A head gasket failure is bad news – very bad news – and will require immediate attention. A head gasket can fail in a number of different ways (coolant leak, oil leak, loss of compression), all of which are harmful for the engine.
The most common cause of a blown head gasket is engine overheating. When the engine gets too hot, the cylinder head expands (thermal expansion), which can crush the head gasket and cause failure.
Once a head gasket has failed it can cause all manner of problems, including:
A head gasket failure may have been caused by an overheating engine (as a result of clogged radiator, faulty fan, etc), but in turn a blown head gasket will also cause the engine to overheat.
Hot exhaust gases can leak into the cooling system, or coolant can leak into the cylinders and be burnt off as steam. Either way, the result is an overheating engine.
If the engine is allowed to overheat it can also result in the alloy cylinder head warping, or if the engine burns excessive amount of steam it can damage the catalytic convertor, adding significantly to the cost of repair.
2 Loss of power
If the head gasket fails in such a way it allows the compressed air/fuel to escape, the compression of that cylinder is reduced. This loss of compression results in a rough-running engine and a notable reduction in engine power.
3 Oil contamination
One of the most famous telltale signs of head gasket failure is the milky sludge or 'mayonnaise' on the underside of the oil filler cap or on the dipstick. Although not conclusive proof of head gasket failure, this is generally a good indicator, and is a sure sign your engine needs to be looked at.
Can a head gasket cause an oil leak? Not exactly, but when a head gasket fails it can allow the coolant and oil to mix, resulting in contaminated oil that will ruin the engine’s bearings if left unattended. If anything, the oil-coolant contamination will cause the engine oil level on the dipstick to rise.
Repair requires a complete engine oil flush as well as a replacement oil filter to ensure all traces of contaminated oil are removed from the engine.
A faulty head gasket can result in blue smoke coming from the exhaust. It's caused by oil leaking past the gasket and into the cylinders, where it is then burnt as part of the combustion process.
The same failure can also cause exhaust gases to enter the oilways and pressurise the crankcase, causing lubrication issues and excessive engine wear.
5 External leaks
If a head gasket has failed between the waterway or oilway and the outside of the engine, the result can be a simple coolant or oil leak.
This may not manifest itself as an immediate problem (other than causing a mess) but if the coolant or oil levels are allowed to drop too far it can lead to serious engine issues.
How is a head gasket fixed?
In the most simple of cases, your mechanic will remove the cylinder head, remove the old gasket, possibly have the head skimmed to ensure a perfectly flat surface when it comes to refitting, then they'll flush out the cooling system – it has to be completely free of oil, or the car will overhead again. If the oil/water gunge in the engine is bad, it’s not uncommon to write the whole car off depending on its age.
But if the job can go ahead, everything is bolted back together, which includes replacing the timing belt or the chain (if necessary).
How long does it take to fix a head gasket?
Anything from six hours to a few days, depending on the severity of the failure. A blown head gasket is one of the biggest failures your car can suffer, and to fix it properly takes time. If you get it repaired by the cheapest rather than the best, you’re going to be back at square one pretty quickly.
How much does it cost to fix a head gasket?
The head gasket itself is a comparatively cheap part - as little as £20-£40 in most cases. However, the labour involved by getting a garage to do it for you will land you with a final bill that's likely to exceed £500.
Every Haynes manual walks you through the process of head gasket replacement as part of the 'cylinder head removal and refitting' procedure. Find your print and online manual here and save yourself hundreds of pounds.
How to prevent head gasket failure
Prevention is much better than cure when it comes to head gaskets. Head gasket failures are usually the result of an engine overheating, so the best way to prevent a problem is to ensure your cooling system is in good condition.
Ensure the system has no leaks, the radiator is working efficiently and the coolant is topped up to the correct level. Also, make sure electric fans are working correctly, and that the thermostat opens at the temperature it should.
If you suspect a head gasket failure you can test for carbon dioxide in the cooling system. This test will show if the compression has leaked into the cooling system, and therefore if the head gasket has blown. However, it won't show if there are any other problems with the head gasket, so the absence of carbon dioxide in the cooling system does not guarantee a healthy head gasket.
Some head gaskets can just fail because they are of a poor design and are not robust enough for the application. Thankfully MLS (multiple layer steel) replacement gaskets are available for most applications and offer improved reliability over the original gasket design.