Engine problem/noise/no compression in cylinders 3 and 4

[Briandale92]

I have a 87 mercruiser 3.0l and i just got a new upper unit put on the sterndrive. I went to take it out for a spin and cranked up fine and idled for a little then went dead. (Seemed to run fine when it ran)I tried to crank it back up while giving it a little gas and when it cranked back up it idled way to high and i shut it off as fast as i could. I tried to crank it back up and nothing. Got to the house and hooked the water up to it and it cranked up but with trouble and was making this horrible noise in the engine. Did a compression test and first cylinder read 190 second 90 and three and four 0. I took valve cover off to see if it was a rod broken or bent and all rods are fine none bent or mushroomed at top. I aslo set valves back. Still will crank but with aweful noise, wont stay running either. What is the most possible thing it could be?

 

[Don S]

Re: Engine problem/noise/no compression in cylinders 3 and 4

You can't see the rods by removing the valve cover. When someone says a broken rod, they are normally referring to a connecting rod that connects the piston to the crankshaft..
Best way to check for that is to stick a pencil or stick in the spark plug hole and turn the engine (BY HAND) to see if you can feel the piston moving. If the engine turns and the piston doesn't move you have your answer.

 

[Bondo]

Re: Engine problem/noise/no compression in cylinders 3 and 4

Ayuh,... I'm guessin' ya mean pushrods, not connectin' rods,...

How'd ya set the valve lash,..??
If ya back 'em off, does the bad noise stop,..??

If not, yank the head, somethin's very wrong, 'n the motors comin' Out anyways...

 

[Ryanlewis2010]

Re: Engine problem/noise/no compression in cylinders 3 and 4

im with don on the pencil if its what i think it is that noise is something bad either two broken rods (highly doubtful unless water ingestion caused hydro lock on abrupt shut down at high RPMs) or something seriously wrong with the head. Any problems with engine B4 replacing the upper? Who replaced the upper you or mechanic? and idk about you but 190 seems really high for a 3.0 mine is supposed to be 140.

Ryan

 

[Briandale92]

Re: Engine problem/noise/no compression in cylinders 3 and 4

Thank you all for the help!! I did the pencil test and in fact the pistons are moving, So i shined a light down in the spark plug hole to find out that the tops of the pistons are cracked.(really bad). Hopefully it wont cost very much. i know it wont be cheap!! just hope i can get it ready for the summer.