Low to no compression

[prairieboy]

I was just doing a little maintenance this weekend and thought why not run a compression test. I ran the compression on one engine and it was 145 mag and 150 PTO. Yes!

Then on to the next. Mag side was almost zero. Maybe 5 to 10 psi. The PTO side was around 150.

Tried again on mag. Nothing. PTO side, good. K so it's not the gauge. Great, meltdown!

I pulled the plugs and could rotate the flywheel with no problems. I brought the mag piston to the top and peeked in with my flashlight but it didn't looked scorched or anything. I compared it to the PTO piston and they both look to be around the same condition. When I rotated the engine there was no rubbing or grinding but the only thing that I was a little unsure of was the mag piston sounded a little loose if that's possible. I'd rotate the engine a little one way and then a little the other way and that's when I heard a little clunk.

So I'm looking for advice. Is there absolutely anything else that can cause no compression on a cylinder other than a hole or even worse meltdown. Could the rings have just broke off? I dunno.

Guess I'm looking for other possible solutions before pulling the head.


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[Dr Honda]

The RAVE could have grabble the rings... and cracked them. BUT, unfortunately... that's not good either.

I would pull off the head, and take a look in.

 

[prairieboy]

When you say not good - how bad then?

I cleaned the raves this spring and they were moving freely. The only thing was that possibly it was put in backwards if that's even possible. Could a water/exhaust hose popped off?

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[ialonso]

When you say not good - how bad then?

I cleaned the raves this spring and they were moving freely. The only thing was that possibly it was put in backwards if that's even possible. Could a water/exhaust hose popped off?

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If the damage is limited to top end, it's not a terrible job to replace pistons and cylinders with engine still on the boat (I'm assuming my 717 top end experience extrapolates to your 787 motor). As long as no parts fell in, you might be OK to not crack the case.

Do not panic yet!

 

[Dr Honda]

When you say not good - how bad then?

..............

It's still damage, and that would mean a new piston, rings, and RAVE. But, those little shards get into the lower.

Regardless... if it's the original engine... it's probably just time for a full rebuild.