Cylinder Compression Test

A compression test indicates if there's a cylinder that is suffering from worn piston rings, a bad valve, head gasket, or if the engine's valve timing is off. An engine cylinder needs a good seal between the piston rings, the cylinder walls, and the valves and their respective valve seats. An engine compression tester is used to test each cylinder's compression. Review the manufacturer's manual and (TSB's) technical service bulletins for any special test procedures.

Compression Test: Valve Timing

The results of a compression test can be quite telling. They indicate a particular area that needs attention.

New piston rings.

Worn piston rings/cylinder walls: After running the first test, squirt about a tablespoon of oil into the cylinder and rotate the crankshaft six turns. If the compression increases, the piston rings are at fault.

Cylinder head valves and valve seats.

Burned valves: If, after injecting oil into the cylinder, the compression reading remains the same, then one or more of the valves or valve seats are bad or not seating correctly. Low compression in one cylinder typically indicates a bad valve. Exhaust valves burn most often due to the hot gases passing through. The incoming air-fuel mixture cools the engine's intake valves.

A cylinder head leaking compression between two adjacent cylinders.

Faulty head gasket: A faulty or blown head gasket leaks compression between two adjacent cylinders. When all the other cylinders are within specifications, and two cylinders next to each other on the same bank are low, suspect a faulty head gasket.

Carbon buildup: Carbon buildup on the top of a piston increases compression readings. It can be seen with a probe inserted into the cylinder.