Engine Compression Testing
This should quickly explain the basics of the cylinder compression test. Injecting oil into the cylinder is a procedure we perform that's part of a compression test. We use injected oil to differentiate between a faulty valve or worn rings as the fault for weak cylinder performance. It brings front and center a phenomenon that occurs when a liquid is added to a poor or worn ring seal, compression increases.
So now we know that when a ringed seal like that of an engines piston is weak adding a liquid like oil will increase the 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. Notice the difference between the engines cylinders that indicate a cylinder with faulty piston rings or a faulty valve. If there is no increase after adding oil to a weak cylinder the valve is faulty and not sealing the cylinder.
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 screw in fitting and compression gauge. 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 cylinders need a good seal between the rings and the cylinder walls and between the valves and their respective seats. Test each cylinders 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 manufacturers 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 crank the rotate the crankshaft three more turns per cylinder. If the compression increases the piston rings are at fault.
- Burned valves: After injecting the oil in the cylinder the compression reading stays the same, then one of the valves is bad or not seating correctly. Low compression in one cylinder typically indicates a bad valve. Exhaust valves tend to burn due to the hot gases passing through while an intake valve is 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.
- Camshaft 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 shown in the image. Before replacing the rings in this engine the camshaft timing must be checked.
- 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.
