, which then opens again when the temperature returns to normal.
- These incredibly hot gasses are retained in the system instead of being blown out the exhaust, thereby increasing heat production.
- The gas bubbles reduce heat conduction from block to coolant, and from coolant to radiator. Bubbles can create destructive "hot spots" even in an engine whose overall temperature is within limits.
- A gas bubble can settle at the thermostat, insulating it from the engine's heat and preventing the thermostat from opening.
- Depending on the volume of gas poured into the coolant, and the geometry and configuration of the cooling system, the gas can create huge pressures which continuously push excessive coolant out to the reservoir, thereby overflowing the reservoir and creating a low coolant condition.
Occasionally "incidental overheating" is reported to Troubleshooters.Com. This means that after the engine warms up, it overheats, and then continued driving brings it back to normal temperature for the remainder of the drive. I would suspect this is a combustion gas caused overheat which happens only while warming up. Another explanation for "incidental overheating" is the gas bubble that settles on the thermostat, insulating it from the engine's heat and preventing it from opening. When something finally happens to dislodge the bubble (a bump, a burp, whatever), the thermostat opens and all gas is blown out into the radiator, eliminating the cause of the problem (until the next time the car stands cool long enough to develop a gas bubble).
NOTE: Theoretically there can be many sources of gas in the coolant. The bubble surrounding thermostat hypothesis can be tested by temporarily removing the thermostat. If the symptom disappears, it's either a bad thermostat or a bubble surrounding the thermostat. Then install a known good replacement thermostat. If the problem reappears, the problem was a bubble surrounding the thermostat.
Incidental overheats that happen early in every drive, and then return to normal temperature, should be investigated for an air bubble surrounding the thermostat.
Never run a car without a thermostat for an extended period of time. Modern cars must be run within tight parameters to prevent abnormal wear and tear.
The common thread in all these head-gasket caused symptom is that they're caused by gasses in the coolant (usually itself caused by head gasket failure). Replacing the radiator or water pump won't fix the head gasket. At best, installing a higher capacity radiator may reduce the severity of such an overheat. The mechanic and customer need to know if a head gasket breach is allowing combustion gasses into the coolant. Fortunately there are two quick and easy tests:
A block tester is a device that can be bought from a NAPA store. It has dye which changes color in the presence of exhaust gasses. From what I understand, you can get about 6 tests from one block tester, and the block tester costs around $45.00. The quote I got from my local NAPA dealer was $45.99. He didn't have it in stock, but said he could have it the next day. The relevant NAPA catalog is called "The PSA 2000 catalog" or the "Balkamp Catalog". The catalog calls the Block Tester a "combustion leak tester kit", so that's probably what you should ask for. From what I understand, it comes with a ball, tubes, test fluid, aspirator bulb and engine adapter (cone shaped device you place in your radiator filler cap). If there's exhaust in your coolant the test fluid changes color.
- Block tester test
- Smog sniffer test
From what I understand, a smog sniffer is an even better method. The smog sniffer is placed above the radiator filler cap, and the HydroCarbon (HC) level is measured. Use extreme care to prevent any contact between the coolant and sniffer probe, as contact with the coolant will ruin the probe. I understand that the car should be warmed up (but not overheated) before performing this test.
Flat Earth MechanicsUnfortunately, many mechanics believe that all head gasket flaws exhibit themselves as coolant in the oil (yellow gunk on the oil cap) or coolant in the cylinders (voluminous white steam out the exhaust). Such mechanics are almost certain to misdiagnose a head gasket caused overheat or coolant loss. I believe this accounts for many of the Troubleshooters.Com reader reports of "they replaced my water pump, radiator, thermostat and hoses, and it still happens". Such diagnosis by replacement is very expensive, and still leaves the owner to pay for the head gasket when someone finally properly diagnoses the problem.
I believe that in all but the most totally obvious overheats, the coolant must be checked for combustion gasses. If your mechanic doesn't believe it's necessary to check for combustion gasses, either because he thinks a head gasket problem can't cause an overheat or because he believes broken head gaskets always exhibit yellow gunk on the oil cap and/or white steam out the exhaust, I recommend you find a different mechanic. Even if the test costs you $60.00, it's likely to prevent expenditures for "diagnosis by substitution". Also, it facilitates a much more accurate cost estimate. Nobody wants to be told they must spend $1000.00 to replace a head gasket after they just bought a radiator.
Did the overheat cause coolant loss, or did the coolant loss cause the overheat?An overheated engine will always spew water out the reservoir, creating a low-coolant situation. Likewise, a low coolant situation will likely cause an overheat (which of course will result in further coolant loss). Was low coolant the root cause, or was it just a symptom? Obviously, the problem will remain until the root cause is fixed.
