Head Gaskets: What They Are, and Signs You’ve Got a Bad One

Head gaskets provide a tight seal between the engine cylinder head and block. When they’re working correctly, they help provide the ideal compression in your cylinders and prevent coolant, oil and other unwanted substances from getting in those cylinders.  

But things can go wrong. Head gaskets have to deal with extreme situations. They work in temperatures as high as 400 degrees Fahrenheit and stand up to high combustion pressures. Conditions in the combustion chamber change between extreme pressure and extreme vacuum, requiring the head gasket to compress and recover fast enough to maintain a seal. 

When one fails, it can be a very expensive repair. There are few common reasons for head gasket failure:

  • Engine performance problems: Issues such as overheating and abnormal combustion can cause a head gasket to fail. Overheating is the most common cause of head gasket failure.
  • Deterioration: The head gasket may deteriorate over time. This is especially true on older vehicles, which typically came with less durable head gasket material.
  • Design flaw: In some cases, the gasket or engine may come flawed from the factory. This can result in accelerated head gasket failure.
  • Incorrect installation: If the cylinder head was removed for repair, it’s possible that the head gasket was damaged during reinstallation.
Get it diagnosed by a professional

Symptoms of a failing head gasket

A faulty head gasket can cause a number of problems with your car. Here's what to look for.

Your engine overheats: A faulty head gasket can let coolant into the combustion chamber. This can keep your coolant from circulating adequately, resulting in overheating.

You have a low coolant level: If a head gasket leaks, coolant can get in the combustion chamber or leak down the side of the engine. Either scenario can cause a low coolant level.

Your heater isn’t working: Your car’s heater relies on warm engine coolant in order to work. A low coolant level, caused by a leaking head gasket, can result in an nonfunctional heater.

Your engine misfires: If coolant is leaking into the cylinders, you may experience incomplete combustion. A faulty head gasket can also cause low engine compression, since the combustion chamber is not properly sealed. In either case, the outcome is an engine misfire. 

You see white smoke coming from the tailpipe: White smoke billowing from the tailpipe can be caused by a bad head gasket that is allowing coolant into the engine. The smoke may have a sweet odor to it. 

Your check engine light is on: The car’s computer constantly monitors engine performance. If it detects a misfire caused by a leaking head gasket, it turns on the check engine light.

Oil and coolant mix together: A head gasket that fails between the oil and coolant passages let the two fluids mix together. This doesn’t happen every time, though.

Engine failure: An engine can’t compress liquids, such as coolant. If coolant enters the combustion chamber, it can result in catastrophic engine failure. The same is true if coolant and oil mix together, preventing adequate lubrication of engine parts.

How to fix the problem

A failed head gasket must be replaced — a job that could cost you thousands of dollars. So, before you get it replaced by a mechanic, it’s important to figure out why the head gasket failed. Otherwise, the replacement gasket is likely to fail as well. 

If the head gasket failed due to an engine performance problem, that must be addressed first. Replacing a head gasket often requires that the cylinder head or engine block be machined at the same time. Otherwise, the new gasket won’t seal properly. 

That could be another expensive repair, so before going forward, calculate if your car is worth it. If it’s an older car with a lot of miles, you may be better off getting a new vehicle. Get an estimate from your mechanic, do the math and decide what’s best.


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2011 Subaru Outback ; cylinder head replacement due to head gasket. Seems that they have NOT figured out how to make this not happen still at Subaru. 160k on car well maintained, still happened. anyone know if Subaru takes responsibility for this? Class action suit?