88 merc 150 piston meltdown

[Blueeyes]

Short recap: I've been chasing a low rpm problem with this engine; only able to get 3700 rpm with a 14.5 x 17 prop. Yesterday I put on a 16 x 13 prop, and low a behold, 5600 rpm! Ran all day long, not too hard, but tested it here and there and trolled a lot. Then, about 5 hours into the day, the engine suddenly seized. After it cooled a bit, I got it started, and could obviously hear the sound of a hole in the piston. I had a spare gas can on board, so pre-mixed 25:1, and limped 2 miles back into harbor from the big blue Atlantic. (kissed the dock!!)<br /><br />Got the boat home and removed the heads. The #2 piston was seriously deteriorated on one side. It looks like it deteriorated to the point that a ring channel failed, causing the ring (or rings) to fragment. There was ring pieces embedded in the piston and head, and lot's of aluminum on the cylinder wall.<br /><br />The only thing I can think of that would cause a piston to deteriorate like that is pre-ignition.<br /><br />Any other ideas?

 

[wilde1j]

Re: 88 merc 150 piston meltdown

Sure, lean fuel mixture caused by crap in the carb.

 

[Blueeyes]

Re: 88 merc 150 piston meltdown

I thought of that too after posting, but...I rebuilt the carbs immediately after receiving the boat. The engine was bought by a family friend 2 years ago as "rebuilt" from a marina, and he used it twice 2 years ago. It sat since then with the same fuel in the tanks and carbs (with Stabil). There was a lot of water in the gas tanks, but the carbs were surprisingly free of gunk. The lean problem certainly could have happened before I owned it. Or, I have read on this board that many older outboards could benefit from a main jet size increase to compensate for higher alcohol content in the fuel. I know the carbs have the original size jets, and I know I'll replace them if I can rebuild this engine myself.<br /><br />I've done a lot to various engines in my life, but never bored a cylinder of a multi cylinder engine. Assuming I have to bore this cylinder, is it OK to go oversized on just one, or will all cylinders need to be bored to the same size? My guess is all, but if one is OK, then that's what I'll do.<br /><br />Thanks,<br />Greg

 

[Hooty]

Re: 88 merc 150 piston meltdown

What kind of engine are we talking about?<br /><br />c/6<br /><br />Hooty

 

[Hooty]

Re: 88 merc 150 piston meltdown

Never mind, I just say what engine we're talking about. Sure, you can go oversize on just one cylinder. Ya sure want to find out what caused the failure though.<br /><br />c/6<br /><br />Hooty

 

[Blueeyes]

Re: 88 merc 150 piston meltdown

Thanks Hooty.<br /><br />It's a 1988 Merc 150, FWIW.<br /><br />I agree. I don't want this to happen again. I'll take that carb apart again.

 

[Blueeyes]

Re: 88 merc 150 piston meltdown

I have some updated info. and need an opinion. I have not decided yet whether to repair this engine, and want to understand the cause of failure as best as I can before making the decision. I removed the pistons the other night, and found that the #2 piston damage was on the 'exhaust' port side of the piston. I could be wrong, but if a lean condition caused this, for some reason I believe that the intake side would have sustained some, if not most of the damage. In this case the intake side is in remarkably good shape. There is about a 75-90 degree section of the piston, aligned with the exhaust port, that was melted/blasted away until the top ring channel failed. About 90 degrees worth of the top ring fragmented. The remaining top ring and the bottom ring are completely bound within the ring channels, presumably caused by the piston overheating and deforming, or due to the aluminum that was removed from the piston sides and transferred to the cylinder wall.<br /><br />Does this information mean anything?<br /><br />Thanks,<br />Greg

 

[Kenny Bush]

Re: 88 merc 150 piston meltdown

Most of the pistons let go at the exhaust port. If your carbs lean out, the pistons will heat up the most at the exhaust port. Also check timing and possibility of Double firing.

 

[Blueeyes]

Re: 88 merc 150 piston meltdown

Bushcat,<br />Thanks for your insight. It makes sense that the exhaust side would be the hotter side. FWIW, I brought the piston to a local Merc repair shop, and got a list a mile long of possible causes. Most I had thought of already. One that stood out was 'cheap gas'. It struck a bell because a friend brought over some 'contributory gas' a couple of days before the meltdown. He's pretty frugal, so I'm pretty sure he would have bought the cheapest gas he could find. My boat has a water separator, and it doesn't have much water in it, even after siphoning out 20 gallons in the past couple of days (through the separator). What is it about 'cheap gas' that could lead to this type of failure. Lots of alcohol? I have a little bit of inside information on what makes 'cheap gas cheap'. It's usually the residual fuel left in the tankers when the primary customer's tanks are full. So they pump off the rest into a 'community' type tank, where multiple brands become mixed. I wanted to ask the guy who said it, but let's just say most of the people at this local shop are about as friendly as a pack of lions, and I was getting the 'here's your 5 second answer, now get out of here and stop wasting my time' attitude. Jeez, I just spent $350 there in the past two weeks, and they stood to sell me a whole bunch of parts to fix this. If I decide to replace the engine, I don't plan to buy a Merc since they are the only Merc parts place within 50 miles of me.<br /><br />Thx,<br />Greg