35 PSI on HPDI cylinder but no damage?

[wbrown]

HELP!

Ok so my bad idle/hard to start/wont stay running that I thought was the fuel pump having trouble ended up being low compression in Cylinder #1.

I have approx 110 on all cylinders except #1 which read 35 PSI. I tried the test several times. Same thing on each one. The spark plug in this cylinder is dry and dark black while the others look normal.

I've had a cylinder drop in this engine before, and feared the worse after my compression test, so I pulled the head. And to my surprise the cylinder wall and piston look perfect. I don't see a single scratch or scoring on either. I thought maybe a leaking head gasket but I can't find any evidence of that on the block surface, head surface, or the gasket. (Although I've never experienced it before so I'm no expert by any means.)

I have a tournament this weekend. Anyone have any ideas? See pics below.

[JoeFriday]

Are you getting good spark to that cylinder? It looks like it is not getting enough juice to burn the fuel.

[Rich Z]

In 3rd pic I see scuff marks on the cylinder wall. It may have choked a reed valve so inspect those also. The head also looks suspicious.

[kry29]

Yea. Looks like some blow by in that third pic. Maybe a bad piston ring

[CJ1]

I would pull the piston and find out why it has low comp. You may get by with a hone job if the scoring is not too deep and a new slug/rings but find out why it failed, reeds like Rich stated, pin issue, injector, ect. I would NOT run it like that, if you stick that piston you will have a pile of junk. CJ

[wbrown]

Thanks guys. I definitely am not running it like this. For one, it won't run. For two, I know better.

Edit: I guess I didn't believe that was scoring. But I'm not an expert. It's going to my mechanic and Im praying it just needs new rings, a hone, and maybe a reed or injector. Piston and cylinder look great to me.

I may have caught it RIGHT as it happened. I was running 5500 rpm back to the dock just fine, come off plane at the dock and she started sneezing and then died. Then very hard to crank and would die a couple of seconds after I got it started. Loaded the boat up.

[1Holeshot]

I think you will find a broken ring when piston is removed.

[jim maloney]

I think you will find a broken ring when piston is removed.

agreed, luckly it doesn't appear that the ring moved into the port or you would be looking at a new powerhead, depends how hard the cylinder material is as to wether you can hone that out, I know nikasill cylinders cannot. likely the pin holding the rings in place loosened and worked its way out

[wbrown]

Slight Update:

Had the injectors flow tested and cleaned. They were all good but did clean up a little.

I started to suspect a reed problem. I looked through the air intake at them best I could but couldn't see any problems. The air box was full of gas.

Its going to my mechanic tomorrow. I further examined the cylinder and there is not a single mark you can feel with your fingernail. The image doesn't do it justice, (the light i had shining affected it) it looks in great shape to me.

[wbrown]

Update:

I know there's several people on here that would find this picture very interesting, so here it is:



The pin did not migrate outward. The hole was enlarged and the pin moved upwards into the piston to allow the ring to turn. As I said before, no damage to the cylinder. I've checked it and my mechanic has checked it, and there's no damage.

New piston and we are top pinning all of the pistons.

[CJ1]

Very fortunate, That was my first suspect for failure. I truly believe it is from light detonation rattling the pins loose. Most of the time they rotate until the ring end ends up in the exhaust port. Then carnage. CJ

[Rich Z]

Lucky you no serious damge was done. That ring spun, broke on a port and went out the exhaust. Thats my theory .

[yam911]

If I were you, with your good luck, I would run out and buy some Powerball tickets for tonight's drawing.

[yam911]

Are you saying that the ring, all of it, was gone or did it simply rotate in the groove?

I am having a difficult time getting it in my mind that all of the ring went out the exhaust or back into the intake. It usually hammers the top of the piston and the cylinder head when it does that. But some strange stuff can happen when parts fail.

[jim maloney]

Update:

I know there's several people on here that would find this picture very interesting, so here



The pin did not migrate outward. The hole was enlarged and the pin moved upwards into the cylinder to allow the ring to turn. As I said before, no damage to the cylinder. I've checked it and my mechanic has checked it, and there's no damage.

New piston and we are top pinning all of the pistons.

yea exactly what I thought, you are a lucky man ! unfortantly moist are not, I can testify to that

[CJ1]

I'll bet the ring was still there, it did not rotate enough to catch a port. Otherwise he would have the normal mess. CJ

[Nova Kaw 650]

Could the pin have been loosened by detonation?

[yam911]

Could the pin have been loosened by detonation?

That is my best guess.

Another photo showing similar damage:

[yam911]

It may be my poor eyes or bad perspective but does it seem that the top of piston, directly above the ring locating pin, is bulged upwards a smidgen?

[wbrown]

I'm not seeing the bulge. I haven't seen the piston in person either.

Can anyone explain why I had 35 psi and the ring was still intact? Will a pin that's moved into the piston increase ring gap so much that you only get 35psi?