Wet Compression Test

Cylinder compression tests are performed to identify any cylinders that may have poor compression. Inject oil into the weak cylinder after a normal cranking compression test has been performed. This is known as a wet compression test. It will indicate whether there's a bad valve, head gasket, or worn piston rings.

Compression test wet injecting oil.

A compression test is performed on the suspect cylinder. If the cylinder’s compression is found to be below specifications, oil is injected. Introducing oil into a cylinder with worn piston rings increases the cylinder's compression. The oil fills the gap between the ring and cylinder wall. If there's no increase after adding oil, an intake or exhaust valve is faulty.

A compression test can be used to compare the compression of one cylinder to the others. An engine’s cylinders need a good seal between the rings and the cylinder walls, and between the valves and their respective seats.

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

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

Burned valves: If results remain the same after injecting oil, one of the valves are bad or not seating correctly. Low compression in only one cylinder typically indicates a bad valve. Exhaust valves burn due to hot gases passing through. Intake valves have the advantage of being cooled by the incoming fuel.

Valve timing : When all of the cylinders are low and inserting oil into the cylinder does not increase compression, the camshaft timing is likely off. The timing belt can slip on the sprockets resulting in staggered and low compression results.

Hole in piston : A hole in a piston will result in no compression in that cylinder. Remove the oil cap or PCV valve from its grommet. Blowby caused by this hole can be seen seeping through these openings.

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.

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