So why have I written information I believe to be partially wrong? To establish a dialog so various people can come together to, over time, solve an incredibly tough class of automotive problem, and to give those already having that problem at least a fighting chance.
Therefore, this information has been superceded by the May 2000 issue of Troubleshooting Professional Magazine, which contains much more information and treats this information as a theory and probable fact, rather than as a hypothesis. This page has been left on the Troubleshooters.Com website to show the history of thinking that went into the head gasket and gas related overheating theory.
The first break happened when one reader had cyclic overheating problems (the temp would go up while driving, then go back down, then up, then down...) and also had white smoke coming out the exhaust. He installed block sealant fluid, and the cyclical overheat temporarily subsided. This is very indicative that the broken head gasket might be the cause of the overheating, as opposed to the usual case of overheating causing a broken head gasket.
Since then, I've paid close attention to all overheating complaints mailed to Troubleshooters.Com, and have noticed that in quite a few there was a reason to be suspicious of head gasket causation.
I believe it possible that these gasses heat and pressurize the coolant and possibly fill it with small gas bubbles, all of which reduces the ability of the coolant to carry heat to the radiator. The engine heats up. Once the pressure becomes too great, the combustion gasses are "burped" out the radiator cap and into the reservoir, at which time the coolant is once again able to carry heat so the engine cools. But the exhaust gasses are still transferring and building pressure in the coolant, so soon enough the process repeats itself.
Nor is cyclical overheat the only outcome of exhaust in the coolant. On some cars the pressure pushes out coolant rather than getting burped out, resulting in a coolant loss. Note that this can happen even in the absense of overheating. Exhaust gasses in the coolant are a documented cause of unexplained coolant loss at steady medium to high speed driving.
If the coolant loss is massive, the loss of coolant results in overheating. Many readers have written me saying they've had the thermostat, coolant and radiator replaced, and a pressure test indicates no leak, yet it's overheating. I advise those readers to check for combustion gasses in the coolant, as explained later.
I just got a particularly interesting email from a reader who found excessive hydrocarbons in his coolant, and was able to eliminate his overheating symptom by loosening the radiator cap. I'd imagine that once the exhaust gasses were free to go, they just cruised right out of the radiator. As long as a cooling system is capable of maintaining temperature below the boiling point under normal circumstances, this is quite believeable.
I've spoken to several mechanic and found that few believe this hypothesis. For one thing, many of these cyclical overheats occured without evidence of coolant in the oil (yellow gunk around the oil cap) or coolant in the cylinders (profuse white exhaust, especially on startup). As one mechanic put it, "a broken head gasket is a two way street". The idea is that once the engine is shut off, the coolant pressure will exceed the cylinder pressure and force coolent into the cylinders and oil.
I'm not so sure. Envision a head gasket defect so small that it acts like a valve, opening only above, let's say, 50psi. The cooling system bleeds at much lower pressures, so the transfer would be only from cylinder to coolant, but not the reverse, even when the engine is shut down.
Most of the readers submitting these symptoms have been to one or more mechanics, and typically had spent several hundred dollars replacing components. Most overheats are caused by clogged radiators, low coolant levels, bad water pumps, inaccurate thermostats and the like, so a mechanic can successfully diagnose and fix over 90% of overheats without ever considering a broken head gasket. And most broken head gaskets leave obvious clues such as yellow gunk on the oil cap or billowing white exhaust. I believe this is why most mechanics reject this hyptothesis.
I don't know how to use this test kit or how good its directions are, but my local mechanic expressed the opinion that an ordinary person could use it to perform the test. Needless to say, for safety reasons it must be performed BEFORE the engine overhats (in other words at the beginning of the day).
I don't know how accurate this test is. As more information comes in from Troubleshooters.Com readers we'll get a better idea.
If, in the future, I find that the Block Tester test is accurate and it is verified that combustion gas leakage frequently causes overheats, I will probably begin recommending Block Testers as a General Maintenance test for overheats and unexplained coolant loss.
Prevention starts with a temperature gauge. Do not buy a car without a working temperature gauge. By the time the temperature "idiot light" comes on, your engine is very close to destructively overheated. If you own a car with only an idiot light for temperature, consider having a gauge installed.
Once you have a working gauge, glance at it at least once every 10 minutes, or any time the car's operation seems strange. Be sure to shut the car down long before the needle goes "in the red" because engines continue to gain temperature after shutdown. It's much better to spend $150.00 for a tow truck than $2500 for an engine rebuild or replacement, or even $1000 for a simple head gasket replacement.
Read Steve Litt's Overheating Guide. It discusses many other tactics and strategies for overheating diagnosis, as well as a discussion on safety.
It's been 3 or 4 days since I put up the hypothesis web page (this page), and I've gotten quite a bit of feedback. Much of the feedback expresses an opinion that yes, it's possible for combustion gasses to get into the coolant without coolant in the oil or steam out the exhaust. Several of those responses were from acknowledged automotive experts on the web.
I've heard from several mechanics who sniff the radiator filler pipe with their emissions probe. If you sniff the radiator filler cap with smog equipment, do it after the engine has warmed up (so warm it up with the cap off), and BE SURE not to let the sniffer contact the coolant, as the coolant will destroy the sniffer according to at least one respondent.
That same respondent said the most sensitive test is the HC (hydrocarbon) test, although a false positive can occur if you've recently flushed your radiator with a petroleum based product. His opinion is that the HC test will reveal the earliest stages of combustion gas transfer, much earlier than the Block Testers will reveal.
Another respondent mentioned an interesting alternative dovetailing hypothesis. He said the concensus on the Mopar lists is that air bubbles in the coolant give rise to hotspots that cause gasket failure due to uneven cooling. Makes sense! Be very, very concerned if you see bubbles in your coolant. So once again we have a chicken and egg question -- did the head gasket create coolant gas, or did the coolant gas cause head gasket failure. Once again, a test to detect combustion gasses in the coolant answers the question.
If my hypothesis is correct, it MUST become common knowledge, because
people regularly spend hundreds of dollars replacing parts willy-nilly
in a vain attempt to fix certain overheat problems. We must find and publicize
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