502MagMPI conversion to Carb
 

I have a 1996 502MagMPI. The MPI (MEFI-1 with cool fuel) system has frustrated me all summer to the point where I’m just about ready to rip it off and bolt on a Carb.
My questions are can I use the existing engine wire harness and distributor without major modifications?
I would assume the IC module internal to the distributor can do the timing advance. What do I do with the ECU? Just unplug it?
What do I need to change on the engine wire harness?
I have the carb and intake off a 1996 454Mag and from what I can tell from the Merc manuals, they were the same for the 502Mag with just different jets and spacer.
I’m hoping I just need to buy an electric fuel pump to feed the carb and be good to go.
Help Please!
I know this has been done before…
Thanks,

I had done exactly what you're trying to do but my mechanic did the conversion. I don't know the details but it wasn't that big of deal.

I'm not sure if the dist is tied to the computer or not, if so just run a aftermarket dist, elec fuel pump and you should be good to go...

Can't you use the existing pulley mounted fuel pump? EFI can be a major pain I agree.

I've read somewhere people have use the existing distributor with built in IC control module.
I rather not buy a new distributor this year; maybe next year.

My EFI just has the cool fuel high pressure electric fuel pump.

I completed the whole job you can call me and I can assist you whatever information you need . I can be reached at 203 877-7560. Jeff

The distributer has no provisions for mechanical advance as this is done completely by the computer program,you will need a different distributer but its not the end of the world,just a std hei would work then set the advance curve,Smitty

thats what I was thinking as most EFI setups manage the advance curve as well, but your fuel pump runs way to much pressure for a carb, could add a regulator but the cost and the plumbing I would just by a std elec pump and be done with it...also if you need a dist I have complete marine setup, everything I'll sell for $100 if interested and a new holley 7 psi fuel pump as well $ 50 Rob [email protected]

Yes Smitty, I know there is no mechanical advance but I thought the IC module inside has an advance curve of 15 or20° built in.
Yes, the ECU controls when it's working right. My original ECU no longer will comunicate with the IC modual but will still control the injectors. F'n black box...:rolleyes:
I know I'm being cheap guys, but I don't want to spend a bunch of cash on stuff to convert to a carb when I may just go back to EFI next year...:cool:

I have and extra cool fuel assembly off of a 500efi if that would work

Have you tried a new MEFI or IC module? Maybe somebody like whipple or Arizona Speed have MEFI 1s traded in.

Don't Shoot the Horse because the Rider Fell Off.
 

Just my opinion, but I wouldn't go through all the trouble and expense to remove the EFI and convert to a carburetor until I had exhausted ALL the options to troubleshoot and repair the original problem.

Sometimes it's something as little as a corroded ground or a pin in the main connector that's not making proper contact.

what he said

Sure, I hear you; I've been trying all summer to get the EFI to work properly.
I've been working with Mark at Precision Marine. He has been vary helpful remapping ECUs and such.
I've got the 3rd MEFI-1 ECU on it now and I've replaced lots of other stuff too.
Yes, I've check all wiring 3 times and gone thru many diagnostic procedures.
It's just this old MEFI-1 stuff is out of date and only available used.
I may convert to a MEFI-3 system next year, but that is even more $$ than going to a Carb...
BOATS...:rolleyes:

I have a carb and intake.
I will need to buy an electric fuel pump.

You guys are telling me I can not use the distributor, right?
I think others have used them, but...

Now, what to do with the engine wire harness?

You never told us the symptoms. What is it doing wrong? No start, rough idle, no power, hot start problems? You might just detail something that one of us has run into before.

I would bet just between MobileMercMan, Raylar, Gellner and myself we have thousands of hours of diagnostic time put in. It is part of our "dues" and the Lord knows we've paid them!

how can it be a mefi 1 with the coolfuel set up ? my understanding was that all the cool fuel set ups were the late ones w/ the mefi 3 ...

I've had a couple of other threads that you guys have chimed in on and been vary helpful. I thank you all for that.
My story is a long one but basically the original ECU failed in a continuous knock mode and had it running rich and the timing retarded. The transom still shows signs of that.
I bought another ECU from a 1994 502MagMPI and the engine ran OK and without fault codes. The idle was not great though and after consulting with you guys I sent the ECU to Mark for reprogramming. That did not work out so great and Mark is checking to see if that ECU has now failed.
Mark loaned me another ECU to try and now I have new fault codes (coolant temp sender). I when through the diagnostics and there is 5vdc at the pins so I'll change the sender and see what happens, but I've about had enough with this 1996 system. I'm not buying another MEFI-1 computer that is for sure.
The engine seems to be OK mechanically; compression test was 150-155 for all. I have not done a leak down yet.
The engine does not run right above 3900rpm, it stumbles and seems like it's not getting fuel or the spark is cutting out.
Fuel pressure is OK.

