View Full Version : Engine compression results and meaning

09-17-2006, 01:05 PM
"Hi All,

I ran a compression test on my three litre I/O yesterday. It is a yamaha assembly - but I believe it is a GM engine. I ran the test dry - I did not add any oil to the cylinders. I let each cylinder compress three times - where the pressure topped out. Cylinder 1 (close to the front) 210psi on first two attempts - then read 140psi on the remainder of the tests. Cylinder 2 - 130psi, cylinder 3 125psi. The cylinder nearest the stern (cylinder 4) initially read 210psi - then stabilized at 135psi for the remainder of the attempts. When I compressed the schrader valve (spelling?) to release the pressure at the end of each test, it seemed like a water spray came out. Question: What do these results mean ? Also, why did cylinder 1 and 4 have such increadibly high readings on the initial tests ? Does these test results mean that my block and head are not cracked, and that I probably have water coming in via the exhaust manifold ?
Any responses will be sincerely appreciated - thank you."

09-17-2006, 01:44 PM
"You need to keep gas from getting into the cyclinders, too. If you have water or any other liquid in the cylinders, the reading will be high because a liquid can't be compressed, only pressurized. This means that any liquid will alter the reading you get.

Pull the spark plugs from all cylinders, crank it over for about 5 seconds and retest. Do this after running the motor so it's at normal operating temperature. Let it cool down, then retest with no plugs in and no gas going to the intake.

Post the new readings."