low compression then high compressionver

[redlinj]

My 3.7 485 appeared to lose a couple hundred rpm's at wot over the last month of the season last year. 157, 155, 163, 55. Yes, 55. Didn't have any oil handy to squirt into the cylinder to see if that raised compression. Nice day today here in WNY so took advantage of it and rechecked compression. 156, 135, 163, and 155. Yes, last year's low cylinder is now OK and a good cylinder is now slightly low. Can compression change like that from just sitting over winter? Do I maybe have a faulty guage (unlikely)? Am I maybe not getting a good seal(screw in type guage) ? Or, perhaps it's the tester and, at 65 i'm just losing it!

 

[Dogdave]

It seams pretty unlikely that the cylinder grew compression. I could have had a stuck ring of valve maybe. I think I would borrow another gauge and check it again on a warm engine and make sure the throttle plates are open when cranking for the compression test.

 

[gm280]

I agree with the above suggestions by Dogdave, as well. Buy or borrow another gauge and retest them all again. And IF the new readings mimic the original readings then at least you know the condition of them now and can either run it like it is or do whatever you think is best. If it is running reasonable, I'd keep on running it and check it throughout the season to see if any significant reading changes happen. JMHO

 

[redlinj]

Today I rented another compression guage today from Auto Zone. Retested each cylinder with both guages. Both guages show pretty much the same readings---all between 120-125. Now these readings were cold with throttle wide open and quite different from the previous days test results. This shouldn't be rocket science but I'm open to any suggestions or theories--even aliens! At this time I guess I'll put this on hold til I can get the boat running up to temp and then start testing again.

 

[Dogdave]

Can you put the ear muffs on the drive to get the boat to temp? I do not know the spec for your compression on your engine but if the readings are within 5 psi i would consider that good. Have you checked the ignition system for full advance? Fuel system for blockage?

 

[redlinj]

My 3.7 485 appeared to lose a couple hundred rpm's at wot over the last month of the season last year. 157, 155, 163, 55. Yes, 55. Didn't have any oil handy to squirt into the cylinder to see if that raised compression. Nice day today here in WNY so took advantage of it and rechecked compression. 156, 135, 163, and 155. Yes, last year's low cylinder is now OK and a good cylinder is now slightly low. Can compression change like that from just sitting over winter? Do I maybe have a faulty guage (unlikely)? Am I maybe not getting a good seal(screw in type guage) ? Or, perhaps it's the tester and, at 65 i'm just losing it!

I can and will put the muffs on just as soon as we get a real temperature break here in Western NY. That should be soon. I'll run her up to operating temp. and try again. 3 separate days of testing and 3 different results. Both guages used had same readings.