My new thread is to see how easily and how much $$ to convert to my carb and intake that I have sitting on the self…

I do have a MEFI-3 computer and harness from a 310HP/7.4MPI, but like I said early, not this year…

1996Merc 502MagMPI has a coolfuel and MEFI-1 computer.

Pat,

I am very interested in this project, as I'd love to go the other direction on my engine. After years of working with EFI both at GM as well as on my own, I'm completely in awe at just how HORRIBLE a job a carb does even when it's tuned "perfectly". Cyl-cyl fuel balance is horrible, resulting in having to run the entire engine way richer than it should need. If you keep the manifold cool for max power, it kills idle and fuel economy. If you size the carb right for part throttle, you lose 20hp on top....etc, etc. All compromises I NEVER had to deal with on EFI.

I wish I was near you. I'd love to be able to help you out. I want to get some experience with MEFI. One possible solution is to keep the complete fuel system, but change to an aftermarket ECU.

If you do go with the carb, I'll be interested in how much driveability you think you lose.

We are changing mine from EFI to carb to gain driveability and pick up the power we're losing by using the stock EFI manifold setup. We mad 501 hp on the dyno and ran out of manifold at 5200 rpm. Our driveability problem has to do with getting the fuel map correct at idle. When the engine was on the dyno, we had to send the ecu back and forth several times over several days to get the program close. We have it pretty well dialed up top, but the idle is just too rich and wants to load up. My engine guy did pick up some software, but without having it back on the dyno, we can't get id dialed. With the carb, we should be able to get it where it needs to be.

Overall, I think you can get EFI closer to dead on, but you have to have the right manifold setup, and you have to have it in the dyno making run after run to get it just right. That can turn into a pretty big financial investment for a 500 hp engine.

Guys I would rather run the EFI for sure.
I am just done putting money into this old MEFI-1 system.
If/when I go back to EFI, I'll use my newer MEFI-3 computer and harness.
I'll also have an O2 sensor bung added to the risers to double check the mixture.

I can't even imagine trying to calibrate any efi system by mail!!! Seems a lot of people around here try, and almost all turn into a nightmare. If you're going to try to modify an EFI engine, you HAVE to have the ability to monitor and change the calibration at the boat. Imagine trying to tune a carb through the mail everytime you needed to change a jet????

I've been hearing good things about this new MEFI TUNE system. I'm anxious to try it out. Some systems are easy to tune and some are clumsy, but ALL are easier to tune than a carb. The only reason people have trouble tuning an EFI is because they try to play around with too many variables at once or they just don't understand how an IC engine works. Of course this assumes the system is functioning correctly, and that's a BIG if when we're talking about electrons :)

You also have to have the right manifold to start with. Trying to make over 500hp with a manifold and throttle that are only made for 450hp max is always going to be an exercise in frustration.

I have in my stockpile of parts an AZ Speed intake, throttle body, camshaft, and other parts from a 500EFI. It has been my plan for a while to rebuild my 502MagMPI into a 500EFI clone. Getting the proper calibration for that combo should not be a problem. We’ll see if I have the time and $$ this winter. Right now, I’d like to just get the thing running OK to finish out the last 6 weeks or so.

I don't know iif it is to any help but how can a MEFI-1 get stuck in knock retard mode? If I recall correctly knock retard is controlled by the knockmodule which in a knock event delays the ignition trigger signal from the distributor IC to the ECU.
The ECU doesn't have a clue if there is knocking going on or not. That is how simple this generation of MEFI ecu's are.

My MEFI-1 ECU stopped communicating with the IC module there for the IC module takes over the timing control. The two fault codes are continuous knock and IC communication failure. What ever you want to call it the timing was retarded and the fuel mixture was way too rich…

He is not doing a custom calibration, he has a stock eng with a stock ecm cal.

efi vs carb
 

i have been also thinking about a change from efi to carb.what i want to know is how much fuel comsumption do you give up w/the carb?

Pat,
Any chance you would let me test your MEFI1 ECM (no charge)? I have a test bench for MEFI 1 and 2 controllers, with knock module and everything. I am very curious about how/why yours failed.

Dave

Sure Dave.
PM me your address, and I'll send it to ya.

For those of you that may be interested, I had another long conversation with Mark at Precision. I’m going to try a few more things using the ECU that Mark has loaned me before pulling the plug. Mark as been extremely helpful and I recommend his company to anyone in the performance boating world.
Also, I can use the existing distributor and wire harness as is with a carberator. Mark is just not sure what the advance curve is on the built in IC module.

