Well, we got the engine out & torn down yesterday........
and #5 is toast. Piston is melted to the top ring land. All other pistons looked normal.... a little sooty as it should be (slightly rich- total rebuild blown EFI 454 MAG MPI by Arizona Speed. Az built this motor and it only has about 8-10 hours on it..... only one cyclinder was fried......all other looked as if they were just put in....????? What caused that lean condition in only ONE cylinder ????? A leaky intake manifold gasket??? A defective injector??? What !!!????? Help me here guys, I don't want this again....Please

was combustion chamber black and piston top light brown-to off white?

the valves were a light to dark brown but the piston was black....all of them......I cannot help but think this is an engine assembly issue (vacuum leak @ #5).??????

Was #5 exhaust valve sucked?

BT

no burned valves. This is something, huh ??

I'm guessing now, but do you think a blown head gasket could permit enough water into the cylinder to raise compression to detonation levels, but still let the water/steam cool the exhaust valve enough to prevent valve destruction? You are lucky... I alway get total destruction of everything it seems.

BT

Welcome to the word of High performance boating. Does you piston look like this one? This was on a motor that was running so rich that the transom was always black & fouled out plugs all the time. After tearing up about 4 motors with that carb I finally threw it away.

As far as injector messing up, possible. But I bet all were lean, just this one detonated first

Sounds like a lean condition to me. Possibly patially clogged injector or badly re-mapped ECU. In either case I feel Az Speed and Marine should make good on this one.
A while ago I was considering doing mods to my stock efi motor. Then after hearing of a few horror stories like this, I decided it was best left alone. Personally, I don't trust someone with programming a fuel curve for my motor. This sounds like a prime example of some computer geek who may have GUESSED wrong.
Can you post a picture of the piston??

Don't you guys think a lean condition would smoke the exhaust valve first?

BT

Not always. I have seen pistons melted first. Besides, what else other than running lean can melt a piston?