'05 Mercury 40 hp, 3 cyl, two cylinders low compression

[halmc]

I have come by an engine as described in the subject, It's an EFI engine.

Weighing a rebuild on the one hand, or a new ~$1500 powerhead on the other. The engine runs great, but uses oil, and shows compression of 180, 140, and 90. . I'm told it overheated. QUESTIONS:

1. Is there any indicator on the engine that was designed to disclose whether the engine did in fact overheat. (they are sometimes installed for warranty considerations) This engine is, of course, way out of warranty, but I'd like to know if it did overheat.

2. I'm finding in parts offerings oversize pistons and rings for this engine, and thus assuming that it uses steel sleeves in lieu of nikisil -- correct?

3. Is there any machine shop on the planet capable of boring the block to an oversize, should that be required? (There sure ain't in my town)

Lastly, I have plenty of theoretical answers to my questions. I'd greatly prefer to hear from those who KNOW any of the answers.

 

[sutor623]

Let me ask you this, is the top cylinder the one with 90psi compression?

If so, that would lead me to believe that it was in fact an overheat. You may get lucky and it be a failed headgasket that is yielding the low compression.
Hard to tell if it overheated or not, but if a seller told you that it did I don't see why you would not believe him, as that is THE worst thing (short of throwing a rod or eating a ring) that can happen to an outboard IMO.

If it is an EFI you could have a service tech hook up to the engine and see how high the temps got during the overheat. Sometimes the paint will turn colors but its hard to tell with the black paint of the mercs.

We don't KNOW anything till we crack her open. But I can tell you ONE thing, RUN AWAY!!!! Unless in fact you are willing to pay out more than you expect to, or maybe you want a learning experience or a nice project at the start of boating season, rather than the end of boating season.

You could have burnt valves, warped head, scored cylinders, scuffed pistons etc. What is the serial number of this engine? What is the asking price of the seller?

 

[halmc]

Engine was "free" to the extent that any item that eats, needs painting, or needs a rebuild is free. I received it as a token of appreciation from a friend for whom I installed a new engine on his boat in place of this engine. My friend bought the boat (along with this engine) from someone who said it overheated. I would have done the R & R of friends engine gratis, anyway, so my basis in the engine is zero.

Boating season here in Fla comes, like in the rest of the country, just once a year. LOL, but it lasts all year.

Bottom cyl is 90psi. No other signs of overheating.

Who knows. A pig in a poke, but it is a rather pretty pig. It certainly would be nice on my currently two-stroke powered pontoon boat.

Good idea on the ECM: can the ECM memory be read w/o the engine running? It's in my shop on an engine stand, but I could easily take it by a merc dealer. Trouble is, in my neck of the woods, taking an engine by a dealer for any purpose means you gotta kiss it off for at least a month. In that time, I can investigate, and haul its remains to the landfill, if necessary.

Do you know whether the alloy block uses nikasil plating, or whether it uses steel or iron liners? If I thought it could be bored and fitted with over sized pistons, I would be much more likely to 'crack it open' than if the block needs replating or replacing.

 

[sutor623]

If you have a borescope you could put it into a sparkplug hole and have a look and see what the cylinder walls look like. Strange to me that the bottom cylinder is the one that has low compression if there was an overheat.

Not sure of the sleeves, but I am pretty sure it can be honed/bored back to square and good to go. Be sure and see what size oversize pistons are readily available for this engine before getting it machined.

 

[halmc]

If you have a borescope you could put it into a sparkplug hole and have a look and see what the cylinder walls look like. Strange to me that the bottom cylinder is the one that has low compression if there was an overheat.

Not sure of the sleeves, but I am pretty sure it can be honed/bored back to square and good to go. Be sure and see what size oversize pistons are readily available for this engine before getting it machined.

Yup, gonna try the borescope. If I'm lucky -- which I some times am -- maybe the bottom rings are stuck. Or something.

What do you think about running the engine as is? I ain't gonna let it overheat, and I would like to know first hand if the oil consumption is significant. Just about everything I know about the engine, I learned second hand, including the compression check.. I did, however, hear and see it start with the boat on its trailer, with the stern well in the water. It certainly stirred up the water well, and I saw no smoke. Ran it for about three or our minutes. No knocks or other issues.


I do have a pontoon boat with a two cyl 25 hp merc four stroke. It would take about an hour to swap the engines out. At the most.

I'd just hate to score a cylinder beyond repair . . .

 

[sutor623]

Without having the engine actually pushing the boat and being able to get it up to speed, is similar to revving your car in the driveway, even if it was stirring up water when backing her in. I made an ode to myself long ago, if I was ever going to pay the price for a "running" motor (with a tail light warranty), it would have to be proven to me that the motor would run in a real world situation (meaning taking a ride on a boat) or I would walk. I'm sure this has saved me plenty of time and $.

In your case you didn't pay much of anything for it, but that doesn't mean its worth the money you are going to put into it!!

I am anxious by nature, so I would NOT run that motor as is. Running on un-even compression is just a disaster waiting to happen, I promise.

First course of action is very simple. A leakdown test. If you don't have a tester and the capabilities, call someone that does and let them do it. They will isolate the valves, headgasket and cylinder walls to see where the issue is. This is a 4 stroke motor, it usually isn't as simple as "stuck rings", although that is possible.

You may get lucky and be in for a simple valve lash adjustment. Could also be as simple as pulling the head and changing the headgasket. Worst case is you have scored cylinder walls and a nice parts motor. I have this same displacement block and if you don't want to fool with it we can talk.

So, figure out what's wrong and then decide if you have a good motor or a boat anchor!! That's my advice and I'm sticking to it!

 

[halmc]

LOL, well, I think it's good advice to stick to. I do have leak down tester and will check it out. I too am loathe to run it as is, cuz I might turn it from salvageable into non salvageable.

One thing to throw into this mix is I love and perhaps need a project. Some folks rescue kittens and puppies: me (apparently) outboards. Just pretty much finished a refurbishing of a pontoon boat (link below) and I'm about ready for some engine work, partly cuz I'm not smitten with the 25hp two stroke outboard that came with it -- and this EFI 40 would be a nice refinement.

If I find a boogered block, I will contact you. Thanks for your thoughtful answers and observations.

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