Need Help!!502 Mpi Hole In Piston
 

WE PULLED JAMIE'S 502 MPI OUT. #6 PISTON HAD A HOLE ALL THE WAY THRU IT. HOLE WAS UNDER SPARK PLUG. PISTON IS HEAT DISTORTED ON TOP AS IF IT LEANED OUT. WE PULLED HEADS OFF OF OTHER MOTOR AND FOUND #6 IN SAME SHAPE. BUT NOT BURNED THRU YET. ALL OTHER 14 PISTONS LOOK PERFECT. MOTORS ARE STOCK EXCEPT FOR CMI EXHAUST, WATER CROSS OVERS,AND ARIZONA SPEED REPROGRAMMING BOXES.MOTORS HAVE APPROX 250 HOURS ON THEM. WHAT COULD MAKE THIS HAPPEN TO BOTH MOTORS SAME CYLINDER. WERE STUMPED WHAT DO YOU THINK OR KNOW? RON

Similar things have been know to happen on small blocks. One cylinder was always either burning through a piston or burning an exhaust valve. I believe Smoky Unick had that problem years ago and assumed it was either poor water flow around that one cylinder, or not enough fuel to that one cylinder causing a lean out... but if your engine is fuel injected... I would have a tendency to think that one cylinder runs hotter. According to the old hot rod guys... the way they circumvented the problem was by lowering the compression on just that one cylinder.... sounds crazy,,,, but that way they could run maximum ignition advance and fuel mixture on the other seven and get more performance than just rejetting and retarding.... Just my guess... good luck...

I have seen one clogged injector do that to an MPI engine. It is unlikely but possible you have the same problem on 2 engines. Or as Reed said there is a cooling problem-possibly due to the crossovers. Do you have any idea what water pressure the engines were running??

I KNOW HE DOES NOT HAVE PRESSURE GAGES

That is the problem cylinder on those engines. There are a variety of cures. Make sure that the bolts are not bottoming out, vent the heads, run a colder plug on #6 and/or a bigger injector.

Just for kicks, check the cam lobes for abnormal wear (degree wheel and dial indicator).

Could it be a programming issue? #6 getting too much advance or lean mixture?

It sounds very unusual to me that both #6 pistons would be running lean. Since both ECU's have been re-mapped, it sounds to me like another Arizona Speed and Marine screw up. How many hours have you put on the motors since having the ECU's done?

I would check the flow of those 2 injectors. Then I would check fuel pressure for both engines. If all that is OK then its the exhaust or reprogramming. Or a combination of both.

I've heard the same thing that the #6 cylinder typically runs lean in the 454/502 motors. I have no idea why.

I also don't think the ECU's are capable of sending a different amount of fuel to each injector. I think the computers are too dumb for that, especially if it is a MEFI1 system. It sounds like the motors in question are pre-serpentine belt applications, so they would be MEFI1. The MEFI3 ECU may not be capable of that either.

Hi, Thanks for the help! The 502's are serpentine belts,the ECU's have about 150 hrs on them, I tried checking the fuel injectors with a pressure gage, the pressure seemed to drop off within 2-3 # of each other. The cross over system was added about 50 hrs ago, We did find exhaust leaks around those cylinders. Jamie

Check your injectors, you proably have a bad one.

Remove the injectors(all of them)Number them upon removal,Bring them to an injector shop & have them flowed.That will give you a base line.If they're all good then you can look elsewhere.It would be quite strange that both #6 injectors are bad.ECM are set for a certian inj. timing on all inj.,can't do just one.Good luck!BOB

a couple idea's that might help out if you are a little hot and are lean is one heat range colder plug plus most of the VST 454's and 502 mag mpi's had a 36psi fuel regulator installed if so then swap it out for a mercury 43 psi fuel regulator the part # is 806808A2
good luck
Mark R

What do the Spark plugs for those #6 cylinders look like? Do they show signs of detonation? (blistering on the electrodes, little balls of aluminum on them?) Also, When you get the motor back together periodically check those plugs and get a magnifying glass and peer all the way down along the ceramic insulator and look for an ash ring near the bottom. If the cylinder is running lean you will notice a difference between the plugs from different cylinders. Ash ring versus no ash ring, but plugs have to be new and only run briefly and then checked. Good luck, I know problems like this are like chasing gremlins. They come back to haunt you. Also, Going to colder plugs may help if the engine is held on boost for a long time. I'm assuming it is supercharged. If one plug gets overheated and the tip begins to glow it will pre-ignite the mixture and cause problems. Check those plugs for blistering.

His plugs are bound to look like a christmas tree glitter ornament - he's got holes burned thru the slugs.

