How to Correctly Check Compression

The service manual does a great job of explaining the proper procedure for compression testing. Many perform this procedure incorrectly. Few realize the engine must be warm so the pistons and rings are expanded from the heat, and only 4 compression strokes are used.

The compression test procedure is important. There isn't a lot of compression in a 134 to begin with. It is a low compression engine. The process is to watch the compression as it builds, and only give it 4 total compression strokes. You are looking for how much compression you get with each stroke. On a warm engine, in 4 strokes, it needs to be 90 - 110 on a L134. The pressure should come up quickly (80% or so) on the first stroke. and after 4 strokes be within specification and within 10 psi of each other.

If it fails this test, a little oil can be added to see just where the leakage is. Squirt a small amount of oil (1/2 oz or so) into the cylinder and repeat the test. Be sure the engine is still warm. Warm it up again if needed. If after inserting some oil into the cylinder it now passes the test, the rings are bad. If there is no or little change after inserting oil into the cylinder, the valve seal is bad and it needs a valve job or, a head gasket is leaking. Maybe even heaven forbid, a crack.