Low compression

[cooter2506]

I checked compression on my 1998 5.0 mercruiser.
#1 30 #2 30 #3 150 #4 150 #5 150 #6 150 #7 150 #8 150. Can anyone tell me what my next step is or what my problem is. IM thinking cam lobe?

 

[searay3]

Re: Low compression

IMHO - With those numbers, further testing is a waste of time. While you may be able to tell if you have ring or valve issues by leak down test, your still gonna have to tear it down. You will see then if there are any issues with the head, tuliped valves, detonated pistons, etc. Pull the head and see what you have. In the process you will visual the cam/lifters/pushrods anyway....

 

[Don S]

Re: Low compression

Is that the actual GM cylinder numbering?

Front
1--2
3--4
5--6
7--8

 

[cooter2506]

Re: Low compression

Yes Don that is the actuall gm numbering

 

[Don S]

Re: Low compression

Pull the valve covers and check, I seriously doubt its the cam. But like searay says, you have to take it apart anyway cause you can't run it like that.
But if you must know, before you pull the engine. Pulling the valve cover and checking cam lift will tell you.
IF that checks out, a leakdown test will tell you if it's valves or pistons.

 

[captmello]

Re: Low compression

I had similar compression numbers in my 350 after the previous owner overheated it badly. I was able to pull the heads without pulling the motor and had them checked. Sure enough, just a couple of warped valves. Had the heads rebuilt. Good as new.

What happened to your motor?

 

[cooter2506]

Re: Low compression

OK thanks guys Will pull valve covers and then leak down test

 

[cooter2506]

Re: Low compression

I looked for the leakdown test in the Adults onlly section and couldnt find it unless I over looked it. Can some one explain how to do it to me.

Captmello--- I bought this engine used from a dealer that said it was torn down and heads checked and regasketed. So I have to idea of its history. But I know I have never had it over heat but that doesnt mea it wasnt before and I dont know if any of the things that the dealer said was actually done either. But that is what I get for buying it used 200 miles away and picking it up and only getting a 30 day warranty. LOL DUH me I guess

 

[Don S]

Re: Low compression

It's not in the AO sticky, google leakdown test, you will find lots of info on it and what it does.
Has this engine EVER worked ok, I noticed you had previous posts about WOT issues.

 

[cooter2506]

Re: Low compression

No Don it hasnt and now I have finally got it figured out just need to fix it. I know should have done everything first and fixed it long before this. Sorry I have left the GREAT DON down. LOL Thanks I will google leakdown test

 

[Don S]

Re: Low compression

You didn't let me down, you let yourself down. Had you done that compression test last July, you would probably have it all fixed by now and ready for spring. NOW, however, you are just finding the problem and have a long way to go and a short time to get there.

 

[Don S]

Re: Low compression

You might as well quit messing with it and pull the heads. As mentioned earlier, you have to take the heads off no matter what is wrong in order to fix it. By the time you play the game of learning to do the leakdown test, you can have the heads off and KNOW for sure what the problem is.

 

[a70eliminator]

Re: Low compression

Are you 100% sure that the schreader valve wasn't stuck on the comp. checker tool or some other possible tool malfunction or operator err. go back and try the again on 1 & 2 cyls. just to be sure your not pulling heads for nothing, I've been tricked into pulling heads before, yes it was a 429 Cobra Jet HP motor, I did not know that the valve cover bolts on each side went directly into it's massive intake runners, I learned the hard way that you cannot run a ford 429 Cobra Jet motor with the valve covers off unless you screw the valve cover bolts on each side back into the head.

 

[TilliamWe]

Re: Low compression

And since the first thing you need to do is remove the valve covers, loosen the valves on those two cylinders a little. Then recheck the compression. If it comes up, all you have are valve issues, and you take the heads off anyway.

Checking your guage is a good idea too.

 

[cooter2506]

Re: Low compression

yes we checked those two cylinders three times each to eliminate the guage as a malfunction. so Im off to pull the heads. Thanks again guys

 

[a70eliminator]

Re: Low compression

If that engine had been sitting for a long time it could be that the valves on the effected cylinders are siezed in their open position and a little oil down the guide and a tap tap with a mallet mignt just do it. So don't jump to any conclusions till you've had a look st that valve train.

 

[cooter2506]

Re: Low compression

I have everything off except the heads themselves. My manual says nothing about using a valve spring compreessing tool to remove. Another words if I remove the stud for the rocker arm the spring will stay copreesed due to the valve spring retainer. I just don want to remove the stud with out knowing for sure.

 

[Don S]

Re: Low compression

Don't take this wrong, but if you can't figure that out, there is no way you are ever going to get it apart, cleaned up, inspected, fixed and back together and running properly.
This isn't as simple as a paint by numbers painting.

 

[cooter2506]

Re: Low compression

Thats ok Don. Heads are off but I dont see anything obvious that is wrong. So I guess the next step is to take it to the machine shop to get new valves put in. I hand cranked engine and didnt see any scaring of cylinder walls either. Anything else I should check While I have it torn down?

 

[TilliamWe]

Re: Low compression

Yes, check your exhaust manifolds for signs of interal water leaking. And your intake manifold too.

And to the naked ey, a tulipped valve still installed in the head, doesn't really appear different than the rest. But when they take them out and sit them next to a good valve, you can tell. Don't be surprised if you have more bad valves than just on those two cylinders. I have several bad valves (not as bad as the worst) on cylinders with good compression.