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.

By any chance you didn't put the wrong spark plugs back in it. A hot plug will do it. My 2 cents JOHN

Loose spark plug will also cause 1 cylinder to run lean.

Do you think I can buy an fuel rail of a later 502 and will it fit or is there an aftermarket product that I can buy.
Harry

NGK BR6FS

Might not be THE problem, but isn't a 741 lift cam to big for a set of stock 502 heads. If you didn't lengthen the valves and strengthen the springs you might have had a bigger problem down the road.

I am actually asking the experts if I am right on this. have heard this from some local guys in the past.

Jeff,

The only thing that is stock is the head it self cause I changed the springs, rockers and valves. Note that mjb is right cause I checked the fuel rail and on the no 7 outlet there is only a little hole that feeds the injector.

Harry

Mjb. The actual hole was 3mm and the best I could drill the hole is to 6.5 mm. Do you think that's enough.

Cool. I don't have fuel injection. Still running the carbs. Good luck.

Hard for me to say if 6.5mm is enough...that's only about 1/4" and still sounds pretty small. I did not drill mine. I used a dremmel tool with a long, slender grinding bit to open up the passage. I think you will be able to remove more material that way as opposed to a drill.
I do not think that there is an aftermarket fuel rail that will just bolt on to solve the problem. You may be able to get a later model mercruiser rail but I am pretty sure that the injectors on the MEFI 1's were different than MEFI 2's and I think the fuel rails are different...but I'm not sure of that?? I think your only quick bolt on option is to modify your fuel rail but maybe someone else in the know can comment.

[QUOTE=mjb;2205023]Hard for me to say if 6.5mm is enough...that's only about 1/4" and still sounds pretty small. I did not drill mine. I used a dremmel tool with a long,

Mjb,

OK, I'll try to a least get the hole about 1/2". But thanks to you I found the problem, cause I couldn't figure out what happened,

Harry

[QUOTE=D Harry;2205070]I don't think you can get it that big. Be careful not to mess up the o-ring seat and take all the plugs and fittings out and clean and blow the sh*t out of the rail. Shavings will be packed in there.

[QUOTE=kennyo;2207574]As a matter of fact, I cannot get it any bigger then 1/4", so I gonna have to chance it like that, or else I was checking if mercury had arranged the problem on the late 502 and I'll buy a new fuel rail, but the problem is that the agent here in Malta dosen't have in stock and so I cannot check it. Does anyone knows if Mercury had arranged it??