Merc 305 No compression

"Hello Everyone:

I have a 2

"Hello Everyone:

I have a 24' cruiser w/ Merc. 305 (220HP).
The engine has about 250 hours.

Winter of 2004/2005 it was improperly winterized, resulting in engine damage.

Took it to a trusted mechanic, and was told that the freeze damage was limited to exhaust manifolds, and that it could be welded.

Took it to a reputable engine welding facility here in Cleveland who verified (through MagnaFlux, X-Ray and pressure testing) that it could be successfully welded.

Got the boat back last summer, and it ran better than ever. Late last summer, while in idle, I struck an underwater obstuction causing the engine to stall. The only visible damage was a slight ding in the aluminum prop, which I quickly replaced.

After that incident, the boat seemed to run and idle fine until the engine was up to temperature and the RPM's was > 3000 RPM while up on plane (under load). Above 3000 RPM, the engine would start to sputter (popping noises) and lose power. Bringing the engine back to <3000 RPM made the symptoms go away.


Tried changing fuel filter, plugs, wires, clean flame arrestor, checked ignition module, tune up, but could not find the problem. Checked for water in the oil, none, also no oil leaks.

Did a compression check and found that one cylinder is only 40PSI.

Here are my questions:

How could the boat run so well (sounded very quite and smooth) under 3000 RPM with no compression in a cylinder?. Would'nt it sound a little rough. Like I said, the problem only occured at >3000RPM and after the engine temperature stablized at 175 degrees (about 15 minutes of running).

Doesn't the symptoms sound like a bad exhaust valve? (un-combusted fuel igniting in the hot exhaust?)

Could it be an undetected crack in the cylinder water jacket?

Could the problem have surfaced as a result of hitting that underwater log, or is it a coincidence? What could've hitting the log done to aggravate an engine problem?

Thanks,
Mike"

Only thing I can think of is t

Only thing I can think of is this: You stopped so fast water entered one of the cylinders and damaged a piston (and/ or rings).
Which cylinder is the low one?

Jeff

"Cylinder #5,
(2nd from

"Cylinder #5,
(2nd from the back on the port side)."

"Jeff:

Thanks for the fast

"Jeff:

Thanks for the fast response.
If it was a bad piston and or rings, wouldn't you expect the engine to be burning oil? I don't blow any smoke, and the oil level is where I put it just before winter lay-up

Mike"

"With 40 pounds of compression

"With 40 pounds of compression, you won't have much combustion happening. If the ring(s) are bad in that cylinder, it's possible that whatever compression is available is blowing the oil back into the crankcase instead of letting it into the cylinder.

Has the motor overheated? If it has, I would suspect a bad head gasket."

"Your collision/grounding caus

"Your collision/grounding caused more damage than a little ding in the aluminum prop... to stop the engine like that would take a lot of force.... all of which was distrubuted, not necessarily equally and undiminished, to all of the drive line components.
Something is obviously broke. I'd start by pulling the head off the side with the low compression. Depending on what you find, this may necessitate more inspections and or troubleshooting.
Don't forget about that poor little outdrive that had the herculian task of tranfering all that energy from the 5000# boat in motion to the underwater obrsturction. I wouldn't be surprised if things weren't quite lined up as good as they used to be. This could be part of your vibration above 300prpm. Drain the oil and see if there is indication of water in the oil or excessive metal shavings on the magnet."

"Well, I had someone (who

"Well, I had someone (who I thought) was very knowledgable about engines look at my problem.

They tore the engine apart and diagnosed a "bad valve" and a "bad head".
They also said that it looked like the piston got hot.
Kinda vague huh?

So I ask "what could've caused that to happen".
They say, "could've been bad fuel"
But wait, would'nt bad fuel effect all cylinders?
Apparently , this guy must think I just fell off the pumpkin truck.


With my limited knowledge , I would think that there was some kind of detonation going on there -
Carbon build up due to unburned fuel? ignition problems?

Thoughts anyone?"

May as well rebuild when your

May as well rebuild when your that far in to it.
305 is a good motor to rebuild parts are a dime a dozen.

"Look at your intake manifold

"Look at your intake manifold for the hose that goes to the PCV valve. If it enters the manifold runner on #5, that's the reason it went lean. Indmar had a problem with the #8 cylinder on some of their motors a while back and the solution was to get a spacer for the throttle body that has a fitting for the hose. This way, the vacuum from all cylinders operated the valve and no one cylinder got too hot. It could have been a bad valve, gas, the cam may be soft and that lobe wore away, etc. There are a lot of things that could cause this, not so easy to tell which it was. Is the spark plug burned, too?"

"Yes, the spark plug for that

"Yes, the spark plug for that cylinder was sort of black.

The engine has no throttle body, it is carburated with Thunderbolt V ignition."