3.0 loss of compression

[inchman]

hi, i have a 1988 3.o mercruiser in my bayliner trophy. i noticed that it was running a bit rough at idle. pulled of the plug leads and it's off on no 3 cylinder. the spark is good. I did a compression test and got 125psi for the other 3 cylinder and 75psi for no3. Im ging to do a leak down test first and maybe check the tapets. has anybody any tips i should know for setting the tapets or possibe cause's before i pull the head off.

cheers Gary

 

[wire2]

Re: 3.0 loss of compression

Pull the rocker cover off, let it idle and observe the amount of deflection of #3 intake & exhaust rockers. If less movement than others, the cam lobe may be worn or the lifter is collapsed.

If it looks similar to adjacent ones, loosen the rocker nut on #3 intake until it makes a "clack" sound, then tighten 3/4 turn. Repeat for exhaust.
If it was out of adjustment, idle will now be faster & smoother.

 

[inchman]

Re: 3.0 loss of compression

thanks for the reply..have these engine's got hydraulic lifters ??

cheers Gary

 

[Skid-joe]

Re: 3.0 loss of compression

Yes.

Good luck.............joe

 

[Wotam]

Re: 3.0 loss of compression

Try the old-fashioned oil test.

Squirt a little light oil in the problem cylinders... if the compression immediately jumps up, it's very likely ring/piston/bore problem.

If the compression doesn't change it's probably a head/valve problem.

Not a perfect test, but usually on the money.