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I've recently learned that a bad head gasket can be a cause for a car to overheat. Could someone explain how this is possible and what's going on mechanistically?

It sounds like this symptom is not always associated with a bad head gasket, and not all head gasket leaks result in this symptom if I understand correctly. So under what conditions would a head gasket leak and engine overheating present itself? Is it when the leak is of such a large volume but not large enough that the engine just doesn't operate at all so at least the engine goes through enough combustion cycles to accumulate enough heat to drive an overheat scenario?

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The head gasket is a flat piece of material (can be made of many different substances) place between the head and block surfaces. It performs several different functions. (I've fully described a head gasket in this answer.) It's main purpose is to keep things in their place (oil, water, combustion, etc).

There are several different types of head gasket failures:

  • Coolant leaks into the cylinder
  • Oil leaks into coolant passage
  • Combustion gasses leak into coolant passage
  • Combustion gasses leak between two cylinders

(NOTE: There may be more, but this list will suffice for this answer.)

When an overheating issue is caused by a blown head gasket, the primary failure which occurred causing this is when combustion gasses leak into the coolant passages (usually coolant is leaking into the cylinder at the same time). When combustion occurs, large pressures occur inside the cylinder which can force the combustion gasses into the cooling system. This creates an over pressure inside the cooling system, which forces coolant out through whatever means is available. That's usually the radiator cap. Inside the cooling system, large pockets of gas form which then allows for overheating of the engine. It's a vicious cycle which will remedy itself rather quickly ... with a dead engine.

Usually things happen the other way around, though. Usually an overheated engine can cause the head to warp, which can cause a head gasket failure. The overheating condition can be caused by any reason (lack of coolant, blocked radiator, pump failure, etc). It still results in major engine damage.

  • Good point, the coolant system is something less than 30psi from what I understand from the range of some pressure testers. The combustion chamber on the other hand has pressures well above that. I can now visualize what's going on, gasses in the cooling system is a big no no just in general since the thermal conductivity of liquid is far greater than gasses, which is why folks tend to bleed/burb the gasses out the cooling system. If the head gasket leak is introducing gasses, hot gases at that, then I can definitely see the cooling system greatly compromised as you say. – jxramos Jan 26 '17 at 2:12
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First, understand that not all head gasket leaks (failures) are the same. The type and design of the engine will have a lot to do with it, as well as the location of the failure within the gasket. Some failures are small and others are catastrophic.

The head gasket is compressed between the head and the block. It seals around cylinders, as well as oil and coolant passages keeping them all separated from one another. It also plays an important job of creating a sealed combustion chamber for each cylinder.

When a gasket fails is can allow for cylinder gases to escape. Sometimes it does not affect anything else. Other times it can allow gases into the cooling system, as well as suck coolant into the cylinder(s). The failure can also allow oil in and, of course, loss of compression and performance.

So, to answer you question very broadly, yes, a failed head gasket can lead to overheating by allowing the cooling system to be compromised.

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