A blown head gasket is bad news. Very bad news. If you think yours is going, it requires immediate attention. A head gasket can fail seven slightly different ways, which we told you about recently, and all of which are bad news for the engine. If there is a sweet smelling white cloud following behind you at all times, you may have a bad head gasket.
Typically head gaskets fail when the head and the engine expend at different rates, and the gasket can't seal the newly expanded gap. This issue is made worse on some motors which use an iron cylinder block and an aluminum head. Moreover, some engines are just designed with poor clamping force from the head bolts, or have heads prone to warping, and a reputation for failure.
Once a head gasket has failed it can cause all manner of problems, including:
A head gasket failure may be caused by an engine overheating one too many times (as a result of a clogged radiator, coolant leak, faulty fan, etc.), but the blown head gasket can also cause the engine to overheat. Hot exhaust gases can leak into the cooling system, or coolant can leak into the cylinders and be burned off as steam, either way, the end result is an overheating engine.
If the car is driven while overheating, it can also result in the alloy cylinder head warping, or steam damaging the catalytic converter, adding significantly to the cost of repair.
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. This sort of failure typically is accompanied by a sound like an exhaust leak.
One of the most famous signs of head gasket failure is the milky sludge on the underside of the oil filler cap or the dipstick, sometimes jokingly called a "milkshake". This is caused by coolant getting into the oil, and vice versa. Although not conclusive proof of head gasket failure this is generally a good indicator and is a sure sign your engine needs to come apart to find the source of contamination.
With antifreeze contaminating the oil, any driving will quickly ruin the engine’s bearings. Repair requires at least an engine oil flush as well as a replacement oil filter, and often times complete disassembly of the bottom end of the engine to ensure the bearings aren't damaged and clear out all contaminated oil.
A faulty head gasket most often results in billowing clouds of sweet-smelling white smoke coming from the exhaust. Thie smoke is caused by antifreeze leaking past the gasket and into the cylinders, where it is turned to steam as part of the combustion process. Less common, but still possible, is a leak from an oil passage to the cylinder, which would cause blueish smoke.
Either of these types of gasket failure will also allow combustion pressure into the cooling system or oil breather system. If a radiator hose suddenly blows off its water outlet, or the dipstick won't stay put, this could be the reason.
If a head gasket has failed between the water or oil passage and the outside of the engine, the result can be a simple coolant or oil leak. This is the least dire version of a blown head gasket, but still serious.
An external leak may not manifest itself as an immediate problem (other than causing a mess), but if the coolant level is allowed to drop too far, it can lead to serious engine issues. The other issue is that leaking oil could get on the hot exhaust leading to acrid smoke, and possibly fire.
A few dollars of prevention is much better than the several thousand dollar cure when it comes to head gaskets. The replacement head gasket itself is not expensive, but the repair is very labor intensive, which significantly increases the cost of repair, especially on modern cars.
Head gasket failures are usually caused by repeated overheating or continuing to drive after the car has overheated, so the best way to prevent a head gasket failure is to ensure your cooling system is in good condition. And if your vehicle does start to boil over, stop, let it cool for at least an hour, and refill the radiator before continuing.
Checking the cooling system is easy: Ensure there are no leaks, that the radiator is working efficiently, the thermostat opens properly, and the coolant is topped up to the correct level. Also make sure the fan (mechanical or electric) is working, has all of its blades, and has a shroud around it to increase the efficiency.
If you suspect a head gasket failure, the scientific test is to check for combustion gasses 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. The old mechanic's trick is to take off the radiator cap, start the car, and look for bubbles in the coolant.
However, these will not show if there are any other failure spots in with the head gasket, so the absence of gasses in the cooling system does not guarantee a healthy head gasket.
Some older head gaskets can just fail because they are of a poor design, and are not robust enough for the application. This used to be more of an issue with older style, metal gaskets, which could only last for so many years of going from cold to hot with every startup before an inevitable failure. Thankfully modern MLS (multiple layer steel) replacement gaskets are now available for most applications, and offer improved reliability over the original gasket design.