Cylinder Wet Compression Test

Cylinder compression tests are performed when a cylinder is suspected of have poor compression. Injecting oil into the cylinder after the cylinders compression has been tested dry will indicate whether there is a bad valve or worn piston rings. It brings front and center a phenomenon that occurs when a liquid is added to a poor or worn ring seal; the compression increases.

Compression test wet injecting oil.

Adding oil to a cylinder with worn piston rings will increase the cylinder's compression. By taking the compression on a cylinder first with no oil and then taking a compression test after injecting an once or so into the cylinder, a cylinder with weak or worn piston rings would increase. If there's no increase after adding oil to the weak cylinder the valve is faulty.

This test is performed after the performance related issues like ignition and fuel are working and performing correctly, but the engine still seems to have a misfire or dead cylinder. Usually before the test all the spark plugs are removed. It's a matter of pulling the spark plug and installing the tubes screw in fitting and compression gauge. Then running a test with a dry cylinder, running a test with a wet cylinder, and comparing the results.

A compression test is used in order to check and compare the compression of one cylinder compared to the others. An engines cylinder's need a good seal between the rings and the cylinder walls and between the valves and their respective seats. Test each cylinder's seal compared to specifications and the other cylinders with an engine compression tester. An engine analyzer or a manual compression tester can be used for this procedure. The engine analyzer is probably something the shop not the individual technician would own. Review the manufacturer's manual for specifications and any special procedures.

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 2 oz. of oil into the cylinder and rotate the crankshaft three more turns per cylinder. If the compression increases the piston rings are at fault.

Burned valves: If after injecting the oil into the cylinder the compression reading stays the same, then one of the valves are bad or not seating correctly. Low compression in one cylinder typically indicates a bad valve. Exhaust valves burn due to the hot gases passing through. The intake valve has the advantage or being cooled by the incoming fuel. A burned valve is usually an exhaust valve.

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.

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 or chain can slip on the sprockets resulting in staggered and low compression results.

Hole in piston: A hole in the piston will result in no compression in the cylinder. Remove the PCV valve from its grommet. The blowby gases caused by this hole can be seen seeping through the PCV valve opening.

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