'86 Merc 115 piston melted

I'm starting to become an expert on this I6 Merc's.... I have/had an '86 Mercury inline 6cyl that had great compression but the trigger coil went bad so I changed that and the stator when I was in there. I was out for a whole day in the boat and went through a tank full of gas and the thing was running great all day and then #4 piston siezed and it through a rod to create just about the worst internal carnage imaginable with an outboard. Upon teardown it looks like the #4 piston melted and caused the problem.

What I think happened is that when I replaced the stator and trigger coil I didnt reset the timing thinking that as long as I didnt touch the adjustments it would still be the same. I now think that it was too far advanced.

The only thing is, the only piston that looked bad was the #4. The other ones all looked fine.

If the timing was off, is it possible that it would only affect one piston instead of all of them? I'm in the process of swapping over another good powerhead and I just dont want to repeat this. I'm going to make sure to adjust the timing properly this time around but I just want to rule out anything else I might be missing. All carbs were rebuilt and it has a new waterpump.

Re: '86 Merc 115 piston melted

It is unusual for #4 to melt/seize. This i usually caused by lack of cooling or lean fuel/air ratio. Normally the lower cylinders of a carb pair run richer and cooler. So you normally lose #3 followed by #1. Maybe the cause was water injection, or ring breakage or something else.

Re: '86 Merc 115 piston melted

Chris1956 said:

It is unusual for #4 to melt/seize. This i usually caused by lack of cooling or lean fuel/air ratio. Normally the lower cylinders of a carb pair run richer and cooler. So you normally lose #3 followed by #1. Maybe the cause was water injection, or ring breakage or something else.

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I thought about a lean air/fuel ratio but the middle carb controls the #3 and #4 piston and #3 looked fine. The rings arent broken either. The only other time I have seen something like this is in a car engine and timing was the culprit.

Re: '86 Merc 115 piston melted

Ok, Then why was the timing for that cylinder different than the other cylinders? since it cannot normally be different, I was looking for a mechanical cause. Mabe heavily carbonized causing detonation?

Re: '86 Merc 115 piston melted

Its a mystery. It couldnt have been carbon. The others all looked great and decarbonizer was ran through it regularly. All compression on the cyls was just about the same and everything. The only things I can think of that can cause it is overheating, lack of lubrication, lean mixture, and timing. I dont think it was the first three and the only thing I didnt check was timing.

I put a spare powerhead on it and just got it started and timed a couple hours ago and it sounds great.

All I know is that it can only happen two more times and then I am out of spare powerheads. LOL

I think these are the kind of motors where you need a couple spare parts motors just to keep one going.

Re: '86 Merc 115 piston melted

I know it's been awhile but I'll still give you my 2 cents. I had a problem like that and could not figure it out and a old machanic told me to check my inter water jacket in the exhaust for leaks, sure enough it had some salt water damage and was getting water in that cycl. leaning it, melting that piston.

Re: '86 Merc 115 piston melted

I have an 87 115 hp inline that I just had the same problem in. I was running steadily across the lake at 5000 rpms and it just completely died. I got it started back up but it was knocking like I had spun a rod bearing.

I just broke it down today and the number 4 piston is melted. It looks like it occurred from detonating too early. The knocking was from the piston tapping the top of the cylinder wall not from being loose but from the melted material. I did get it stopped before it did too much damage and think I can save the powerhead. The only damage was on 4.

Over the winter I rebuilt the cards, fuel pump, water pump, new rectifer and a new stator but I did not reset the timing when I put it back together. This thing ran so good it was unbelievable. It purred at an idle, had tons of power. The only thing I could figure is it was the timing but I still don't know.

The manual says I can bore .015 over without doing all the cylinders. Hopefully I will not have to have it done any more than that, I am taking it to a machine shop in the morning to get it done. This is the first outboard I have ever worked on. I am going to go ahead and replace all the rings and bearings while I have it down but from there I don't know if there is anything else I should do. The reeds seemed to be fine as it did run fine until it died.

Should I put new reeds in it? Is there anything else I should replace while I am in it? Any help would be great.

Re: '86 Merc 115 piston melted

I continued breaking down the remainder of the block today so I could take it in. I found 2 loose bolts on the Intake cover right next to the #4 piston. I was carefull when removing it not to mess up the gasket and found evidence (Oil between gasket and aluminum) of it leaking. Could this of been pulling more oxygen into the cylinder making it run too lean in turn melting the piston?