Blow-by and compression numbers?

[rupp0003]

I noticed oil spurting out of my crankcase vent and into my flame arrestor. Enough of it that it became a slow drip. I Did a compression test and the numbers seem ok #1 140 #2 140 #3 135 #4 150. I did the compression test cold as per mercruiser service bulletin 97-25. My question is why are most compression Tests done at operating temp. ? And why am I getting so much blow by if the compression seems ok? The motor has been acting odd, stalling out after about a half hour to hour repeatedly. It's a 1988 mercruiser 3.0

 

[Don S]

Re: Blow-by and compression numbers?

Cold is one thing, but that doesn't mean after it's been setting a long time, keep that in mind.

140 to 150 doesn't sound bad for a compression test on an 88 engne. A leakdown test would be better for finding out how much blowby you have past the rings.

 

[rupp0003]

Re: Blow-by and compression numbers?

The boat had been sitting for about 30 hours. Do you think that would make any difference?

 

[Don S]

Re: Blow-by and compression numbers?

No problem with 30 hours. But still suggest the leak down test.

 

[rupp0003]

Re: Blow-by and compression numbers?

Ok thanks Don I will try that next.