The answer is found using a pressure test, together with head gasket tests. There are four places your coolant can go:
#4 can be ruled in or out by observing the oil and oil cap. #3 can be pretty well deduced by observing the exhaust, especially on startup. #2 can be deduced with a block tester or smog sniffer. And #1 can be deduced by a pressure test.
- Out an external leak from a hose, water pump, radiator, etc.
- Out the reservoir as a result of excess pressure from an overheat or combustion gas leakage
- Into the cylinders through a broken head gasket
- Into the oil through a broken gasket
In a pressure test, the mechanic removes the radiator cap (after the engine has cooled, obviously), and replaces it with a cap attached to a pump with a pressure gauge. He pumps it up to a pressure above your system's rated pressure, and observes whether it can hold that pressure for 2 minutes or more. If not, there's a leak. The next step is visual observation to find the location of the leak. If it can't be seen, the next step might be to place an ultra-violet reactive dye in the coolant, pump up the pressure, and look for leaks under an ultraviolet light.
While it's possible for a pressure test to push coolant through a break in the head gasket into cylinders or oil, often such leakage requires a greater pressure than that developed by the pressure test. This is why a successful pressure test does not rule out a broken head gasket.
Obviously, any leaks must be fixed. Once it's confirmed that there are no leaks, other causes can be confidently investigated.
The Thermostat Bubble Connection
By Steve LittA.S.E. certified master truck and auto mechanic Dennis Buler emailed me, mentioning he's solved many overheats by drilling a 1/8" hole in the thermostat. Dennis explained that if any gas, be it air, combustion gas, or steam, gathers around the thermostat, that often the thermostat can't detect the coolant heat, and therefore doesn't open. If only the thermostat would open, the gas would be "blown" into the radiator. And of only the gas were "blown" into the radiator, the thermostat would open. Catch 22.
Dennis explains that his 1/8" hole provides an escape route for the gas without passing so much coolant that the engine runs too cool. In other words, the thermostat will simply open less in order to maintain temperature.
Reading Dennis's email, my mind drifted back to all the emailed symptoms where the car overheated in the first few minutes, then magically dropped back to normal temperature throughout the rest of the trip. Could it be that when cold, air surrounded the thermostat. The thermostat therefore didn't open at the proper temperature. However, when the engine temperature reached overheat levels, maybe the thermostat opened, blowing out the air. The opened thermostat would allow the cooling system to do its job, returning to normal temperature. The partially open thermostat would prevent a gas buildup. Until the car was shut off long enough that the thermostat would completely close :-)
Dennis's findings might suggest that running for an hour with no thermostat might be an excellent test. If overheating occurs with a known good thermostat but doesn't occur without a thermostat, it's a strong suggestion for Dennis's air bubble scenario.
I don't know enough yet to make drilling the hole a suggestion. I don't know its side effects and risks. But it sure sounds reasonable. Add that to the fact that Dennis says he's seen several foreign thermostats come from the factory with a 1/8" hole, and the fact that another Troubleshooters.Com visitor mentions in an email that VW Rabbit owners often drill such a hole to eliminate engine airlock.
If you want to try it (and once again, I don't know enough to recommend it), you can see the location of Dennis's proposed hole in the diagram below. Here's how Dennis describes the location verbally:
"i drill the hole in the flat disk part like half way between where the gasket would seal and the center opening mechanism"
||Top view of thermostat. The violet dot at the 12 o'clock position of the rim is where Dennis recommends drilling the 1/8" hole.|
|Side view of thermostat|
|Bottom view of thermostat|
By the way, Dennis also heartily agreed with my hypothesis that head gasket flaws, even those that don't manifest symptoms of steam out the tailpipe or yellow gunk on the oil cap, can cause overheating for the reasons I've stated in this issue's "Examining the Two Cooling Chicken-and-Eggs" article.
Steve Litt can be reached at Steve Litt's email address.
Dennis Buler is an A.S.E. certified master truck and auto mechanic and a volunteer at allexperts.com. His URL is listed in the URL's section.
A Possible Overheat Troubleshooting Strategy
By Steve LittThe overheat troubleshooting strategy I'd like to recommend involves the Universal Troubleshooting Process, which is listed below to refresh your memory:
The remainder of this article describes my suggestion on how to put the Universal Troubleshooting Process to work in diagnosing an overheat.
