Compression Tests

[murnmill]

What would be an acceptable compression variance (+/- psi) for a 1989 Mariah? thanks!

[haugy]

If I remember right, and I probably don't, I wanna say the range you would like to be in is 120-140 PSI for an engine of that age.

It's 8.5:1 ratio.

[suprasam]

I think you want to stay within 10% but not sure it works the same on a standard motor versus a 2stroke.....I always thought it was compared different

[ts6mnewbie]

I am looking at a 1990 comp right now with a questionable variance myself. The PCM manual does't say what the compression psi
should be but it does say that if the lowest reading is within 75% of the highest it is ok. That seems like a lot to me. The only
other info I found was that allegedly these motors can still run good at as low as 90 psi. The boat that I'm looking at is between
100-125 psi. Still waiting to test it on the water.....

[michael hunter]

Those readings are not bad. Can you post each cylinders reading and how the test was run? How many hours are on the engine?

[murnmill]

the 1989 Mariah I'm interested in reads;

Compression is:
#1 145
#2 150
#3 150
#4 145
#5 135
#6 145
#7 145
#8 145

All within 10%. Not sure why they are so high. It has the 330 hp PCM. Higher HP = Higher PSI??? idk.

[DAFF]

The 1989 Mariah numbers look great to me.

Usually if you have a cylinder with a low reading add some oil to it. If the number jumps up to normal readings the issue lies with tired or worn rings. No change then the issue is the top end, head or head gaskets. Anything over 100 is good in my eyes, unless they are all in that ball park. I have seen many engines with a tired hole which run just fine.

[docdrs]

those numbers look good, # 5 has the greatest diff and its only 10% the rest are virtually the same

[lively]

i would agree , 125 psi or more is fresh with a stock bore , no worries there .... i always like to check valve lash and make sure im good there before compression comes on the
plate