compression for mercruiser 5.7 alpha 1

Hi everyone,

New to iboats forums and had a question on compression for my 1994 donzi 21 5.7 mercruiser engine. Did compression test and results were 115, 120,110,115,119... but 1 cyclinder tested at only 90. Do i need to have my engine rebuilt? Seems like the engine runs fine but just worried because thought all cylinders should be +/- 10.

Thank you

Re: compression for mercruiser 5.7 alpha 1

Ayuh,.....

All of those #s are at the Bottom of acceptable....

Re: compression for mercruiser 5.7 alpha 1

I'm of the philosophy of "If it ain't broke, don't fix it". Why were you doing a compression test? Bored?

In a perfect world all engines would have all cylinders ~10% to eac hother.

If you're having performance issues, then by all means, go for it. But it's not going to be worth the money or effort to do a rebuild over something like that.

More likely than not, if you just have 1 low cylinder, it's a leaking exhaust valve. If it's going to bother you, maybe just pull that head, and take a look. It's pretty easy to replace a valve or two, VS. a complete rebuild...

If it was me, I'd just leave it alone, unless it becomes a proble. If it gets to that point, you're still going to have to pull it out and rebuild it, but rebuilding it now doesn't gain you anything besides loss of a good part of boating season.

Don't know where you're located, but here in Utah, boating season is precious time. If anything, I'd do a rebuild over the winter, if I was going to do it.

Others may disagree.

Re: compression for mercruiser 5.7 alpha 1

That 90 is low compared to the others. 30psi lower is cause for alarm. Something is not quite right.. could be as poster above said but could be something else. Something is not 100% right and continued driving may cause internal damage. Try running seafoam through it or put some oil down that cyl and let it sit for a while to see if maybe it's carbon build up. Then test again to see if it brings the number up.

Re: compression for mercruiser 5.7 alpha 1

I didn't notice how low those other numbers were as well... A rebuild wouldn't hurt, that's for sure! Like the above guy said, try a couple of squirts of oil in each cylinder before compression testing, and see if that brings your numbers up.

If it does, your rings are shot. If not, it's probably just top-end issues, and freshening up the heads will probably bring it all back.

Maybe throw a set of Vortec heads on it, if you're inclined...

Re: compression for mercruiser 5.7 alpha 1

90 psi is out of limits. 100 minimum.
Might want to have a read at this Mercruiser service bulletin and do the followup test in order to determine the exact problem.
http://www.boatfix.com/merc/Bullet/97/97_25.pdf

Re: compression for mercruiser 5.7 alpha 1

try doing a valve adjustment and see if you get some of that compression back.

Re: compression for mercruiser 5.7 alpha 1

thanks for everyones input. No, i wasnt bored lol i just bought the boat and person who sold me the boat said he checked the compression and all cylinders were around 150. just had to see for myself.