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Pat Goss: Ask The Expert

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Head Gasket Headaches

Every month I receive far too many inquiries about overheating and possible head gasket failures. Frequently the car has been to multiple shops where it has been tested seven ways from Sunday, with no conclusive answers. This is understandable when you realize that schooling on this matter is often right out of the stone ages.

To simplify things letís begin by dispelling some of the more common misconceptions. Like, when a head gasket fails, coolant will mix with the oil or oil will mix with the coolant. Unh-uh, those symptoms only occur in about 2% of the head gasket failures I encounter.

Much more typical is overheating, a fluctuating temperature gauge, forceful discharge of coolant from the radiator, or no heat from the heater. It could be any of these symptoms or, any combination of them.

The next fallacy is, cooling system pressure testing is a good way to identify a failed head gasket. IT USUALLY IS NOT. The most common form of head gasket failure allows combustion gases, under extreme pressure, to leak past a defect in the gasket and enter the cooling system. Because combustion pressures are much higher then cooling system pressures, exhaust gas will displace some of the coolant and may cause it to be forcefully expelled from the radiator.

Exhaust gas is not an effective coolant consequently the vehicle often overheats. Gases are poor conductors of heat, so every time a pocket of gas passes over the coolant temperature sensor, the temperature gauge fluctuates. And when the heater core becomes filled with gas, thereís no heat from the heater.

But why is pressure testing not the answer? Head gaskets form an airtight seal between coolant and the pressure of burning fuel. The maximum safe pressure, which can be applied to a cooling system, is less than 20 pounds while combustion pressures (on the other side of the head gasket) are in the hundreds of pounds. Plus the molecules of coolant are enormous compared to molecules of exhaust gas.

Therefore itís typical that the higher pressure and smaller molecules of gas can be pushed through a gasket defect much too small for the lower cooling system pressure to push large coolant molecules through.

If pressure testing is not valid what is? An exhaust gas analyzer is quick, accurate, and easy! The engine is warmed up with the radiator cap removed and held at a fast idle while the exhaust gas sensing Commercial Three Baybe (piece normally placed in the tailpipe) is held about an inch above the open radiator filler neck. If the hydrocarbon readings on the tester increase noticeably the engine has a bad head gasket.

Note: EXTREME caution must be used to prevent even the smallest amount of coolant from entering the sample tube. If coolant is inadvertently drawn into the exhaust gas analyzer it can instantly cause hundreds to thousands in damage to the tester.

The exhaust gas test takes less than ten minutes and is highly accurate. Naturally it would be prudent to start with a pressure test, which would disclose massive leaks but if pressure testing is inconclusive Commercial Three Bayceed immediately to the exhaust gas test. End of story!

© Copyright 12/27/02 Pat Goss all rights reserved, 525 words.

Date Updated  Friday, December 27, 2002

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