A compression test measures how much pressure each cylinder builds when the engine cranks, using a gauge screwed into the spark plug hole. Healthy cylinders read close to each other; one low cylinder points to a leak past the rings, a valve or the head gasket. A leak-down test then pressurizes that cylinder with shop air and shows where the air escapes, which pins down the cause.
How to do a compression test
- Warm the engine up, then switch it off.
- Disable the fuel and ignition systems as your service manual describes, usually by removing the fuel pump fuse and ignition fuse or coil connectors.
- Remove all the spark plugs so the engine cranks evenly.
- Screw the gauge into the first cylinder, hold the throttle wide open, and crank the engine for about four to six compression strokes.
- Record the highest reading, release the gauge and repeat for every cylinder.
Reading the results
- Compare cylinders: the spread between them matters more than the absolute number. A common guide is that all cylinders should be within about 10% of each other; use your engine’s specification where available.
- One low cylinder: a ring, valve or head gasket problem on that cylinder.
- Two neighboring low cylinders: often a head gasket leak between them; see signs of a blown head gasket.
- All low: general wear, incorrect cam timing or a test problem such as a closed throttle.
Wet compression test
Squirt a teaspoon of engine oil into a low cylinder and test again. If the reading rises noticeably, the oil is sealing worn rings; see piston rings. If it barely changes, suspect a valve or the head gasket.
Leak-down test
Bring the cylinder to top dead center on its compression stroke, connect a leak-down tester to shop air and the plug hole, and read the percentage of air escaping. Then listen for where it goes:
- Air from the intake or throttle body: intake valve.
- Air from the tailpipe: exhaust valve.
- Air from the oil filler or dipstick tube: piston rings.
- Bubbles in the coolant: head gasket or a cracked head.
As a general guide, low single-digit leakage is very healthy and readings that climb past roughly 20% deserve attention, but compare cylinders and follow the tester maker’s guidance.
What next?
A misfire on a low cylinder is common; see engine misfire. Valve and head gasket problems are head work; worn rings or damaged pistons mean the short block. When the repair is too large, compare a replacement short block or long block on our engines page; short block vs long block explains the difference.
More guides: low compression in one cylinder · compression ratio vs compression test · bent pushrod symptoms.
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