A blown head gasket is bad news. Very bad news. If you think yours is failing, it needs immediate attention. A head gasket can fail in about seven slightly different ways, and all of them are bad for the engine. If a sweet-smelling white cloud follows your car everywhere, you may have a bad head gasket.
Below you will find why head gaskets fail, the five most common signs, how to test for a blown gasket, and how to prevent it from happening.
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Why Head Gaskets Fail
Head gaskets usually fail when the cylinder head and the engine block expand at different rates, and the gasket can no longer seal the new gap. This problem is worse on engines that use an iron cylinder block with an aluminum head. Some engines are also designed with poor clamping force from the head bolts, or have heads that are prone to warping, and they have a reputation for failure.
Once a head gasket has failed, it can cause all kinds of problems. These are the five most common signs.
5 Signs of a Blown Head Gasket
1. Overheating
An engine that has overheated one too many times (because of a clogged radiator, a coolant leak, a faulty fan, etc.) can blow its head gasket. But a blown head gasket will also make the engine overheat. Hot exhaust gases can leak into the cooling system, or coolant can leak into the cylinders and burn off as steam. Either way, the result is an overheating engine.
If you keep driving the car while it overheats, the alloy cylinder head can warp, or steam can damage the catalytic converter. Both add a lot to the cost of the repair.
2. Loss of Power
If the gasket fails in a way that lets the compressed air and fuel escape, the compression in that cylinder drops. This loss of compression makes the engine run rough and noticeably reduces engine power. This type of failure usually comes with a sound like an exhaust leak.
A rough-running engine has many other possible causes too. On VW group cars, see our guide to rough idle fixes for the VW Golf, Passat, Jetta and Audi.
3. Oil Contamination (the “Milkshake”)
One of the best-known signs of head gasket failure is a milky sludge on the underside of the oil filler cap or on the dipstick, sometimes jokingly called a “milkshake”. It is caused by coolant getting into the oil, and vice versa. It is not conclusive proof of a failed head gasket, but it is a good indicator, and a sure sign that your engine needs to come apart to find the source of the contamination.
With antifreeze in the oil, any driving will quickly ruin the engine’s bearings. The repair needs at least an engine oil flush and a new oil filter. Often the bottom end of the engine must be completely taken apart, to make sure the bearings are not damaged and to clear out all the contaminated oil.
4. White Smoke from the Exhaust
A faulty head gasket most often causes large clouds of sweet-smelling white smoke from the exhaust. Antifreeze leaks past the gasket into the cylinders, where the combustion process turns it into steam. Less common, but still possible, is a leak from an oil passage into a cylinder, which causes bluish smoke.
Both types of failure can also push combustion pressure into the cooling system or the oil breather system. If a radiator hose suddenly blows off its outlet, or the dipstick will not stay in place, this could be the reason.
5. External Leaks
If the gasket has failed between a water or oil passage and the outside of the engine, the result can be a simple coolant or oil leak. This is the least dangerous type of blown head gasket, but it is still serious.
It may not cause an immediate problem (other than a mess), but if the coolant level drops too far, it can lead to serious engine damage. Leaking oil can also drip onto the hot exhaust, which causes acrid smoke and possibly a fire.
How to Test for a Blown Head Gasket
The scientific test is to check for combustion gases in the cooling system. This test shows whether compression has leaked into the cooling system, and therefore whether the head gasket has blown.
The old mechanic’s trick is to take off the radiator cap, start the car, and look for bubbles in the coolant.
Warning: Only remove the radiator cap when the engine is cold. The coolant in a hot engine is under pressure and can cause serious burns.
However, these tests will not show other failure points in the head gasket. So if you find no gases in the cooling system, that does not guarantee a healthy head gasket.
How to Prevent Head Gasket Failure
A few dollars of prevention is much better than a cure that costs several thousand dollars. The replacement head gasket itself is not expensive, but the repair is very labor-intensive, which makes it costly, especially on modern cars.
Head gasket failures are usually caused by repeated overheating, or by continuing to drive after the car has overheated. So the best way to prevent a failure is to keep your cooling system in good condition. If your car starts to boil over, stop, let it cool for at least an hour, and refill the radiator before you continue.
Checking the cooling system is easy:
- Make sure there are no leaks.
- Check that the radiator is working efficiently.
- Check that the thermostat opens properly.
- Keep the coolant topped up to the correct level.
- Make sure the fan (mechanical or electric) works, has all of its blades, and has a shroud around it to make it more efficient.
Some older head gaskets fail simply because of a poor design: they are not strong enough for the application. This used to be more of a problem with older metal gaskets, which could only survive so many years of going from cold to hot with every start-up. Thankfully, modern MLS (multi-layer steel) replacement gaskets are now available for most applications, and they are more reliable than the original gasket designs.
FAQ
Can I keep driving with a blown head gasket?
It is not a good idea. Driving while the engine overheats can warp the cylinder head and damage the catalytic converter, and driving with coolant in the oil quickly ruins the engine bearings. Stop, let the engine cool, and have it checked.
Is a head gasket expensive to replace?
The gasket itself is cheap. The cost comes from the labor, because the engine has to be taken apart. That is why preventing overheating is so much cheaper than the repair.
Source: 5 Signs You Have a Blown Head Gasket (and How To Prevent It), Pomona Swap Meet blog
