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Mercruiser 2004 5.0 Compression Test Advice

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chadg6

New member
I'm looking at buying a 24 foot 2004 deckboat with a Mercruiser 5.0 Bravo 3 (260 HP) that has 715 hours on it. The compression test results were as follows: 141, 149, 131, 135, 127, 149, 133, 137. The percentage difference is 15% between the highest (149) and the lowest (127). The engine starts and runs well, with no oil smoke. I've gotten varying opinions on whether I should be concerned. One mechanic suggested that conventional wisdom was that any difference more than 10% should raise a concern. Another mechanic said that the 10% rule make sense for 2-4 cylinder outboards, but for a V8 auto-type engine, 15% was good, and up to 30% was acceptable (I checked the mercruiser service manual, it states that compression should be within 30% or more intensive maintenance should be performed). I'm looking for some additional advice as to whether 10% is the best rule of thumb for V8's, or 15% for a newer boat with 700+ hours is acceptable, and why? Thanks! Chad
 
Re: Mercruiser 5.0 Compression Test Advice

did you do the test on a warm motor all the plugs out and the throttle fully down ?

Did you do a vacuum test also ?
 
Re: Mercruiser 5.0 Compression Test Advice

I'm looking at buying a 24 foot 2004 deckboat with a Mercruiser 5.0 Bravo 3 (260 HP) that has 715 hours on it. The compression test results were as follows: 141, 149, 131, 135, 127, 149, 133, 137.
Click to expand...

Ayuh,... If you like the boat,.... Buy it...

You're beating yourself up over Nothing...
 
Re: Mercruiser 5.0 Compression Test Advice

chiefalen said:
did you do the test on a warm motor all the plugs out and the throttle fully down ?

Did you do a vacuum test also ?
Click to expand...

Chiefalen,

The compression test was done by a mechanic in IL, and I am in NY. No vacuum test was done. My understanding is that it was done with all plugs out and throttle down, though I was not there observing it. Not sure how warm the motor was at the time.
 
Re: Mercruiser 5.0 Compression Test Advice

The reason i say warm motor cause the oil would have circulated and have coated the walls. The result could be skewered.

This ain't gonna be a cheap boat, and i'm looking out for you not the owner of the boat. Plus i don't have any financial interest ether way.

Me i would want to be there watching and running the boat myself. Who is this mechanic doing the work for you ?

127 ain't that bad could come up after running the boat a little while. just bothers me it is all over the place the numbers, my pos 4 cyl 140 are all the same and is a 85' this 6 year old boat should have numbers that are the same or 1 or 2 ether way, bothers me.

me a vacuum test will tell more.

Sorry it's the best i can tell you. i hope it works out for you.
 
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