dead502

I have no pics as of yet. but it looks like a typical severely overheated piston and cylinder. I am going over to look at the motor again today. a friend at the locale chevy dealer told me to take a good look at the head gaskets they used as there are many choices and if the rear water passages were covered up or restricted it could cause the problem... the rod appears to be ok.

stevesxm

he's right about that but my understanding is that when that happens the heads roast... not the block... and you would have had to see that on the gage... but either way he is correct and that needs looking at.

EVERYTHING needs looking at. once may be random but not twice.

rule # 1... nothing happens for no reason

BillK

dead,
Like the others said, take a bunch of pictures and post them. I would almost have to say that it is something external for it to be burning the same piston both times. Injector, ignition system crossfiring, vacuum leak etc etc etc.

Bill Koustenis
Advanced Automotive Machine
Waldorf Md

RHM253

Like stevesmx and BillK said, everything needs to be looked at. This could be as simple as a bad plug wire, or even the way the wires are routed. It isn't uncommon for the #5 and 7 to crossfire. It was running lean I have found the #6 cyl is the one that goes lean on the EFI's. #6 is the cly I try to tune to when remapping. I would also check the ring end gap very closely. None of this may help you, but good anyway.

Smitty

iTrader: (1)

Check the fuel rail. I believe there was a problem with the early model EFI motors. The #7 port was too small on the rail and would not provide enough fuel under a large load and thus go lean and burn up that cyl.

Do a search here, you will find it.

Found the link for you:
http://www.offshoreonly.com/forums/g...il-defect.html

dead502

Thanks for the info Smitty. I will foward it to the builder.

stevesxm

if it was the rail, wouldn't the original motor have done it BEFORE rebuild ?

dead502

CORRECTION!!!!!!!!! I DID FIND A PROBLEM WITH THE FUEL RAIL. the number seven injector feed hole is much smaller than the rest. I don’t know if this is causing my problem but its definitely a defect Mercruiser should have published. the hole for number seven is 3/16 in diameter and the rest are 3/8. that’s a huge difference! I wouldn’t run this POS fuel rail a in a any motor! Its JUNK. There’s not enough material in that area to come close to matching the other holes. (I tried and butchered it) anyone know where I can get a updated one on the cheap????????? Good call Smitty!!



see pic here http://home.rr.com/mfox

Fixxxer22

intake leak at suspect cylinder causing lean condition? low fuel pressure? it is the last injector on that bank.

Boat Tech

Maybe it was starting to happen slowly over time because I do remember back in the mid to late 90s a few big blocks coming in with something wrong on the #7 cyl, And I believe we just ended up fixing the engine and made sure every thing was set up right to specks and I think at the time after talking with merc I am sure of, Because we always did call them a lot back then. we Thought back then it would have been a bad vacuum leak on the intake manifold and {I think} And if i remember right we just put a little extra sealent in that area, But I do not remember myself merc ever saying it could be the fuel rail to us any way, But I guess it could have been who you talk to might not have wanted to say any thing or just did not know at the time. This is intersting to me because last august I bought a 1996 454 mag mpi 21 ft boat with 170 hours on it with [OEM] spark plugs in it and when I changed them the # 7 plug was a different color then all the rest, all the other ones had a blacker richer color to them and #7 has way less of a running rich look to it and the center post in side is way reder in color then all the rest and it is not RUST because if you look real close at all of them you can see a little red on all the other plugs but this one just stands out in the crowed. And this engine is all stock. And it's hard to see but there is a small sign of pitting on the plug also. you can tell this plug was not getting the same fuel volume as much as the others where. I just thought I would put that out there to see if it helps here any. Good luck with it.