Mercruiser 5.0 compression question

[littlebookworm]

I have a 1986 SunRunner with a Mercruiser 5.0/230hp. Engine is shot. Marina mechanic has a 1983 5.0/228hp complete with all accesories. Oil and under valve cover looks good. Engine well-maintained, supposedly. Pulled as part of repower. Carb was replaced with a marine Holley. (I've got a rebuilt Rochester from my previous boat.) Did compression check. Seven cylinders 100-105 lb. #6 cylinder 70 lb. Squirted oil in cylinder, no effect. My assumption: engine basically good, needs valve job; one valve either leaking or hanging up. Engine worth buying as core and accesories. I used to be a mechanic so I'll do disassembly and reassembly. I'll have machine shop do valves themselves. I don't have the tools anymore. Your comments/recommendations?

 

[Don S]

Re: Mercruiser 5.0 compression question

Run away, the engine is gone. 100 is the MINIMUM you should see, should be up around 130 to 150.
Here is a Merc service bulletin you should read.

http://www-alt.mercurymarine.com/mnetdata/Service/Cruiser/Bullet/97/mc9725r1.pdf

 

[littlebookworm]

Re: Mercruiser 5.0 compression question

Don: Thanks for your quick reply. Over the last few weeks, I've been reading the questions and answers on this site and have learned quite a bit. I value your and Bondo's advice and knowledge. I read the info. you recommended. One fact did strike me, however. I had tested the compression using a battery that was, frankly, very low on charge connected with booster cables. It turned the engine over very slowly. (I had been using it to run the radio and to test power circuits on the boat without recharging it.) I know I should have hard-wired the battery in, but the cables were in the boat, which is tarped over. How much difference do you think using a fully charged battery, hard-wired in, would have made in the readings? I've got the battery on a charger right now and can redo the compression test some time after Christmas. Do you think it will make much difference? Hy.

 

[rodbolt]

Re: Mercruiser 5.0 compression question

with that spread? I would either buy it as a core or walk away.

if its saltwater cooled I would simply run away.
but a leakdown test would be a more accurate assesment of ring and valve condition.

 

[Don S]

Re: Mercruiser 5.0 compression question

I've got the battery on a charger right now and can redo the compression test some time after Christmas. Do you think it will make much difference?

Do another test, then you will know for sure. Just be sure to follow that service bulletin exactly.

 

[Bondo]

Re: Mercruiser 5.0 compression question

Run away, the engine is gone. 100 is the MINIMUM you should see, should be up around 130 to 150.
but a leakdown test would be a more accurate assesment of ring and valve condition.

Ayuh,............

Unless you get the #s mentioned by Don in your Test with a Fully Charged battery,............
And you can Confirm that it's a Valve using the Leak-down Test mentioned by Rodbolt,.....

I'd keep looking........

Then I'd go find a 350 to put in that hole...............
It's a Direct Swap, with Free HP......

 

[MikDee]

Re: Mercruiser 5.0 compression question

I agree with all the above ^^^ especially the last 2 sentences by Bond-o

 

[Haut Medoc]

Re: Mercruiser 5.0 compression question

I would not waste my time.....
I agree with Bondo.....
You could get a brand new long block & BOLT EVERYTHING YOU ALL READY HAVE ON FOR LIKE 3k IF YOU WERE TO DIY.......
Sorry for the caps.....:redface:​

 

[mikeneal]

Re: Mercruiser 5.0 compression question

I will add to the list of folks advising to stay away, those #s are horrible. If it is free then take for a core, otherwise tough to beat a 350 CU drop in. You will ahead on $$ and time, even though you can do work yourself. (still have machine shop time)