A blown head gasket is one of a motorist's nightmares, the mere mention of it can send the car owner's blood cold. A repair requires the removing of the cylinder head and whilst the cost of part itself can be minimal, the labour involved is great and the costs commensurate. However, if left unchecked, a blown head gasket can potentially lead to further damage to other parts of the engine and should be addressed at the earliest opportunity.

So, what is the head gasket and what does it do? The gasket is a slim metal template that sits between the engine block and the cylinder head and forms a seal between the two. Essentially, it performs two important functions. Firstly, it keeps the combustion process within the engine and secondly keeps the coolant and oil separate as they move between the engine block and the car's cylinder head.

There are several ways of diagnosing a problem with this part of the car engine. The classic indicator of a problem with head gasket is the exhaust gases being produced by the car. When a head gasket is damaged, coolant can get sucked into the cylinders and is ejected via the exhaust pipe as white steam. The exhaust gases might also smell unusually sweet due to the anti-freeze contained within. However, this is not always the result of an issue with the head gasket but if suspected, a hydrocarbon test of the coolant will prevent a misdiagnosis.

Another easily checked symptom is a build up of mayonnaise-like white sludge under the oil filler cap after the engine has been running. This indicates that coolant is mixing with the oil, which is then emulsifying. However, it should be noted that temperature differences may also lead to this symptom as condensation may gather under the filler cap which also emulsifies when mixed with oil.

The coolant system is also often an effective indicator. The coolant system of a car is normally a closed system. If the head gasket is leaking then air can enter the coolant system. Try running the car up to temperature and then carefully remove the coolant cap. Rev the engine and check the expansion tank for bubbles. These would indicate air entering the system and may suggest an issue with the gasket. The coolant system might also lose fluid. It should be noted though that several other problems with the motor cooling system can also result in these symptoms showing.

A loss of engine power is also a common result of an issue with the head gasket. If the gasket is blown then compression will be reduced in the cylinders, resulting in a noticeable loss of power. A garage will be able to carry out a compression test to verify if this is indeed the cause of the loss of power.

Whilst these symptoms all suggest a leaking head gasket, it should be worth noting that some, if not all of them can also be attributed to other problems. The best advice is to look for a combination of these symptoms and make a diagnosis of that basis. Then take your car to a local garage and advise them that you think the problem is a blown head gasket and want this confirmed. This will prevent unnecessary work and cost if the gasket is not the problem. Hopefully, such tests will confirm that the engine problem is something different and hopefully cheaper to repair. This recently happened to me when I noticed mayonnaise on the filler cap and was losing coolant. A trip to the garage resulted in the head gasket being passed fit and a hole in the radiator explaining the loss of coolant. Extreme cold morning temperatures were leading to condensation under the filler cap, explaining the build up of the milky deposit. This shows that it is possible to misdiagnose a head gasket problem.