LAST week I advised a reader with an Opel Astra that has 300000km on the clock to watch out for early warning signs of head gasket failure.

LAST week I advised a reader with an Opel Astra that has 300000km on the clock to watch out for early warning signs of head gasket failure.

The next day I received a call from a friend asking me what exactly are the early warning signs of head gasket failure.

The head gasket is a thin, pre-cut template of steel or composite material which fits between the cylinder head and engine block. Its purpose is twofold: Firstly, it provides a watertight seal around the openings of the mating passages by which a coolant circulates through the head and block, and in the same way it provides an oil-tight seal around the openings of the passages carrying the engine oil. Thus it prevents cross-contamination of oil and coolant. Secondly, it seals off each of the combustion chambers to prevent any leakage of combustion gases under compression. A head gasket fails, or "blows", when it starts to leak due to cracking, burning or warping. These conditions are often caused by pre-ignition or detonation (pinging), overheating, improper installation of the gasket during an engine overhaul, or a slightly warped cylinder head. Incorrect tightening of cylinder head bolts or re-using stretch bolts are common mistakes made during installation. Like many other car ailments the chances of head gasket failure increase as the vehicle gets older. Some engines are also more prone to head gasket failure than others. For the benefit of all owners of high-mileage vehicles, here is a brief list of possible symptoms of a head gasket that's beginning to fail.

l Coolant leaking into an oil passage will cause a thick, creamy residue (similar to mayonnaise) inside the oil filler cap. In later stages the oil on the dipstick may look like chocolate milkshake.

l White smoke from a fully warmed-up engine can be an indication of coolant leaking into a combustion chamber. If antifreeze is used in the coolant, you may have sweet-smelling steam coming from the exhaust pipe.

One spark plug that's cleaner than the others may indicate a coolant leak into that cylinder. The symptom of a coolant leak will be a loss of coolant without any external leaks, drips or stains being evident. It will often be accompanied by hard starting, and rough running .

l A compression leak from a combustion chamber into a coolant passage is the most common way for head gasket failure to start. It will raise the pressure in the entire cooling system, thus hindering the flow of coolant which relies on a pressure difference created by the water pump. Mysterious overheating will follow, often accompanied by steam and coolant being pushed out of the radiator overflow tank. Sometimes a radiator hose or heater hose will let go before the pressure relief mechanism in the radiator cap opens. When the engine is switched off, the pressure in the cooling system becomes the dominant factor and forces coolant along the same path into the combustion chamber. There are tools to test for combustion gas in the coolant, but the home mechanic can check for a smell of fuel in the radiator header tank and watch for any sign of bubbles in the coolant.

l A compression leak between two adjacent cylinders will cause loss of engine power and rough running.

l A leak from an oil passage will usually show up as an external oil leak originating from the gasket area. Occasionally oil will be forced into a coolant passage, causing "mayonnaise" to form under the cap of the coolant reservoir.