1995 Mercruiser MCM 5.7 Alpha EFI TBI S/N 0F357172

Customer brought boat to me and had been sitting for 2 years without being winterized. Removed engine, all components and reinstalled on new reman block.

Reassembled, tried to start it out of the boat, but the TBI wasn't spraying fuel. Checked fuel pressure, etc, Has good fuel pressure, but injectors not getting pulse signal from MEFI 1 computer.

Hooked up to laptop, no codes. I have checked everything I can think of and I'm suspecting the computer, but my experience has been in the past that suspecting the computer is the lazy way out, and that usually isn't the problem anyway. I had another boat here with a 5.7 setup out of a Volvo Penta, I swapped the mefi 1 computer out just to see if it would spray fuel, still no fuel. Am thinking it shouldn't care whether it is a Mercruiser or Volvo Penta

Now I'm triple guessing whether the computer is bad at all, or whether I'm missing something stupid simple. Trickled gas down throttle body and engine fires right up.


On a side note, the ignition coil is making a weird buzzing sound for about 2 or 3 seconds when key is first turned on, like it's sparking constantly without the engine even cranking over. If I'm not mistaken the computer gets the reference signal from the ignition to fire the injectors, I replaced the pickup coil, coil and sensor on ignition and it still does the same thing.


Any help would be greatly appreciated. Thanks

I'm not sure on how these work, but can you 'short' or 'jump' the fuel injector? see if it opens? Bench test it/them?

is there a way to verify it's getting power/signal?

can you mount a new carb to it, and call it a day

Already done that, the injectors are working, just not getting a signal from the computer.
I'm going to fix it, whatever it takes, but no I won't put a carb on a customer's boat.
That will just open up a whole new can of worms.

Thanks

Chesp:
Cheapest = free. Many part stores including autozone have free loaner tools.

Edit in, i misread / speed read your post.

Take a look at wiring diagram. There is a fuse for injectors. https://www.google.com/search?q=5.7+...a5mTnERVorof6M

Heres a few things to.maybe help you along the way in your testing:
1. The injectors are powered all the time with full ignition voltage so you should see 12/13 volts to ground on one side of your injector plugs when keyed up, if not, go backwards until you find out why.
2. The ecm grounds them in milliseconds as it sees rpms to fire the injectors, if you put a digital volt meter on a injector plug acrossed both leads, you should see somewhere between 1 and 3 volts as you crank it and it pulses. If you find the common wire to both injectors, it will be your power wire. If you go to battery power and one of the other two wires, you should see it make the 1 to 3 volts while cranking. This will confirm your ecm.IS firing injectors, they just don't have power
3. If you have a scan tool or mefiburn scanner pro, you should see rpms on your screen while you crank it, at least 200,300, if not, its not getting a rpm pulse from something in the distributor.
4. We have used a MSD tester/ signal generator in place of the pickup coil in distributor to pulse ignition and injectors diagnosing stuff like this without cranking the motor. You literally plug the two test leads on a 7 pin ignition module from the tester where it normally goes to the reluctor/pickup coil in distributor and turn the digital rpm to say 500. The fact you have spark though tells me thats most likely working.
i suspect the ignition 12 volts to injectors as culprit.
youll also see a injector pulse width when cranking it over, i doubt you have a bad injector driver in ecm as it would fire one of them and run terrible as they normally fire them seperatelynin batch fire, my guess would be on a tbi it would fire each injector like itbfiresceach batch on a mpi but i may be wrong, Smitty

Since you have another MEFI system on hand still?, try swapping the wire harness that runs between the ignition coil and distributor as it has a noise filter circuit type diode inline of the pink wire...If that part fails the engine "might not" start under normal conditions....ohms test the pink wire(s) in both directions as well.


Item number 17, Genuine Mercury & Mercruiser parts. DISTRIBITOR AND IGNITION COMPONENTS


PS, please bear in mind that I'd just like to generally mention that in some cases while swapping big $ item engine components like ECM's, it's considered safer to put a thought to be bad ECM type component on a known good engine to see if the engine will still start, run, etc. this way should the bad engine have something like a wire short, etc. it won't damage someone's else's good engine high $ component.


Noid light injector wire harness tester kits.


Let us know what you find, Good luck.

I will be removing this same motor from a boat in the next week or 2, but currently still completely together. If you need me to take any pictures of the harness when I disassemble to compare let me know!

Do you know what this diode/noise filter actually does? Had this discussion somewhere else and other than "you have to have it" , there was never a real answer why. The same wire off a chevy 5.7/5.0 TBI pickup truck does NOT have the diode, Ive used them on the dyno with no issues but makes me wonder if the higher constant rpms of a marine engine vs a tbi 5.7 would overload/overheat something, Smitty

Round table talk: Yeh, i don’t see why a v8 needs one.
The Merc 3.0 l 4cyl has one with EST distributor and carburetor. It is a known failure point.
But I’m pretty sure that is a wire resistor or to keep the ignition system voltage down as to not overheat at lower engine speeds. Pretty sure they use it since the module is also used in v8’s. The module controls the dwell. 4cyl’s have longer time between cyl firing then v8’s, therefore coil longer time charging and can overheat at low/lower engine speeds.

btw: again, just a guess on 4cyl’s. I dealt with one of these some years ago but did not check to see if it is an actual noise filter or a resistor or ??

Can see it on diag for 3.0 4cyl w/est diagram:

I’ll do more digging.

This is interesting. Kinda stinks timing wise as I just (within 2 weeks) winterized and put away 2x3.0 carb’d with est powered boats and 1 with a 305tbi engine with est of coarse.