Got some more info from the Subie dealer... full compression test resulted in 190-200 psi in all cylinders. BUT they did somehow find a wiring problem on the #4 injector (golly what a shock!)

 

I questioned the service manager as to why I was told yesterday that the compression on the #4 was 150 psi and now today it is 200 psi. She told me "that a compression test on a single cylinder is not as accurate as one done on all 4" To which I replied "uh huh"...

 

Now i am assuming that she meant that once they tested all 4 the numbers were different... I am hoping she is not implying that they tested all 4 simultaneously...4 testers, 1 in each cylinder... would it make any kind of difference vs 1 cylinder at a time? I cannot see how it would make that kind of difference provided all the rest of the cylinders still had spark plugs in them...

 

Thoughts?

David

She told me "that a compression test on a single cylinder is not as accurate as one done on all 4"

To my knowledge, the only acceptable meaning that sentence can have is that the compression in one cylinder is a more meaningfull info when you can compare it to compression in the other cylinders.

150, 150, 150, 150 does not say the same thing as 190, 190, 150, 190.

But like youself I don't see how taking a compression reading of the other three cylinders - be it simultaneously or not - could alter the readinds on no 4 cylinder.

A compression test should be done at wide open throttle. It it isn't the numbers will be lower. Either that or they only removed the one spark plug the first time and they removed them all the second time, allowing the engine to spin faster. Just guesses.

there are lots of variable to compression tests (block temperature, outside temperature, etc), which is why you remove all spark plugs and open the throttle all the way. you then have an even basis for comparing cylinder to cylinder.