Head Gasket

Automotive illustrated glossary

Engine cylinder head

Head gasket. Click for larger picture

Engine cylinder block

The gasket installed between the cylinder block and the cylinder head of the engine is called a head gasket. A head gasket seals the combustion chambers as well as oil and coolant passages between the cylinder block and the cylinder head. A head gasket also has to be able to withstand some movement between the cylinder block and the head, because when the engine warms up, the cylinder head and the cylinder block may expand at different rates, especially when the cylinder block and the head are made from different materials (e.g. cast iron block with aluminum head).

Head gasket problems

The head gasket failures are often caused by overheating, for example, when the engine is low on coolant or when the radiator fans don't work. There is also a number of other factors that can contribute to head gasket problems: the detonation (engine knock), clogged EGR system, design flaw, etc.

Here a few of the signs of possible failed head gasket:
- The coolant in the engine oil (the engine oil has a "coffee with milk" color).
- The engine oil in the coolant (the coolant looks dark and dirty with some oil in it).
- White steam (smoke) with sweet (antifreeze) smell from the exhaust.
- Bubbling it the cooling system.
- Loss of coolant with no visible leaks.
- Overheating.
- The starter clicks, but won't crank; the starter cannot turn over the engine due to a hydraulic lock caused by coolant in one of the cylinders.

Not all of these symptoms could be present at once and some of these symptoms can also be attributed to other problems such as a crack in the cylinder head, leaking intake gasket, etc. A head gasket problem may get worse gradually: in the beginning, the car may just run a little rough and sputter when started cold, as few drops of coolant seep into the combustion chamber through a small damage in the head gasket. Over time, as the leak gets bigger, the coolant level may drop significantly causing the heater not to work and the car may start overheating.

If you suspect your car has a bad head gasket, it's important to have the car checked out as soon as possible because driving with a bad head gasket can cause more damage to the engine. Take it to a repair shop where a mechanic could do a cooling system pressure test or check if the exhaust gases are present in the cooling system. A cylinder leak-down test can also tell a lot.

Broken piston from badly overheated engine When the head gasket fails, it's important to find what caused it to prevent future problems. For example, if the radiator fans don't work, the engine will overheat again and the newly replaced head gasket will fail very fast. It's also important to determine what else is damaged. Because if the vehicle was driven with the coolant present in the engine oil, the crankshaft bearings could go bad due to lack of lubrication. The coolant leaking into the combustion chambers can damage a catalytic converter and cause a hydraulic lock, which may result in broken pistons and bent connection rods. If the engine oil has gotten into the coolant, the whole cooling system will need to be flushed. If the engine has been severely overheated, there is a good chance that it has more damage, such as broken pistons, warped cylinder head, damaged seals; even the cylinder block could be warped. So, before spending money on repairs, consider all other options. Sometimes, for example, when the engine was severely overheated, it might be cheaper to replace the whole engine with a used or a rebuilt unit.

Replacing the head gasket

The head gasket itself is not very expensive, so it's always better to use a good quality part rather than cheap replacement. Replacing the head gasket though is a big job as a cylinder head(s) with manifolds has to come off. The cylinder head needs to be checked and machined if needed; the timing may need to be reset and valves readjusted. There are few other parts that usually replaced along with the head gasket: intake gaskets, valve cover gaskets, head bolts, etc. It may be also a good idea to do the valves and change the spark plugs once the head is out.
The head gasket replacement in a repair shop could cost from $700 - $800 and up to $2000, depending if the cylinder head(s) needs to be machined or some other parts need to be replaced.

Replacing the head gasket

If you are planning to do the job yourself, replacing a head gasket is a very difficult job, but not impossible. I've done it myself in a home garage few times. With some skills, a repair manual, necessary parts, good tools and a friend who can help you with heavy lifting it's a full day's job. Of course, this is if the cylinder head checks out OK and doesn't need to be machined and there is no other damage.

If you are looking for the information how to replace a head gasket in your car, there is a website where for a fee you can get an on-line access to the repair information for your car. To learn more visit this page:
Where to find technical information about your car.