When you either decide that you have "blown" enough money on the current system or sta around and watch everyone else go out boating...this is all you need to do. I have done this to several of my friends boats here and they all perform flawlessly now.

1. pull the MPI stuff off and toss it in the yard
2. unplug all the knock sensors, this sensor & that sensor. Dont cut them off because some of the plugs loop back. Tape them up or cover them in silicone to water tight them...toss the ECU. (Basically all you want to be connected is the hot wire running from the alternator and your temp wires to the temp senders)
3. install your intake and carb
4. pull the stock distributor out and toss it.
5. Order from Jegs or Summit ...
a. MSD-8560 distributor (install the black bushing inside the distributor and the smallest springs on the weights)
b. MSD-6460 brain box
c. Holley "RED" electric marine fuel pump


You have any questions...holler and I wil help you out...
[email protected]
817-657-5662

No need to. Very dependable and has a spark curve module in it that very good for most performance motors. If you want different timing curves, rev limiter, and stronger ignition it is very easy to modify to hook a Crane HI-6M box into.

It's been a while, but I think the advance is only about 16-20 degrees or so. Just set the total where you want it and the initial will be wherever it ends up.

The swap from the 502MPI to carb is relatively easy. Been done by many.

Thanks for all the replys guys.
I found a little time between the rain drops yesterday to change the TS and check the wiring for the 4th time.
Tomorrow I will test run again and hopefully there won't be any error codes in the ECU. I will also double check the fuel pressure under load.
If it doesn't work out, I'll likely be pulling the EFI off next weekend...

You could use a Mallory marine mag pickup distributor or the ready to run MSD and you would not need any box. Check timing a few times the first year then you won't need to touch it for 10 years.

Yup, I also like the DUI ready to run distributors.
The existing EFI distributor does have an IC module with spark/timing control so no box required either.

I ran the boat again last night after changing the TS. The TS error code came back though. The diagnostic procedure now tells me to change the MAP sender and then if that doesn't work change the ECU. I’ll change the MAP sender because it's about 3 min to do it, but???:confused:

The engine seems to run so well up to about 80% of the throttle. If I push it farther the engine stumbles.
I can hold the throttle at 80% and trim up and she'll rev from 3800 to 4200+ but as soon as I give it a little more, she dies...:(

Maybe I should change the throttle position sensor too? Hell, that would be just about everything…:rolleyes:

The fuel pressure is a solid 34lbs at all rpm...

Anyone want to buy a Merc EFI system cheap...:grinser010:

Change the Throttle Position Sensor I just did on two of my motors. One read it was opening 67% and the other motor only opening 82% After we swapped in new TPS sensors they are both reading around the 94% range. This might help

I have a friend, (BigSheesh) in the same situation about to go through the same thing due to intermittant shutdown of one engine I'll try to get him to step in here...

I changed the MAP and ran it on the hose with no change. I still have error code 14. Changing the TPS is the only sensor I have yet to change. I have another TPS, but Mark suggested running with it disconnected first. I also have another wire harness I could try but…
The season has maybe 3 weeks to go here so I guess I can debate if I go with a Carb or MEFI-3 all winter. I’m not going to bother changing it now.
Thanks for all the replies.

Well guys we changed the TPS and tried disconnecting the dash from the ignition circuit. No change, the engine still idles and runs perfectly to about 4000 rpm and then stumbles like its loosing spark or starving for fuel.

I called my marine EFI engine consultant, Mark Boos @ Precision Marine EFI.
I told him what went down last night and then asked him about what else I can try.

From the fuel delivery side-
Even though my fuel pressure looks good, it does not mean that the fuel is not being aerated by a cavitating pump or a cracked inlet hose or pipe. I need to try running the return line back to the tank directly rather than to the fuel filter block.

From the electrical side-
If I think the ECU or the injectors may be loosing power when the ignition circuit has higher load, I can wire 12VDC directly to the injectors bypassing the relay that normally powers them. The engine will probably not shut off quickly when I turn the key off, but it's a test. He has seen fuses/breakers or relays act funny as the load on the circuit increases.
I can try changing the coil too, but he thought I already did. I can't remember so I'll just change the thing again.
As for changing the ECU wiring harness, sure, but that is a much bigger pain in the azz than the rest so that is a last ditch effort.

Thanks to Mark and the rest of you for all the help.
I plan to keep at it without buying more electronic parts I don't already have in my stockpile. Hell, I've got 2-3 more weeks before the freezing weather hits...:rolleyes:

Replies and other ideas are appreciated...:cool: