Merc 5.7 compression results

[Onexl85]

Looking at good reading on 7 cylinders and no compression on the 8th.
What would cause this?

 

[Eric 3.7LX Alpha 1]

Hole in the piston....

Bent valve not closing fully

 

[Bondo]

Ayuh,... I use the whip of my compression tester to blow compressed air back into the cylinder,.....

Roll it up to Tdc, 'n Blow,....

You'll Hear where the airs comin' out,....

The carb, a bad intake valve,....
The exhaust, a bad exhaust valve,...
Through the oil fill, a holed piston,...

Has it been runnin' poorly at high load conditions,..??
A lean condition is probably the #1 cause of detonation,....

 

[Onexl85]

At idle, runs rough, at around 2000 rpm it smooths out, feels great runs smooth, boat will do 34 if I'm pushing it.....bayliner Ciara 2355

 

[Onexl85]

I have my marina looking into it, I guess I'm just trying to get a feel from you guy about what I'm in for with this thing. Possible rebuild?

 

[nateo]

I have my marina looking into it, I guess I'm just trying to get a feel from you guy about what I'm in for with this thing. Possible rebuild?

yep. Just went through exact same issue. I tried just replacing the bent intake valve and been turning wrenches every since, mine still ain't fixed. There's a good chance there's more internally that's wrong with it. What's the back story, when did the trouble begin? If it's a old engine you may want to just go straight for the kill and total rebuild with new bearings, gaskets, pistons, and cam, etc. (or find new engine)