Occasionally, the compression in the cylinder will cause a leak to form in the gasket and the gasket will have to be replaced, or severe damage can take place (a "blown" head gasket). This problem has been exacerbated by the use of aluminum rather than iron cylinder heads; while lighter than iron, aluminum has a much greater thermal expansion rate, which in turn causes a great deal more stress to be placed on the head gasket.
Engine makers have responded to this by adding a non-stick coating such as Teflon to the surface of the head gasket.
If the gasket fails, a variety of problems can occur, from compression loss (leading to power reduction, or a rough engine), to exhaust gases being forced into the cooling system, leading to the engine overheating and increased engine wear due to the motor oil being mixed with antifreeze. Coolant can leak into the cylinders, causing the exhaust to issue steam[3] and the catalytic converter to be damaged. If a very large amount of coolant does this, hydrolock can occur, causing extensive engine damage. Sometimes, all that may happen when a head gasket is blown is excessive steam erupting from the tailpipe; yet the engine may act and drive like normal, until all the coolant is gone and the engine overheats.
A damaged head gasket can cause compression to leak between cylinders.
The condition of a head gasket is typically investigated by checking the compression pressure with a pressure gauge, or better, a leak-down test, and/or noting any indication of combustion gasses in the cooling system on a water-cooled engine. Oil mixed with coolant and excessive coolant loss with no apparent cause, or presence of carbon monoxide or hydrocarbon gases in the expansion tank of the cooling system can also be signs of head gasket problems. A good sign of head gasket failure on water cooled engines is the presence of a substance that resembles mayonnaise in the oil, often to be seen on the dipstick, or oil filler cap. However, the presence of this substance is not conclusive proof of head gasket failure, since oil could mix with the coolant via other routes.
Driving with a blown head gasket (if possible) can cause additional extensive damage due to overheating or loss of lubrication.
Blue exhaust smoke may be indicative of oil burning. White exhaust smoke is an indicator of coolant burning, another sign of head gasket wear.