Hey all,
I need to ask what's the proper way to do a Cylinder Compression Test? I know i have to pull out the fuel pump relay thats in the glove box so that the pump doesnt keep pushing fuel and that it wont allow the truck to start. but now do all the spark plugs have to be out at the same time? or can i take one plug out at a time, then do the test on that cylinder, and then put the spark plug back in and move on to the next cylinder. also does the gas peddle have to be nailed? i have a 95 CPI vortec engine....

 

Pull all of the spark plugs, defeat the fuel pump (pull the fuse or relay), hook up the compression tester to a cylinder, spin the engine over for 2-3 seconds, and repeat.

If you find a cylinder that is low, try squirting some motor oil in through the spark plug hole and spin the motor over a bit, then try to read the compression again. If it gets better, you have worn or broken rings. If it doesn't get better, you could have a bent or otherwise leaking valve.

 

You can take out the plugs one at a time too, that way you dont mix up the order the wires go on. Just unplug the coil wire as well as pull the fuel pump fuse.

120+ pounds is decent compression. 140 is good. Anything under 120 is not good.

I rotate the motor about 5 times, you can tell from the sound it makes of a revolution.

 

It is much easier on the starter to have all of the plugs removed. You should be able to keep track of plug wires if you are attempting to do a compression test.

And I have to disagree with the <120psi being bad. It all depends on the motor. My 2000 Bravada had 115psi on all 6 cylinders and ran perfectly fine. My '74 K5 has 105-115psi and, again, runs perfectly fine. Even putting oil on the rings did not cause a substantial increase in pressure. As I said, all engines are different.