uneven compression on mercruiser

[rolmops]

I just tested the compression on my 1984 3 liter 140 horse engine and the readings are 90-150-120-150. Does this mean that the block is to be scrapped or could there be a simple solution for a simple problem?
I just put some marvel mystery oil in the 2 holes with low compression hoping it is stuck rings is there anything else I can do?
Cornelis.

 

[achris]

Re: uneven compression on mercruiser

If you put oil in and the compression didn't change, then it sounds like a head/valve problem....

You did do the test with the engine warm and the throttle full open, yes?

Chris........

 

[Don S]

Re: uneven compression on mercruiser

How to do a proper compression test.

View attachment 97-25 Compression Test.pdf

 

[rolmops]

Re: uneven compression on mercruiser

When I added some oil the compression in both low cylinders went up to 150,so by what you say does this mean that there is a fix?

 

[Bt Doctur]

Re: uneven compression on mercruiser

Either you did the first test wrong or with the increase after adding oil, I`d be looking at a ring issue.

 

[Bondo]

Re: uneven compression on mercruiser

When I added some oil the compression in both low cylinders went up to 150,so by what you say does this mean that there is a fix?

Ayuh,.... It means yer motor is really Tired, 'n needs to be Refreshened...

The Fix will be a rebuild, or used replacement...

Talk to yer local Machine Shop, or look on-line...

I think with an '84, you can use any 3.0l up to '90...

 

[rolmops]

Re: uneven compression on mercruiser

Bond-o,what is the going rate for a replacement engine out there by the Thousand Islands?

 

[Bondo]

Re: uneven compression on mercruiser

Bond-o,what is the going rate for a replacement engine out there by the Thousand Islands?

Ayuh,.... Honestly don't know.... I'm not really in the "Biz",..

I just live within it's fringes....
I just gave away an ole 2.5l in 'bout the same shape as yer's,... Runnin', but tired....

Andy Debranski, at Best Machine in Wat'n does some darn nice work, 'n is reasonable....
I donno if he's got any cores, but I'm pretty darn sure he'd rebuild yer's...

 

[tpenfield]

Re: uneven compression on mercruiser

Isn't there some 'stuff' you can spray down the carb that fixes problems like this? ( probably not )

My advice would be to do a cylinder leak test to get a better understanding of the problem with the two bad cylinders. Compression tests are not specific enough in isolating problems. So, it is too much of a guessing game to be tearing into an engine.

 

[Don S]

Re: uneven compression on mercruiser

The oil in the cylinders and the compression coming up tells you the rings are not sealing. Time to pull the head and find out why. Rust from leaky manifold/riser? Broken ring? The cylinder wall will tell the story.

 

[rolmops]

Re: uneven compression on mercruiser

It seems that the marvel mystery oil did its thing. Today the compression was way better in both lower cylinders. Maybe it was just sticking rings. Anyway,the bellows are changed (because of my extra large hands it cost me a lot of skin) but I figured out a quick way to get those bellows to slide over the metal housings.My special tool is a nice long Allen wrench with a pair of pliers clamped onto the long end.The drive shaft bellows were easy probably because of their size,but then the exhaust bellows taught me a good lesson.Always do the transom side first and the bell housing second. With the elongated Allen wrench I pulled the bellows from inside the exhaust housing in about 1 minute.
So now the bellows and the impeller are taken care of.
I run the engine on the muffs for about 10 minutes at a low rpm and I must say that it is nice to have a very quiet engine instead of a loud 2 stroke.
Rolmops.

 

[Don S]

Re: uneven compression on mercruiser

Just curious. Did you do this compression test without running the engine just prior to the compression test, after the boat sat all winter?

 

[tpenfield]

Re: uneven compression on mercruiser

Just curious. Did you do this compression test without running the engine just prior to the compression test, after the boat sat all winter?

That would be my guess . . . :facepalm:

I have found, at least on my BB V8, that you need to run the engine for almost 1/2 hour, else you get questionable results.

 

[rolmops]

Re: uneven compression on mercruiser

Just curious. Did you do this compression test without running the engine just prior to the compression test, after the boat sat all winter?

I did it the original first time ever i/o mechanic way.I turned over the engine a few times each time with all the plugs out. In retrospect it is probably a very stupid thing to do, but ignorance and eagerness sometimes take over from common sense.
Cornelis.

 

[achris]

Re: uneven compression on mercruiser

I really love rollmops (pickled herrings). I catch the herring (that's also fun) and put them to pickle. Once they're done, they usually don't last a week... All eaten...

Cheers,

Chris....

 

[rolmops]

Re: uneven compression on mercruiser

I like rolmops a lot myself. I grew up on a US Airforce base in the Netherlands and that is where I started eating it,much to my mothers dismay.
Last night I pulled the engine for a thorough overhaul and today I pulled the head. as far as I can see there are no scratches on the cylinder walls,but I do think that the head gasket is blown near the low cylinder. Tomorrow I will take the measurements as to what rings to buy and of course I will replace the head gasket and the manifold gasket.While the thing is open I will replace as much as I can on the crank shaft. I also saw that the exhaust manifold gasket is blown between the exhaust of cylinder #2 and the intake of cylinder #3

 

[rolmops]

Re: uneven compression on mercruiser

today,my apprentice ship on the I/O mercruiser continues.I pulled one of the pistons, and was amazed to see how easy that is,compared to a 2 stroke outboard.(no needle bearing). On the pistop head it said "030" does this imply that it is an over bore or does the number mean something else ?
thanks,Cornelis.

 

[Pete104]

Re: uneven compression on mercruiser

That's .030" over. Why are pulling pistons if you only need gaskets? Now you need another gasket!

 

[tpenfield]

Re: uneven compression on mercruiser

today,my apprentice ship on the I/O mercruiser continues.I pulled one of the pistons, and was amazed to see how easy that is,compared to a 2 stroke outboard.(no needle bearing). On the pistop head it said "030" does this imply that it is an over bore or does the number mean something else ?
thanks,Cornelis.

I assume you pulled the piston to get a look at wear on the lower end of the engine? (I did that too)

Seems like that engine has been rebuilt at some point in its life . . .

 

[rolmops]

Re: uneven compression on mercruiser

That's .030" over. Why are pulling pistons if you only need gaskets? Now you need another gasket!

I am pulling the pistons because this rig will be 15 to 20 miles off shore with yours truly fishing in it probably by myself. Every bit and piece in that engine will be inspected and if I can prevent anything from going wrong at the wrong time,I will do so. That head gasket is the least of my worries.