Low Compression 2003 Merc 90

[hsi111]

Hi, I have a 2003 merc 90 elpto (sn 0t663974). It has 115 lbs compression on 1 and 3 and 30lbs on #2. I don't know anything about the history of this engine. I recently bought the boat knowing the problem and now I need to fix it. My question is, is there anything besides bad rings that could cause low compression and how would I troubleshoot anything else. Thanks, Rick

 

[bilge rat jim]

Re: Low Compression 2003 Merc 90

Seeing as how the problem is internal, you will have to disassemble the engine for repair, so why not take it apart and have a look? Could also be a scored cylinder or a small hole in the piston (but that will usually give a 0 psi reading. Before getting out the tools, you might try loading the bad cylinder up with a carbon removing treatment like Ring Free+, wait overnite, and see if the compression comes up significantly.

 

[hsi111]

Re: Low Compression 2003 Merc 90

Thanks, I'll try that first. I was already planning on putting rings in but just curious if there was anything to try first.

 

[jbjennings]

Re: Low Compression 2003 Merc 90

Thanks, I'll try that first. I was already planning on putting rings in but just curious if there was anything to try first.

Usually with that kind of compression it requires more than just a re-ring----from my meager experience....

 

[Texasmark]

Re: Low Compression 2003 Merc 90

This thread is about 2 wks old, but not answered and "beins" (recognize the term down "theah", grin) it is the off season, even down theah it is Christmas.

I have an '02 90. Would be nice to know the history but if you don't you don't. My first question is why do you read such a diverse pressure.

I can't fathom the idea that one cylinder can be that far off. #2 does no extra work and is not down in the cowling where you could have measurement problems. Poissibilities would be a hole in the cylinder caused by corrosion or a broken ring possibly. With loop charging and only 2 pressure pulsed rings that could be a possibility. However Merc goes to a lot of effort to control corrosion in it's engines and a pin hole wouldn't cause that kind of disparity. Broken ring? Why?

Pull the plugs and look at the piston crowns. Tell me (us) what they look like, especially #2 vs the rest. What do the plugs look like...assuming you are running surface gaps (NGK BUHW-2)? If lube or OT problems all should be effected.

Beins that you listed MS. I assume you are in Mississippi and may have obtained the engine from the coast; like Biloxi or Gulfport. So maybe the PO wasn't careful to wash out the engine after outings.

Before I "blew my brains out" so to speak, squirt some 30w oil in the cylinders, ensuring that you get good coverage around the piston. A little pump up oil can would do the trick. Then measure compression again. Post the results.

Let's go from there. PM me if necessary to get my attention, not to deprive the other readers of the forum of the info.

Mark

 

[emckelvy]

Re: Low Compression 2003 Merc 90

I do recall that some of the older "Big Bore" 3-cylinder blocks had trouble with the cylinder sleeves shifting/rotating in the bores. Is it possible the same problem is happening on a later-model block?

Or did Merc redesign the blocks to prevent that issue?

Just a thought, can't think of too many other reasons why the same cylinder in 2 different motors should have the same problem, except for coincidence or maybe a manufacturing error/failure...........ed