liquid asset

lost compresion ( down to 45 psi ) on a cylinder or two on a 2000 stock 454 mag mpi with 90 hours , 4rth time out in new boat the week before the 4rth wtf what gives, any ideas

Outdrive1

Awfully low hours for a 454 mag to have those kinds of problems. Is it possible that you had some freezing this winter and possibly cracked a head? Maybe you just need new head gaskets, has the motor overheated recently? Bad impeller recently? Any water in the oil? What made you do a leak down test in the first place? Was it running bad? It could be a bad valve but in two different cylinders at the same time I would guess its a head problem.

Outdrive1

4th time out on the boat, does that mean you recently purchased it from someone? Maybe 90 hours isn't the whole truth? That motor should be bullet proof for 4 or 5 hundred hours easy or more if it's maintained properly. Not that you can't have something fail but it's just not very common.

articfriends

iTrader: (1)

Is it ticking too? If they go lean the valves can mushrum sometimes but not necessarily fall of and lose compresion. Detonation or overheating could take out the rings/ringlands on the pistons that inherently run lean. Is it 2 pistons or 1,are they adjoining? Was there anything on plug or did it look lean?Smitty

Dave_N

Was it one cylinder or two? Which cylinders?

Dave

liquid asset

It all happened suddenly, the motor does appear that it could have overheated in the past, the corsa sticker are missing on this motor but not the port motor, this ussally means they baked off, So far on the inside cyliders, the front 2 where 145 the third was 45 and the 4rth was 125, I was tired of dicking with it, so I have not done the outside cylindrs comp check, there is no noise it starts up, a little hard to start , but not bad, and sounds like a four cylider and has no power. I dont know what to look for on the plugs but they where black and smelled like gas, and that side had way more soot than the port, so I thought it was running rich, also the scan tool read 90.43 hours and no codes, Another thing was I never ran it hard not over 3,500 or 4,000 that day and only a couple of 5,000 runs since I bought it for less than 5 minutes, I am used to running Poker runs at 5,000 all day with no trouble but have a 2 year old daughter with me most of the time, and have babied it so far

liquid asset

I dont think freezing would be an issue, since it came from So Cal

Dave_N

Inside bank on the starboard engine would make the 45PSI cylinder #5. When water intrusion is an issue, #5 is often the first cylinder to have a problem. Stuck exhaust valve and/or rusty valve seats. But these problems normally show up on a startup after sitting for a while, not all of a sudden while underway.

Time for a wet cranking compression and compression leakdown test.

Dave

mwdill

got a buddy that has the same problem, lost comp in the left front, 145 in all the rest 40 in the dead one not sure what it is yet, 99 454 mag 120 hours.

liquid asset

that motor has been harder to start since I got it , not bad but always seemes to be a little flooded when starting and then clear up with a little throtle