A blown head gasket can leak in a few directions: combustion gas into the coolant, coolant into a cylinder, or one cylinder into the one next to it. The symptoms depend on which way it's leaking.
Here's what to look for and how to prove it before you tear into the engine.
The symptoms
- Steady white smoke that smells sweet on a warm engine: coolant burning in a cylinder
- Losing coolant with no outside leak
- Overheating, especially if combustion gas is pushing into the cooling system
- Low compression on two cylinders next to each other
Combustion leak test
This test checks the coolant for combustion gases. If it's positive, combustion gas is getting into the cooling system. Combined with coolant loss and no outside leak, that points right at the head gasket, or a crack in the head or block.
What a leak-down test shows
Pressurize the cylinder with the radiator cap off. If bubbles come up steady in the coolant, air is getting from the combustion chamber into the cooling system. That's a head gasket or a crack.
If air comes out the next cylinder's spark plug hole, the gasket is leaking between those two cylinders.
Before you put a new one on
A lot of engines use multi-layer steel (MLS) head gaskets. These have a factory coating and go on dry, unless the manufacturer says to use sealer. Adding spray sealer can actually cause leaks.
MLS gaskets also need a smoother deck surface than old-style soft gaskets, since the thin steel can't fill in a rough surface. Check the surface finish spec in service information.
Find out why it failed
Head gaskets usually fail for a reason, like an overheat. Figure out what caused it and fix that too, or the new gasket won't last.
Cheat sheet
- Sweet white smoke on a warm engine = coolant burning
- Coolant loss with no outside leak is a big clue
- A combustion leak test finds exhaust gases in the coolant
- Leak-down bubbles in the coolant = head gasket or a crack
- MLS gaskets go on dry and need a smooth deck
Common questions
What does a blown head gasket smoke look like?
Steady white smoke with a sweet smell, on a warm engine. That's coolant burning.
How do you test for a blown head gasket?
A combustion leak test checks the coolant for exhaust gases. A leak-down test with the radiator cap off shows bubbles if air is getting into the coolant.
Do MLS head gaskets need sealer?
Usually not. They go on dry unless the manufacturer says otherwise, and extra sealer can cause leaks.
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