502 MPI Nightmare!!!
 

Hello all,
Hoping someone can help with me some insight. I swapped out my good old trusty 454 330's for a set of 502 MPI's. One motor runs great and the other not so good. Boat has been laid up for weeks now and if I didn't sell my 330's, they'd be back in!

The motor starts great and idles great and runs strong up to 2400 RPM's and then will just start popping and not going any further. This is with and without load. All data has been checked on computer by a qualified mechanic and everything checks out. Compression test good on all cylinders. Fuel pressure checks good. Firing order checked. Plugs and wires all new. Valves were done before motor went in.

One thing that doesn't check is that plugs show all cylinders running a bit lean. ECU was swapped from the good motor so it's not that. I'm at a loss and so is my mechanic.
Any insight would be greatly appreciated!

Replace the rotor and cap. Take one plug wire off at a time and place on new cap or label each wire so you dont effit up. If it has not been replaced, that is likely the problem.

Chris

Baja,

Once you have successfully eliminated the diagnostic process of fuel/air/spark, the root cause the issue is likely mechanical. In light of the symptoms, a broken valve spring would be at the top of that list. The inability of the spring to control the mass of the valve beyond a finite RPM level, both under load and unloaded, the resulting backfire, are all symptomatic of this condition.

Bob

It could also be clogged fuel injectors.

Rotor and cap has been done
I was thinking fuel injectors, but I've tried a different set from another intake that I know is good, and all plugs look exactly the same - lean. So I don't think that is it.
I'm starting to think that the shop that did the valves did something wrong.

Maybe this has been checked, but what is the timing advance at 2400 compared to the other motor. If they are good then it has to be one of the other suggestions. The popping usually indicates a fuel problem or valve trane. I would check the vacuum and compare between the 2 also, and maybe swap some of the sensors.

sounds like injectors to me...known issues with them...

Thanks for your feedback guys! I do have one set of injectors being tested and cleaned. Still don't think that's it. Like I said, I've tried two sets now. The second were on a boat last year that ran great. And I can't see all eight going bad at the same time. But ya never know.

I did get some input today that if any of the valves were not done right, it would cause a blowout in the cylinder which would in turn trip the ECU to lean out, thus causing all plugs to indicate running lean.

I've heard so many different things at this point, my head is spinning!!!
Any thoughts?

Can you run an oxygen sensor in the exhaust? That should verify what it is doing for sure, then you can run things down until you find the culprit. I read an article about ethanol fuels, They say enough ethanol in the fuel & it can fool the computer by actually making the engine run lean. Randy

The ignition pickup or control module (same thing) can be bad on the distributor. Swap distributors from one to the other and check it.

Also in case you do find out that this was the culprit see my post here:
http://www.offshoreonly.com/forums/g...odule-icm.html

Double check that it's not in power reduction mode.

(copy and paste below from the merc book for MEFI-1)
-----------------
The two discrete switch inputs and the ECT (engine coolant temperature) sensor are used
by the EFI system to identify abnormal conditions that affect engine operations. If engine oil
pressure drops too low, or if drive unit fluid level (in the reservoir bottle) becomes too low, or
if the engine coolant temperature becomes too high, the ECM will engage the “power reduction mode”. The “power reduction mode” allows normal fuel injection and full power up to
2,800 rpm. Above 2800 rpm, fuel delivery is limited to 1 injector (TBI models) or 4 injectors
(MPI models) until rpm falls below 1200. The engine will then resume normal operation with
all injectors firing. If the problem goes away (while power reduction mode is engaged), the
engine immediately resumes normal operation.

We've got two of our very best guys working on it for you. We'll find it!

O yes it can be the injectors!! it can also be caused by low fuel pressure....anytime a efi engine is lacking fuel in the cylinders it will make one hell of a popping noise through the intake...most likely the screnes on top on the injectors are pluged up and not letting fuel through!!..

Could be, but if I understand correctly it cuts out at that rpm regardless of load. If it were fuel flow it'd rev higher with no load than with a load.

I am thinking valve spring as well. It would run fine up to a point and then float. You would also have an okay compression test as well. I had a 465 HP do this kinda thing to me and I finally checked and for a broken spring.

Possibly a worn camshaft or collapsed lifter, just my .02

I had the same problem on a 502 efi some years ago ,it turn out to be a clog fuel strainer that is located in fuel rail behind regulator

make sure plugwires go to right plug and they are separated, especially 5 and 7. just had this.

Well folks, this one finally got solved and the OP should be picking the boat up tonight. Thanks for all of your suggestions, but nobody was even close. I'll let the OP fill you in on what we found, but I will pass on one bit of advice / hint that our Service Manager passed on to me many, many years ago:

"All stupid electrical problems are a bad ground."

I will say that the two mechanics at our marina who worked on this boat checked absolutely everything - over and over again. Although they kept running into brick walls, neither of them gave up and every morning they'd show up with lists of new things to check for. When I heard the boat (in our launch ramp for the millionth time) fire up and actually rev past a couple thousand RPM for the first time this morning, you should have seen the smiles on these two guys faces. There were high fives all around. The key here - persistence and product knowledge.

Those grounds can get ya everytime!