Compression test results

C

cmariner32

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exMember
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Jan 20, 2002
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7269
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Last year I did my port engine with good even results. This year I did the starboard engine with not-so-good results:

135-8 135-7
120-6 115-5
120-4 110-3
145-2 135-1

The engines are '89 454 Crusaders with about 1350 hours. Both engines seem to run fine-in fact the starboard engine has been relatively problem free since I got the boat last year. The 3-4-5-6 compression numbers concern me. I poured some oil into the #4 cylinder and rechecked with no improvement. I see no smoke coming from the exhaust at startup, however I do see some minor blue smoke from the port engine-thinking that is just worn valve oil seals. Neither engine uses any oil. The test was done cold, all plugs out and carb open. I would appreciate anyone's thoughts-cause for concern or fahgettaboudit until it gets worse?
 
I did a compression test on my '92 Merc 454 2-3 years ago. Readings were between 137-150 but then engine was warmed up prior to doing it. I would not lose sleep over it if the engine seems fine when it's running. If anything, I would warm the engine up and check the compression.
 
If you have center riser exhaust manifolds, that would go along with the center cylinders low readings. They also indicate water getting into the manifolds and collecting in the exhaust port and rusting the valve seats.
 
If adding oil to the cylinders doesn't up the readings, the rings are fine. Chances are, the valves aren't sealing quite perfectly. If you wanted to do anything about it, it would probably be down to pulling the heads and having them rebuilt.

However, if it runs fine, you're probably ok to leave it alone for now.
 
Following up with a vacuum gauge test can help pinpoint the cause of those results.
http://www.secondchancegarage.com/public/186.cfm
One of scenarios 5-10 is most likely to result from your posted tests. Pulling the heads for valve jobs & reinstalling should make a noticeable difference in compression , perfomance and economy, but it sounds like they are still easily usable.

Charlie raises a very good point about a possible leak issue. How old are the riser/elbows? Any signs of manifold to riser/elbow rust stains? Any difference in the appearance of the plugs to the low cylinders? They are FWC engines right? Obviously used in SW from your listed homeport .
 
What charlie said. I would look at the risers but at those levelsdo no repairs to the engine.
 
The manifolds are the old log-style manifolds so they do not have the center risers. The engines are fresh water cooled so any issue would be at the risers only-which seem to be OK although I have no idea how long its been since they have been changed. The boat was in fresh water for the last 13 years so I'm tempted to think that they should be fine for another year or two now that I am in salt water/ The plugs looked fine except one-#1 had some oil fouling on the threads. Thank you all for your thoughts. I'm feeling a little more confident now that I should just leave well enough alone.
 
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