Signs that *suggest* a blown head gasket (but could be other issues):
* White smoke from the exhaust: Especially if it's thick, milky white and persists even after the engine warms up. This often indicates coolant is leaking into the cylinders. (Oil burning will produce bluish-gray smoke.)
* Overheating: The engine repeatedly overheats, even with sufficient coolant. A blown head gasket can allow coolant to leak into the cylinders or even externally.
* Coolant loss: You're losing coolant, but you can't find external leaks. The coolant may be leaking into the cylinders or oil.
* Oil contamination: The oil looks milky or has a chocolate-brown sludge (emulsified oil and coolant). This is a strong indicator.
* External coolant leaks: While not *directly* indicating a blown gasket within the engine, leaks around the head gasket area are suspicious.
* Loss of compression: A compression test will show significantly lower compression in one or more cylinders. This is a good way to narrow down the cause, though low compression doesn't automatically mean it's a blown head gasket.
* Sweet smell in exhaust: Coolant is mostly water, but contains ethylene glycol which has a slightly sweet smell. You might detect this in the exhaust fumes.
Tests that can offer *some* evidence (but not definitive proof):
* Coolant pressure test: Pressurizing the cooling system can sometimes reveal leaks, but it won't necessarily pinpoint a head gasket failure.
* Block test: This uses a test kit to check for combustion gases in the coolant. A positive result strongly suggests a head gasket failure, but a negative result doesn't guarantee that there isn't a problem.
Important Note: Any of the above symptoms could be caused by other problems, such as a cracked cylinder head, warped head, cracked engine block, or other coolant system leaks. A mechanic will need to perform a proper diagnosis to confirm a blown head gasket. Relying solely on these clues for a repair decision can be costly if the underlying issue is different.
