5.7 compression test- one cylinder at 150 psi, worry?

[westcoastcalvin]

Doing a mechanical on a 2006 2600SE we are considering purchasing and the mechanic indicates cylinder 6 compression is 150 psi- all others 165. Is this a problem we should be worried about?

[jamanati]

In the cylinder that showed 150 psi, squirt a little oil, and re-test. If the psi changes, it could be a sign of leaking rings. If it doesn't, it could be either a bad head gasket, bad valves, or possible block problem. There shouldn't be more than a 10% delta across cylinders. These numbers are on the border.

[mrxitmt]

they say that a 10% varience is ok

[westcoastcalvin]

Originally Posted by jamanati

In the cylinder that showed 150 psi, squirt a little oil, and re-test. If the psi changes, it could be a sign of leaking rings. If it doesn't, it could be either a bad head gasket, bad valves, or possible block problem. There shouldn't be more than a 10% delta across cylinders. These numbers are on the border.

A number of mechanics have told me not too worry about it. The boat hadn't been in the water for a couple years. Plan to run it in the water and re-check. This make sense?

[mmwjr]

Welcome aboard.

150 psi is considered a good compression. Low compression is lower than 130. The cylinder should be within 10%.

If anything is weak I would think a leak valve and not the rings with a reading this high.

Sounds like a good plan go me. Also check oil and gear lube after the run.

[westcoastcalvin]

Originally Posted by mmwjr

Welcome aboard.

150 psi is considered a good compression. Low compression is lower than 130. The cylinder should be within 10%.

If anything is weak I would think a leak valve and not the rings with a reading this high.

Sounds like a good plan go me. Also check oil and gear lube after the run.

After running the boat the re-test came up the same- 150psi. The boat only has 360 hrs on it, which seems low hours to start seeing a 10% vriance in cylinder compression, no?

[mmwjr]

Engine build tolerances can result in a 10% variance right from the start. I would not be concerned. A gas engine typically last ~2000 hours so 360 is ~ 20% of the overlife.

[njones]

150 psi certainly sounds like a healthy cylinder to me. I wouldn't worry too much about it.

[canada randy]

I wish I had 150 psi. My 1995 7.4 L has around 105 psi each. I then did a leak down test and they came in at around 8 % . I was ready to pull it out and rebuild it. Then I found a Mercruiser Service Bulletin from 1997 that said as long as they were above 100 psi all was good? Sure sounds low to me. It runs good. A little low on power for a 7700 lb boat. It ain't broke so unless I come into a bunch of spare money then I guess I won't fix it.

[mmwjr]

Originally Posted by canada randy

I wish I had 150 psi. My 1995 7.4 L has around 105 psi each. I then did a leak down test and they came in at around 8 % . I was ready to pull it out and rebuild it. Then I found a Mercruiser Service Bulletin from 1997 that said as long as they were above 100 psi all was good? Sure sounds low to me. It runs good. A little low on power for a 7700 lb boat. It ain't broke so unless I come into a bunch of spare money then I guess I won't fix it.

How many hours are on it? What shape is the exhaust in? If the exhaust is allowing water to leak and it's get back to the engine burning water will damage the exhaust valves causing them to not seal properly.

However with all cylinders being low one would tend to think worn rings. So until the power is too low or she is puffing smoke I would hold off.

[canada randy]

Hours are unknown. No hour meter if you can believe that. I thought all boats had em. The leakdown test indicated any leakage was at the rings. She doesn't smoke. Runs very good actually. Just not as much power as I'd like. I had the exhaust apart to change the flappers and all looked good. Fresh water boat so it doesn't eat up exhaust like salt water.