All internal combustion engines have an engine block and one or more cylinder heads. The mating surfaces where the block and head meet are filed flat for a close, precision fit, but no amount of careful machining can make them completely water tight or be able to hold back combustion gases from escaping through the mating surfaces.
To seal the block to the heads, a head gasket is used. The head gasket has many things it is sealing against. First is the combustion pressure on each cylinder. Oil and coolant must be made to easily flow between block and head and it is the duty of the head gasket to keep these fluids from leaking out or into the combustion chamber, or each other as the case may be.
Basically, the head gasket is made of soft sheet metal that is stamped with ridges that surround all leak points. When the head is placed on the block, the head gasket is put between them. Many bolts, called head bolts are screwed in and tightened down causing the head gasket to crush against and form a tight seal between the block and head.
Head gaskets usually fail if the engine overheats for a long period of time causing the cylinder head to warp and pass pressure on the head gasket. This is most prevalent on engines with cast aluminum heads, which are now on about all modern engines.
Immediately coolant or combustion gases leak past the head gasket, the gasket material is usually worn to a point where it will no longer hold the seal. This causes leaks in several possible areas like:
- Combustion gases leaking into the coolant passages causes excessive pressure in the cooling system.
- Coolant leaking into the combustion chamber could cause coolant to escape through the exhaust system, often causing a white cloud of smoke at the tailpipe.
- Other problems include oil mixing with the coolant or being burned out through the exhaust.
Some engines have more susceptibility to head gasket failure than others. There are instances where blown head gaskets on engines that just started to overheat and were running hot for less than 5 minutes. Advice for this kind of situation is, if the engine shows signs of overheating, find a place to pull over and shut the engine off as quickly as you could.
A head gasket replacement starts with the diagnosis that the head gasket has failed. There is no way for a mechanic can tell for certain whether there is additional damage to the cylinder head or other components without first disassembling the engine. All he knows is that fluid and/or combustion is not being contained.
A good way to tell if a head gasket has failed is through a combustion leak test on the radiator. This is a chemical test that shows if there are combustion gases in the engine coolant. Another way is to remove the spark plugs and crank the engine while watching for water spray from one or more spark plug holes.
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