troc

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

Team Archer

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

troc

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

AllDodge

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?

Trash

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

and at what rpm?

What is the base timing set to?

BUP

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

lakematdude

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

MDG_Jason

troc

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?

troc

yes sir with my rinda scanner