Five-O, HO Compression

Doing a compression test on a stock 1990 5.0 HO. What kind of numbers should I expect? Recently rebuilt. Warmed up, plugs removed, throttle body open......................

Each compression gauge reads differently because the springs in the valves are different. If it was over 170 psi on my gauge it would surprise me.
Paul

Thanks for the comeback PaulS1950. My 145 to 150 leads me to believe the cam may be

Absolute minimum would be 101 psi. Your 145 to 150 sounds fine. What is the reading for all 8? Like PaulS1950 said, it's more important to look at the spread than at the max psi. All should be relatively close to each other. After my rebuild, all cylinders were within 5 psi from one another.

The compression should build up quickly. Low compression on the first stroke, followed by gradual increasing pressure on successive strokes, indicates worn piston rings. Try adding 3 squirts of engine oil through the spark plug hole. If the compression increases significantly then the rings are worn. If the compression does not increase significantly, then leakage is occuring at the valves or head gasket.

If 2 adjacent cylinders have equally low compression, it's a possibility you have a blown head gasket between those two cylinders.

If one cylinder is 20% lower than the others, and if there is a slightly rough idle, a worn exhaust lobe on the cam could be the problem.



<font size=-1>[ This Message was edited by: 89Trooper on 4/30/07 2:56am ]</font>