I did read above that the motors are NOT supercharged, just some minor bolt on upgrades.

With this in mind, running stock compression and nothing fancy, stock heat range plugs *should* be just fine for the application.

I also don't believe that the ECU is capable of different injector timing for a particular cylinder but could be mistaken.

As far as looking at the plugs, I am more interested in knowing what the OTHER plugs look like. We are pretty confident that #6 is/was detonating, but we don't know if it was at WOT or during cruise. We also don't now if the other holes were burning correctly or if they were lean or overadvanced. I'd certainly like to be able to stare at a fresh set of plugs out of it after a hard pull..

Bob280silencer Thanks for the response, Do you know of a injector shop, if so can you give me there name and address Jamie Treblas

The injectors are hard wired together in parrallel in 2 banks of 4 on the 454/502 merc engines and so it is not possible for the ECU to have different injector timing for a particular cylinder.

JT,There's got to one closer than New Orleans.Ask a local speed shop or carb shop BUT,if you strike out give me a call 1.504.486.7070 weekdays and I'll hook you up.Good luck,BOB

Reed stated that the number 6 cylinder runs hotter on big blocks , I have heard this also. So maybe both motors ran lean due to regulator/pump/programming issues and those were just the first cylinders to go because they run hotter. Just a thought

Why did you have your computers reprogrammed. Did you do this because of your new exhaust?

JT,There's got to one closer than New Orleans.Ask a local speed shop or carb shop BUT,if you strike out give me a call 1.504.486.7070 weekdays and I'll hook you up.Good luck,BOB

I tuliped an exhaust valve on my starboard engine (#6 cylinder) and when I checked the jetting on the rear carb I discovered that the 'genius' that recalibrated the carb had left a bit of the gasket from the bowl floating around. Guess where it ended up..... that's right..... in the main jet to the venturi right over the #6 cylinder. But to have both of your #6 cylinders pistons damaged leads me to believe that it is an 'engine management' issue. Ie: fuel injection system too lean for that hot running cylinder and or too much advance or a combination of all those things. Although the fuel and air mixture went through the blower and then into a plenum before it went into the intake runner. I have to wonder how much the restricted jet added to the burned valve. After it was repaired I didn't do any rejetting but never had the problem again. I was very careful to check the plugs after that happened though. My jetting for the engine was on the rich side to begin with. Good luck guys.

How do the other piston look? Is there any pitting on the outside above the rings? How do the rings look? Are they missing any of the finish?
How many RPM's do you run at WOT? Do you run WOT for a long time? Some EFI's Rev limiters cut fuel and can cause the engine to lean out, and if the engine is held at WOT bouncing off the Rev limiter detination can burn the pistons.

With the modifications you have done on these motors, they will want more fuel. You might want to check if the boat has anti-siphon valves. They can be very restictive.

Have taken motors down further, have problems with # 6 cylinder gold color top of piston warping, bottom of all pistons in both motors show signs of heat , gold color. I think I was running WOT on rev limiters, could this been leaning out ?:confused: :confused:

I'm no computer guru by any stretch, but I've been told that early (mefi3)? controllers leaned out the mixture instead of adjusting timing when on the rev Lim. . If so that may be your problem. and if so, what a dumb way to slow down a motor.

Dan

Going with the last modification......Are you running a restrictor in the T-stat housing? T-stat and Bypass hose? With no restrictors of any kind, WP will be low. Not enough for a good fill causing the rear cyls. to run hotter. Low WP can cause for steam pockets in rear of heads.
My feelings are a WP gauge is a must whenever a crossover is used. No if, ands or buts about it. ;) You have to know what you have.

Iam running a 120 T-stat and a by pas hose, but no WP Gages. Iam thinking about adding Water prasure gages and fuel preasure gages. Which ones are the most important:confused:

They both are.... trick answer for a trick question :p

I was talking to a computer expert all of the ECU's cut the fuel supply off when you hit the rev limiters. The problem is my boat weighs over 8000 # I can't feel it hitting the rev limiters. If they were set at 5300 like they were supose to be, I might not have this problem.:hothead:

I agree with teambaja,befor it goes back into the boat ,DYNO. on the dyno you can look at individual E.G.T. and see the real story of each cycl.

I had a set of rebuilt 502 EFI's put in my Sunsation last winter. After 65 hours both motors broke at the same time. I do believe the #8 cylinder on one engine melted a hole the size of a nickel and the other engine had the #4 and 6# pistons melt down around the engines. Blew oil everywhere and now both motors are sitting on the floor again. Engine builder cannot figure out went wrong and neither can any of his peers. Very strange, why both motors? Everything is stock and I hadn't had the boat wide open the last three times out so it wasn't even being pounded.