1996 Evinrude 112 compression loss....re-gain

For no apparent reason, my engine started idling very rough. With my experience, I went straight to a compression test and got 120 psi on three and 65 psi on fourth (bottom right, aft looking forward). Took the head off and noticed a single piece of metal (origin unknown) "welded/fused" to the top of the piston near the cylinder wall. Cylinder walls were absolutely smooth...rings appeared to be intact. A small shiny "witness" mark was apparent on the head which matched the metal location. Since I was on vacation, I trailored the boat to a local shop and the mechanic said it was a piece of aluminum and there was little to be done short-term. He said to get a head gasket, add more oil to the gas and run it.

Before replacing the head, I move the piston as far out as possible and filed the "metal piece" down to a smooth surface (equal to the piston face), and filed the edge away from the cylinder wall. Put the head back on (new gasket), added extra oil and ran it for the rest of vacation. Much to my blessing, the boat kept running better and almost felt normal the last day of vacation.

When I got home from vacation, I again checked the compression and much to my surprise, it read 120 psi. I have a 19' Wellcraft CC, and I?m getting 5300 - 5400 rpm @ 38 mph. Have run the boat without incident approximately 25 hours.

I scanned my manual since this "miracle cure" in attempt to figure out what could have happend, and would like to ask the question: Could this piece of metal been a piston ring positioning pin that broke off? If the pin broke, and the rings were allowed to "align" gaps, this would explain why I lost compression?.and if the ring gaps again separated, why I re-gained compression. I doubt my theory, but can?t come up with anything better.

Right now, start is great, idle is great, performance is great...but the future worries me.

Your thoughts??

Re: 1996 Evinrude 112 compression loss....re-gain

I really doubt the piece of metal you found was a piece of the ring. If it was, the piston and the cylinder wall would be destroyed. How did the spark plug look when you took it out? Could it have been missing a piece of the electrode?

Re: 1996 Evinrude 112 compression loss....re-gain

Thanks for the help. All plugs looked normal....nothing missing.

I agree with you concerning the rings....especially since I re-gained good compression, and the wall was perfect.

I was more referring to the piston ring locating pin my manual was talking about. Each ring to suppose to "straddle the pin" during ring installation. I was wondering if this "pin" could be the origin of the mysterious metal and the cause of my compression loss (and subsequent gain) due to the ring now slipping and aligning with the gap of the second ring.

I know its a thin theory but...I've never see this before.

Re: 1996 Evinrude 112 compression loss....re-gain

Any seasoned mechs out there heard of the piston ring locating pin breaking?? Please read thread and comment. Hoping to help others as well.

Thanks,

Re: 1996 Evinrude 112 compression loss....re-gain

Possible the head gasket was damaged on that cylinder, causing the low compression. Any chance the piece of metal in the cylinder was from the seal ring on the head gasket?

Re: 1996 Evinrude 112 compression loss....re-gain

Good idea...but no, the head gasket was perfect. I did a pre, and post head gasket comp-test when this happened with equally negative numbers.

Thanks for the ideas and I was quite sure this problem was not "typical". I will strike this up to some incredibly good fortune and add to this thread if I find something in the future.