A compression test tells you which cylinder is weak. A leak-down test tells you why, by showing where the air is escaping. You'll usually use them together.
Here's how each one works and how to read the results, which is exactly what the A1 likes to ask about.
Compression test
You crank the engine with a gauge in the spark plug hole and see how much pressure each cylinder builds. Low on one cylinder? Something's leaking. But a regular compression test can't tell you if it's the rings, the valves, or the head gasket.
The wet test trick
Squirt a little oil into the weak cylinder and test it again. The oil temporarily seals the gap between the rings and the cylinder wall.
If compression jumps up a lot, the rings are the problem. If it barely moves, look at the valves instead.
Leak-down test
Bring the piston to top dead center on its compression stroke, so both valves are closed. Then pump shop air into the cylinder and read how much leaks out as a percentage. Service information gives the limit for that engine.
Then listen to find out where it's going. That's what makes this test so useful.
Where the air comes out
- Throttle body or intake: the intake valve isn't sealing
- Tailpipe: the exhaust valve isn't sealing
- Oil fill cap: it's getting past the rings or a damaged piston
- Bubbles in the radiator: head gasket or a cracked head or block
- Next cylinder's spark plug hole: the head gasket between those two cylinders
Why TDC matters
If the piston isn't at top dead center on the compression stroke, one of the valves might be open. Then you'll hear air at the intake or exhaust and think you have a bad valve when the cylinder is actually fine.
Cheat sheet
- Compression test finds which cylinder, leak-down finds why
- Wet test: big jump = rings, little change = valves
- Do the leak-down at TDC on the compression stroke
- Intake = intake valve, tailpipe = exhaust valve, oil fill = rings
- Bubbles in the coolant = head gasket or a crack
Common questions
Which is better, a compression test or a leak-down test?
They do different jobs. Compression finds the weak cylinder, and leak-down tells you where it's leaking. Use them together.
What does air at the oil fill cap mean on a leak-down?
Air is getting past the piston into the crankcase, so it's the rings, the piston, or the cylinder wall.
What does a wet compression test tell you?
If compression jumps up after adding oil, the rings are leaking. If it barely changes, look at the valves.
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