Mercury 1994 2.0L 150

[FORBESAUTO]

So I've read that the engines with the compression relief could read low on a regular compression test. Is it possible that my low reading on only #3 could be caused by that? If not, then I'll have to look for another cause, but if so, why would the others read normal?

The compression reliefs will cause your compression readings to read lower psi readings than actual compression ratio would show without the reliefs due to bleeding off through exhaust port at cranking speed. But the readings should be consistent across the board.
When you hear that statement, what's meant from it is for example, the max pressure you can safely run 91 octane in is 140-150 psi on a non compression relief mota. But on a compression relief motor the same maximum compression ratio would show much less psi on a compression gauge, for example around 135 psi. Even though it actually has the same compression as the non relief motor.

[FORBESAUTO]

If you wanted to go ahead and pull that piston out to check it and rod bearings, you can pull it out by just removing head and intake. If you decide to, note that the rods are crack cap design and have to mate up properly and in right direction on reassembly. And once you get it out you need to reinstall cap and torque it down to prevent cap from jingling on it and other surfaces from knocking the tiny high spots from the crack procedure down. The mating surfaces should be dull in color, if any shiny spots on mating suface not good meaning high spots have already been knocked down. Also once torqued should not be able to feel any lip on bearing surface.

[Glastron1987]

So by removing intake does that refer to the plate assembly and reeds, or is that the front half? If it's the plate assembly and reeds then I guess you're saying I should be able to reach a socket through the front half to get at the cap bolts.

[FORBESAUTO]

So by removing intake does that refer to the plate assembly and reeds, or is that the front half? If it's the plate assembly and reeds then I guess you're saying I should be able to reach a socket through the front half to get at the cap bolts.

Thats correct, once reeds are removed you can rotate crank to put rod near opening in front and remove bolts and push piston out. The bearings will not come out opening in front half, but can take them out through rod slot through cylinder.

[FORBESAUTO]

I agree with the others that you need to get it all he way down to clean and inspect. Just giving you an option to see what your up against without complete disassembly. The mains are lubed through bleeder lines, small fittings and passages, check valves, and around front half bolts to a small in main journal on center mains. If any of these passages have gook in them it won't last. But this way you can see rod journals to see if has any pitting that would require crank replacement until you have time and space for complete tear down.

[FMP]

By the time you finish taking one out , finding what ever falls somewhere, then bitching it back together you'll have the PH off the leg and apart.

[Glastron1987]

I agree with the others that you need to get it all he way down to clean and inspect. Just giving you an option to see what your up against without complete disassembly. The mains are lubed through bleeder lines, small fittings and passages, check valves, and around front half bolts to a small in main journal on center mains. If any of these passages have gook in them it won't last. But this way you can see rod journals to see if has any pitting that would require crank replacement until you have time and space for complete tear down.

Excellent explanation for why I need to break the motor down. Will do when I have the space cleared. In the meantime I do want to do what I can to determine why #3 is low. Thanks guys.

[Glastron1987]

Upon closer examination of the head (gouges) and head gasket, I think what FMP indicated before may be correct. At one time #3 may have eaten a reed or at least something difficult to digest, but that does not appear to be the cause of the current compression issue. To me the gasket looks to be in good shape on both sides.

Also interesting, after clearing off the carbon from the piston tops, it is evident that pistons #1 and #5 have similar stamp/number markings and #3 does not have any markings. This leads me to believe that there has been engine work before. I’m not aware of how pistons are marked but it seems that either #1 and #5 have been replaced and #3 was not, or #3 was replaced and #1 and #5 were not.

In any case, my next step will be to try to pull out #3 and inspect for any further clues to the cause of the compression issue.

[Glastron1987]

Well got #3 piston out.
Observations:

1. One of the rings was stuck in a section with a bunch of carbon and stuff, but freed easily just by nudging it a bit by hand. My inexperienced assessment on this is that running the engine a few times would have worked this out, but could also have been a factor in the low compression.
2. Some scuffing on both sides of piston. My thought it that this is not great but probably normal wear and tear and not a great contributor to the compression issue.
3. The connecting rod does not seem to be loose or anything at the top so I think that bearing is good. The bottom of the rods and cap seam OK to me with just a slight discoloration inside the cap. Not quite sure what to make of the mating surface, but it seems it can be lined up properly.
4. The most worrisome thing is the bearing on the crank journal. Not sure if it is normal to have a section missing out of it, but to me that’s what it looks like.

[W2F a V-King]

Maybe just me, but care needs to be taken with rods/caps. The very irregular surfaces between them are easily damaged making re-alignment sketchy at best if the peaks are flattened or damaged.
When I pull rods off the crank, I carefully clean with tooth brush and carb spray or the like, blow dry then carefully re-assemble them and torque the caps back on for storage. jmo.

Might be a bad angle in the picture, but it appears to me that those caps are not aligned properly.

[Gordon02]

The rods/caps are "cracked" sets and only fit one way and must be perfectly aligned. The factory manual emphasizes the importance of matching all edges of the rod and cap. The cap and rod match lines are almost indistinguishable and will not catch a fingernail when properly bolted. James is spot-on about those surfaces having to be clean. The bolts will not guarantee that the cap and rod will fit perfectly, so checking the outside edges is critical during reassembly.

This not matched -



These edges are smooth and perfectly matched. You cannot feel the match lines - when properly matched and tightened, they almost disappear.

Here is a 2.4L that I recently assembled -

[Glastron1987]

This is with the cap cleaned and better aligned. My impression is that it is probably OK.

[Gordon02]

Here is the page from my manual that describes the "fractured cap" and using a pencil lead to be sure all external mating lines are invisible -

[Glastron1987]

That will help, thanks.

[Glastron1987]

Got the bearing out and it actually appears fine. Also, I see how to align the cap now. One question is whether the cap bolts can be reused and should loctite be applied? I guess that's two questions.