The head gasket is vital for the functioning of an engine, acting to seal both the combustion chambers and the coolant and oil channels between the head and the block. For this reason any damage or leak caused to the gasket can cause fatal problems for your car, and if not remedied soon may result in a large repair bill.
There are a number of ways in which a head gasket may fail, and a number of reasons that may be the culprit. Below is a list of the most common reasons that may be encountered:
- Poor head gasket / engine design – some gaskets are more prone to failure due to weak gasket design, bimetal engines which may cause thermal stress, or uneven pressure caused by poor bolt positioning. Aluminum cylinder heads are often prone to damage due to their high thermal expansion rate. This high expansion rate of the gasket causes a weakening in the integrity of the material itself. There are a number of vehicles that are known to be prone to head gasket failure. Although these vehicles will have undergone extensive testing the issue is often not discovered for a number of years, and usually when the car is out of warranty and at a cost of the owner.
- Detonation (knock, spark knock, pinging) – detonation, or spark knock, is another reason why head gaskets may fail and can occur due to a number of different reasons including poor design, overheating or an accumulation of carbon deposits. Put simply, detonation basically occurs when the remaining end-gas (remaining fuel / air mixture) spontaneously combusts, leading to a sharp spike in pressure in the combustion chamber. This pressure spike may over time cause cracks and fractures and a loss of compression, leading to engine damage and ultimate failure.
- Pre-ignition – unlike detonation which occurs after the normal ignition of gas, pre-ignition occurs when the fuel/air mixture ignites prior to the spark plug firing. Again, there are numerous reasons why pre-ignition may take place, including carbon deposits in the chamber, an overheated spark plug tip or incorrect spark plugs, overheating or a lean-fuel condition. Unlike with detonation, which an engine can live with for some time, pre-ignition will cause fatal damage in a very short period of time due to the high temperatures and pressures involved.
- Installation error – Of course there is always the possibility that human error may be at fault, and head gasket installation error can often lead to damage. This could include not following the correct procedure or bolt sequence, or indeed using the incorrect torque specifications on bolts or even reusing old bolts; not having a flat or smooth block or head or the wrong surface finish. As you can imagine, a poorly fitted head gasket will inevitably lead to poor performance and damage.
- Overheating – this may be the result of a faulty water pump or thermostat or fan, a blown hose or coolant leak. Any extreme engine temperature may cause the head gasket to blow. Overheating may also be caused by pre-ignition as described above.
Where Can A Head Gasket Blow?
As we have seen there are many ways in which a head gasket can become damaged, but there are also different places where the gasket can blow, and these can result in differing symptoms.
- Failure between cylinders – engine will misfire and rough idle.
- Compression leak to crankcase – where the head gasket fails between the cylinder and the oil return port causing air/fuel or gases into the crankcase, leading to misfire, smoking and a loss of power.
- Head gasket blown to coolant port – the result will be coolant in the oil.
- Coolant leak to outside of engine from the oil return will result in an oil leak and low oil levels. – The head gasket blown to outside of the engine will result in a coolant leak and low coolant levels.
- Compression leak to coolant– where the head gasket fails between the cooling jacket and the cylinder, causing bubbles in the radiator, coolant overflow and overheating.
- Coolant leak to outside of engine – the head gasket blown to outside of the engine will result in a coolant leak and low coolant levels.
For a detailed description of the different symptoms that may be encountered with a blown head gasket, view our Blown Head Gasket Symptoms article.