Can someone me an idea what happened
 

I just refreshed my 502mpi engine, were I changed the cam to a 741, roller rockers, ported the intake and heads, took the injectors for a clean and checked and bearings. Now since I put the engine in the boat, it wasn't running very well between idle and 2000 rpm, but the engine was running rich, so I put a DDT scanner and it showed that the map sensor, changed it but it wasn't running smooth. Now after 6 hrs of running, I heard a tapping noise, took the engine apart and found the no 7 piston blown.
Any idea what happened, cause I don't want to rebuilt the engine every 6 hrs!

Thanks

lean condition , detonation , my guess

lean sounds like it.
Check the injector prior to assy.
Check the rear intake gasket. Put a nice 3/8 bead of silicone down front and back of block.
What did the spark plug look like?

You can search past posts for more info on this subject. It depends on what year 502MPI you have but many of the earlier MPI's have a factory defect in the fuel rail on the #7 cylinder. The hole feeding this injector is MUCH smaller than the other 7 holes. With a stock cam it really wasn't much of a problem. Therefore I do not think mercruiser ever publicized the issue very well. The problem comes when you put in a bigger cam causing the #7 cylinder to run lean. When I did mine, I opened up the feed hole to the #7 injector with a dremmell tool and I have had no problems to date.

mjb

Do you know the yr range?
Is there a seriel # range or anything?
Thanks

Thanks Mjd.

The 502 engine that I have is a 1996 model Gen VI V belt. I'll have to check that cause I think that should have been the problem then. How much bigger did you make the hole??

Harry

Vegas,

The plugs are full of black soot as if it was running rich and that could have been caused by the bad map sensor, but the pistons are black also, except for number 7 which melted.

Harry

You did have the ECM recalibrated for set up, didn't you????

You don't mention that or who did it in any of your posts.

Yes, I did the ECM recalibration was done by Tyler Crockett

My engines where 1997's and they had the defect. I'm not sure when Mercruiser fixed the problem but I know it was after 1997. You have to be VERT careful when you open up the hole not to score the matting surface of the o-ring. I opened mine up as much as I could and still leave enough mating surface for the o-ring to seal well. Hard to give you any definitive measurements though. I'm guessing that I doubled the x-sectional area of the feed hole...but it's still smaller than the other 7 holes....but large enough to prevent a lean condition with my bigger cams.