Cooling system problems are a common theme in many of the letters sent to Motormouth, and mysterious coolant loss is one of the most challenging. There is no steam coming from under the hood, no puddles under the car, no soggy carpets inside. So where is it going?
Very often, we suspect a bad head gasket is allowing the coolant to escape. But without seeing the car, diagnosing a bad cylinder head gasket is usually difficult. The head gasket goes between the engine block and the cylinder head and seals the combustion chambers, coolant passages and oil passages.
We say “usually difficult” because there are two dead giveaways.
The first is steam billowing from the tailpipe. If the head gasket leak allows coolant to enter the combustion chamber, it departs via the tailpipe. We’re not talking about some wispy steam like the kind you see in the morning on cold days. We’re talking a serious cloud that would be the envy of mosquito abatement trucks. Quite often the steam has an acrid odor. Minor leaks, however, are not so dramatically revealed.
(Incidentally, it also is possible for pressure from the combustion chamber to leak into the cooling system. A ballooned radiator, or leaking radiator, is a good clue.)
The other dead giveaway is on your dipstick. Instead of seeing oil there, the fluid looks like a chocolate milkshake. The oil will be the same brown color as the milkshake because of the coolant and oil mixing together. The coolant leaking into an oil passage causes this.
There are other clues, but they are subtler and may require professional tools. If you can replace your own spark plugs, you can spot one of the signs of a head-gasket leak. If one of the plugs appears much cleaner than the others, coolant may be leaking into that cylinder.
If your engine has a miss first thing in the morning but is fine the rest of the day, it could be an early warning sign of a head-gasket leak.
A professional tech can find small leaks by pressurizing the cooling system overnight. If the pressure has dropped to zero by the next morning, there is a leak somewhere. By disabling the ignition and cranking the engine, then removing the spark plugs, the tech might find one wet with coolant.
If a combustion leak into the cooling system is suspected, your technician may be able to sniff for signs of hydrocarbons or carbon monoxide in the cooling system. No, not with his nose, with an exhaust gas analyzer. Holding the analyzer probe over the open radiator, it can detect exhaust gases that have gotten into the cooling system.
Sometimes, a leak in the cooling system will cause your engine’s temperature to spike. That is because the system should be under pressure, which raises the boiling point, and a leak reduces the system pressure. When a leak lowers the boiling point the coolant boils more readily, which makes it less effective, and the engine runs hotter. The leak could be anywhere in the system, including the head gasket. A professional technician has the tools to pressurize your car’s cooling system and check for leaks, internal and external.
Not all mystery leaks are head-gasket failures. A defective radiator cap may allow coolant to flow from the system when the engine warms up but fail to allow the coolant to return from the overflow jug as the engine cools.
Using an adapter on a cooling system pressurization tool, your technician can test the pressure cap. But if you suspect it may be the culprit, it is probably just as cheap to replace the cap.
Quite often when the head gasket is damaged, so is the cylinder head, which may be warped. Sometimes, the head can be straightened or flatness restored. Sometimes not. On an overhead cam engine, warpage usually affects the cam supports for the camshaft. Sometimes this can be corrected, but other times the head will have to be replaced, and this can be expensive.
The No.1 cause of head-gasket leaks is overheating. So routine cooling-system maintenance can prevent costly breakdowns.
Before condemning a head gasket, your technician should eliminate other possible causes. Replacing the head gasket also is expensive. To help prevent such repairs, maintain your car’s cooling system by having your coolant (antifreeze and water mixture) replaced according to schedule.
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Bob Weber is an ASE-certified Master Automobile Technician, having recertified every five years since 1978. Contact him at MMTribune@netscape.net
