Cylinder Compression Test

A compression test will indicate if there's a cylinder that is suffering from low compression, a bad valve, 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 the particular area that needs attention.

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.

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. Intake valves are cooled by the incoming fuel.

Faulty head gasket: A faulty or blown head gasket will leak 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 will increase compression readings. It can be seen with a probe inserted into the cylinder.