Low compression in one cylinder means that cylinder is not sealing: a burnt or bent valve, worn or broken piston rings, a head gasket leak, or a damaged piston. A wet compression test and a leak-down test tell you which. The cylinder usually misfires, especially at idle, and the engine runs rough; see engine misfire.
Low compression in one cylinder: causes
| Cause | Clues |
|---|---|
| Burnt or bent valve | Wet test shows little change; air escapes through the intake or exhaust |
| Worn or broken rings | Wet test raises compression; air escapes into the crankcase |
| Head gasket leak | Two adjacent cylinders low, or bubbles in the coolant |
| Damaged piston or ringland | Low reading with heavy blow-by; common after detonation; see EJ257 problems |
| Timing problem | Several cylinders low together after timing work; see timing belt |
How to pinpoint it
- Dry compression test: compare all cylinders; a cylinder well below the others is the problem.
- Wet test: add a small amount of oil to the low cylinder and retest. If compression rises, the rings are worn; if not, look at valves or the head gasket.
- Leak-down test: pressurize the cylinder at top dead center and listen: air at the intake points to an intake valve, at the tailpipe to an exhaust valve, at the oil filler to the rings, and bubbles in the radiator to the head gasket.
See compression and leak-down tests for the procedure.
Symptoms
- A steady misfire on one cylinder, often with codes such as P0301 to P0308
- Rough idle and shaking; see rough idle
- Loss of power and higher fuel use
- Blue smoke if the rings are worn; see engine burning oil
What counts as low
The absolute number matters less than the spread between cylinders. Compare every cylinder in the same test with the throttle held open and a charged battery, and check the specification for your engine. A cylinder far below the others needs a leak-down test; all cylinders low together suggests worn engines overall, a timing problem or a test error.
Fixes
A burnt valve or head gasket means cylinder head work. Worn rings or a damaged piston mean the engine must come apart, and at that point a replacement engine often costs less than a full rebuild with machining. Outboards and diesels follow the same tests, and diesels are especially sensitive because they rely on compression heat to start; see diesel won’t start.
Repair or replace?
Compare a head repair or rebuild with a replacement in engine replacement cost and is it worth replacing an engine?, and see our engines by engine code.
More guides: what is compression ratio?.
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