Seals and gaskets only are designed to withstand the heat generated in the normal operation of an engine. When the engine overheats, the seals begin to melt, dry out or crack -- all of which can cause leaks. Valve stem seals, for instance, crack after an engine overheats. This leads to a variety of oil-related problems, including increased emissions. Performance also may be compromised if oil residue gathers on the spark plugs or around the intake valves.
Extensive damage occurs in an engine that runs in an overheated state for a prolonged period of time. This is the reason almost all vehicles are equipped with a heat gauge or with a sensor that warns the driver if the engine heat is too high. When an engine overheats, the vehicle needs to be turned off immediately to avoid permanent damage.
Cracked Head Gasket
If the engine gets hot enough, it can crack the head gasket. The heat in the engine causes metal to expand. This puts pressure on the head gasket, which is neatly fitted into place. Eventually, that pressure cracks the gasket. When this happens, water leaks into the cylinders of the engine, which causes damage to other parts. A cracked head gasket is time consuming to repair because the entire cylinder head needs to be removed to access it.
Warped Cylinder Head
If the engine gets extremely hot or if the heat lasts for a long period of time, it can warp the metal of the cylinder head. This is more common on modern vehicles with aluminum cylinder heads because aluminum warps more easily than the iron used in older-model cars. A warped cylinder no longer properly seals to the block. The head gasket eventually fails in this situation if the problem is not caught and remedied. The warped head alone causes ongoing problems with overheating, and the vehicle may not idle properly.
Once an engine gets hot enough for the coolant to boil, a number of events may take place, all of which might mean the end of the engine. If the coolant is boiling in the radiator, it may blow a hose off its clamp, blow a hole in the side of the hose or cause the radiator to explode. What's worse, in an engine this hot, the pistons might swell. This damages the bore, which means a very expensive engine repair -- if it is repairable at all.
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