- Get the Attitude
- Make a Damage Control Plan
- Formulate a Symptom Description
- Reproduce the Symptom
- Do the General Maintenance
- Narrow it Down
- Replace or Repair the Defective Part(s)
- Take Pride
- Prevent Future Occurrence
Get the AttitudeOverheats and the threat of broken head gaskets or broken/warped heads can tax anyone's state of mind. Remember that no matter how bad it is, you'll probably get out of it for $2000.00 or less. Maybe a lot less, like less than $100.00 if it's a bad hose. And after this you'll never let the car overheat again. Accept what comes your way, and vow simply to diagnose the problem in a systematic and accurate way.
Make a Damage Control PlanSafety, safety, safety. Cars can burn you -- especially overheated cars. Understand that you never remove the radiator cap while the engine is hot. Open only after it's cooled to the point where you can comfortably leave your hand on the radiator for 5 seconds. Even then, be sure to cover the cap with a towel before removing the cap, to prevent spraying. Always wear safety glasses when working on a car, make sure long hair is pinned up tight to prevent it getting caught in the machinery. For the same reason, remove jewelry and even rings.
Make sure to stay well away from an open carburetor, or even the air intake, when running the car. Backfires can cause third degree burns. Take care not to short the battery or cause any sort of spark, as batteries can explode, throwing acid at anyone near. Never work under a jacked up car unless it's been blocked up very securely, and even then keep in mind that the car could be hit by a careless driver or dislodged by an earthquake or extremely strong wind. Wear strong shoes to prevent injury if you should drop a heavy part on your foot.
Resolve to not let the temperature go into the red during testing and repair. Make sure to shut down the engine long before it gets to the red, remembering the engine will heat up more after shutting down.
For further thoughts on safety, see the Troubleshooters.Com Overheating Guide. The time to think of safety is BEFORE beginning your repair.
Formulate a Symptom DescriptionDoes it overheat? Under what conditions? Is it more likely to happen in stop and go traffic, or at a steady 65mph? Is there evidence of coolant loss before the overheat? Do you see dripping coolant anywhere? Billowing steam out the exhaust? Yellow gunk on the oil cap? Continuous bubbles in the radiator? Does the fan ever spin? When?
When did you first notice the symptom? What else happened at that time? Were later occurrences different than the first (an initial low-coolant caused overheat could crack the gasket, after which the cracked gasket could cause overheats even at full coolant). Has the symptom been changing with time? What repairs and maintenance have been done on the vehicle?
Reproduce the SymptomNEVER DELIBERATELY CAUSE AN OVERHEAT!!! Instead, demonstrate that there is no reasonable "stopping point" on the temperature. Well maintained vehicles always stay well to the left of the red on steady, flat drives. A temperature gauge that goes 2/3 toward the red is very good evidence of a problem. Be sure to shut it down when it reaches that 2/3 point to avoid it actually going into the red.
Subsymptoms such as steam out the exhaust, yellow gunk on the oil cap, dripping coolant, should also be verified at this time.
Do the General MaintenanceGeneral maintenance consists of things that should be done regardless of problem, and easy and obvious steps. Make the following observations while the vehicle is not hot or running. Note that these steps combined should take a few minutes, but they can save costly and embarrassing mis-diagnosis:
Now some observations with the engine running:
- Coolant full?
- Coolant mixture OK? Hydrometer reading ok, color ok, no oil in coolant?
- Radiator deposits? Drain enough coolant to see some of the tube ends.
- Radiator cap OK (no cracks and swelling)
- Radiator has clean air flow path? Bugs? Leaves? Clogged air conditioning coils in front of it?
- Radiator dented or otherwise obviously damaged?
- Hoses OK? Feel them.
- Coolant in oil? Yellow gunk on oil filler cap?
- Water pump turns easily? Without excessive shaft play? Any signs of leakage?
- Belts OK, with reasonable tension?
- Evidence of freeze plug leaks?
- Head gasket bolts appear loose?
- Fan(s) run when hot?
- Thermostat work? (detect increased pressure by squeezing hose, or observe coolant flow in a radiator drained of a few inches of coolant)
- Heater work? Heater hoses hot?
- Radiator flow? View flow after draining a few inches of coolant. More rigorous tests require removal of the radiator.
- Bubbles? Look for bubbles or foam in the coolant in the radiator. If so, there might be a head gasket problem. The block tester or smog sniffer will tell.
- Steam from exhaust? If so, suspect head gasket problem.
Next, do the two chicken and egg tests:
I'm well aware that the combustion gas test has a significant cost attached to it, but I have still included it in General Maintenance. The cost of misdiagnosing a broken head gasket is so severe that I believe it warrants the combustion gas test in all but the most obvious cases. Even in cases where a definitive cause is found, such as a broken fan belt, remember that the overheat could have consequently broken the head gasket. Obviously, if steam out the exhaust or coolant in the oil reveals a broken head gasket, you know you have a broken head gasket without doing the combustion gas test.
