7.4 mpi timing issues
 

Hi , this one is for you EFI gurus Im involved with a 7.4 MPI gen VI that was initially reported to be running fine. the other inboard was removed due to a mechanical failure and my buddy thought it would be a good time to freshen up both After the rebuild the "good" motor only lasted a few hours before detonation hurt a piston, this is where I come in. It was clearly suffering from massive blowbye . I pulled the motor and sent it back to the same machine shop , they put a piston in it, ran it on the dyno with a carb and mechanical distributor and it made 315hp I suggested to put MPI and ecm on and rerun it. It would only make 275 hp Swapped ECM from other motor,same output. Hooked up my scanner with engine running ,it confirmed that timing was advancing but not enough (both ECMs) double checked with a timing light.
no codes,new distributor module, fresh gas,fuel press at spec Moving the distributor will make more HP but base timing will be way off and Im wondering if the first mechanic "bummped" the timing to make it run correctly??? any suggestions, tests I missed? Thanks for listening, any help is welcomed , cort

Are you running the Merc. MPI system ? If so you have to put the computer in base timing mode to set timing you need a Rinda tool to put the computer in base timing. Regards Dan

thanks Dan, Yes using the Rinda and base timing mode. Just cant get enough timing from the ECM....and had a few sets of eyes verify lol

What signs are there for blow by?
If it was only one piston hurt and the others looked fine then its one fuel injector. Loosing one injector means you lost 1/8 total hp and if others are having some issues then more loss.
Have the injectors been cleaned and flow tested?
How is the fuel pressure?

As an aside what is the total timing your are seeing under load?

and at what rpm?

What is the base timing set to?

you did put the scan tool in service mode ?? for setting and checking the timing Im just checking

What year / model is the 7.4L ? ... The L29 design intake plenum or the 454/502 design intake plenum/manifold....Sort of thinking it may also be an air intake design difference between a "good carb setup" vs an EFI plenum type used for HP ratings shown...,
https://www.perfprotech.com/blog/boa...marine-engines

yes ,fuel pressure was 42 psi , known good set of injectors swapped into manifold , the timing wont advance enough to make any real hp, I am starting to wonder if my buddy mechanic advanced the distributor to try and get the boat to run? right now it wont make enough power to hurt itself on the dyno very puzzling?

yes sir with my rinda scanner

yes sir, My friend 14apache said you were the man to talk to lol

Yes sir I used my rinda scanner

about 28 deg at 2700 then the timing starts to peel off down to 19deg as RPMs increase

Is this a MEFI 3 ......is the knock sensor connected ?.......seems the ECM thinks there's a problem and is pulling timing out

I'm thinking you need to pull the motor apart and find out what was not put together correctly. Even a carb motor with 088 heads should make more then 315hp with fresh rebuild

I did , scanner matched timing light on the balancer . timing starts to increase ,then just peels off as rpms increase I swapped out the knock sensor, but im wondering if i should check voltage on the ECM pertaing to knock/advance? I believe its J1-1? waiting for the manual I ordered to show up to verify I appreciate the responses!

With the MEFI3 (EDIT once its running), you can just remove the knock sensor wire off the sensor and move it out of the way. If power increases, the sensor is picking up a knock and retarding timing. If it does increase power after wire is removed "SHUT IT DOWN" because your destroying the motor

I think im going to find the ECM input that controlls knock next. TWO ecms doing the very same thing...starting to run out of ideas lol

agreed , ran some good fuel on the dyno and did exactly that...even with sensor wire disconnected the knock module is still in the circuit, no?

Id pull the front cover and check the timing marks, pull the distributor, remark #1 and restab the dist .............I dont think its an ECM issue, the engine isn't happy

FYI, here is the timing chart vs manifold pressure for 454 mpi mag. Not a lot of timing.



https://cimg1.ibsrv.net/gimg/www.off...802666323d.jpg

That is great thank you ! I will be dynoing this week Looking at inputs at ECM that control timing I will report back thanks again to all

Try another Ignition Module. A known good one if possible!

Did that , same timing curve