Compression results

[renton]

I know that if you squirt oil into a low compression reading cylinder and it goes up that shoes its rings and not valves. My question is how much should it go up? Mine goes from 120psi to 135-140psi. Not a big amount. Could it still be bad valve ?

 

[scotts90ranger]

Actually 15-20psi is a fair amount I would suspect, that's around 15%. If you wanted to get a real feel for the engines health you could do a leak down test which takes a special gauge setup and an air compressor but that also gives you an answer to where the leakage is going, if air comes out the oil fill it's coming past the rings, if it's going to the throttle it's intake valve if it's coming out the exhaust exhaust valve. I got my leak down tester on Amazon for like $30

 

[RonD]

Putting oil in any cylinder will make compression go up higher, doesn't matter if it was high or low

If adding oil doesn't bring a low compression cylinder up to other cylinders levels then its a valve issue

All piston rings leak air, metal on metal is NOT a good air tight seal, lol, they all leak

Adding oil to a cylinder seals the rings, temporarily, so they don't leak as much

i.e. if average compression is 140psi
And a low cylinder is 120psi
Then adding oil to the low cylinder with bad rings would make if come up to 140psi if not a bit more
So by deduction if adding oil doesn't get cylinder up to 140psi then must be a valve issue