- Pressure test OK?
- Block tester or smog sniffer reveal combustion gasses in coolant?
The few minutes and dollars spent on these tests greatly increase the chance of an accurate diagnosis and estimate. Once again, remember that overheats are likely to cause consequential damage, some of which in and of themselves can cause overheats. It's therefore common for an overheat to be found to have multiple causes.
Then there's a modification. I cannot recommend this, as I don't have enough information on it. But the possibilities sound intriguing. One Troubleshooters.Com visitor, a diesel technician with an ASE certification, says that he often drills a 1/8" hole in the thermostat. The hole is too small to affect the temperature regulation, but it's big enough to bleed out any gas accumulated at the thermostat. Such gas, whether it comes from combustion gasses or other sources (sucking back on a dry reservoir, for instance), could shield the temperature sensing part of the thermostat from the coolant temperature, thus making the thermostat "think" that the engine is cool, so the thermostat remains closed. Overheat city! The little hole bleeds off air before the engine overheats. The same T.C visitor told me in the last couple years he's seen several thermostats, especially for foreign cars, with the hole already there. Hmmmm!
Narrow it DownIf the General Maintenance was done properly, there's probably little you need to do in narrowing it down. Resolve any uncertainties with tests designed to narrow the scope of the problem, always keeping in mind that overheats often have multiple causes, especially in the case of head gasket problems.
Does it overheat more in stop and go traffic, or cruising at 65mph? The former implicates the fan or shroud, the latter tends to rule out the fan and shroud.
Advice to the MechanicInform the customer of all defective or semi-defective equipment you've found. Explain that it's his choice, but fixing a head gasket and leaving the partially clogged radiator could result in another busted head gasket a week later. Explain that replacing the radiator without replacing the head gasket could result in another overheat, and maybe this one will bend the heads. Explain that driving with a broken head gasket risks a broken starter and flywheel if coolant gets in the cylinders.
Advice to the CustomerDon't try to get off cheap in cooling system repairs. Overheats cause major consequential damage, such as broken head gaskets and even broken heads. Broken head gaskets in turn can cause broken starters and flywheels. Replacing a flywheel on a transverse engine is *very* expensive.
First ascertain the condition of the head gasket. If you're driving a cheap car, you may wish to junk it or sell it cheap upon hearing of a head gasket problem, especially if accompanied by a bad radiator. However, remember that all cars need repairs, and the car you replace it with might also need expensive repairs. So if it's a good, reliable car in reasonable condition, it's often best to bite the bullet and do the repair, even to the tune of $2000.00.
Once you've committed to making the repair, replace *all* bad and semi-bad components. If the water pump leaks or has excessive play in the bearings, replace it even if it might not have been the primary root cause. If the radiator is partially clogged, replace it. You don't want that radiator clog to rear its ugly head on a 3000 foot climb.
I consider a finding of clogged radiator to be an opportunity to get a heavy duty radiator. I spare no expense on radiators. A high capacity radiator can compensate for a host of other problems. When my Dodge Coronet radiator needed replacement, I had Harry at Valley Radiator build me a 17x25 4 row monster. It cost me $400.00, but let me tell you, when I drove it across the country in one of the worst heat waves ever, I was darned glad I spent the money. If you don't have a radiator shop you trust enough to custom build you a thyroidal radiator, or if you have a recent car that can't take a custom radiator, ask for the stock radiator that comes with the car's "towing package". Such a radiator is designed to dissipate the heat generated while towing a trailer up a 3000 foot climb. The extra $100 or so will be greatly appreciated as the years go on.
Sometimes you have the option of repairing the radiator. If you already have a heavy duty radiator, and there's a real opportunity to restore it to like-new condition, maybe you'll choose to repair the radiator. But you would still have deposits and diminished capacity after repairing the radiator, I'd personally choose to buy a new radiator instead of attempting the repair. A top functioning radiator is your best defense against long steep climbs or long drives in hundred degree weather.
Replace or Repair the Defective Part(s)Once mechanic and customer have reached a decision on the strategy of the repair, the parts are replaced and the old parts are given to the customer if requested. As a customer, I personally always request the old parts.
Owner TestingThe mechanic himself should have driven the car long enough to see it stabilize at a running temperature. If it doesn't stabilize, the repair is not complete.
Mechanic TestingFirst, drive under normal conditions to verify that the temperature stabilizes at a reasonable figure. If it does, this takes guts, but I recommend a trip over a peak like the Grapevine (but no air conditioning please, cars aren't designed to take that kind of abuse). If the temperature doesn't stabilize, shut it down long before the temperature gauge redlines. When it cools, fill the coolant, turn around and limp home. Consult the mechanic.