7.4 mpi injectors stay on
 

2000 2855 ciera sb 7.4 mpi
currently when key on not running driver side bank of injectors open up continually spray fuel, flooding cylinder with fuel to 3 of four injectors at all time.
all circuits to ecu have been checked for continuity and resistance, zero faults found.
have replaced with 3 sets of injectors no change in results, 2 sets of new distributor and coil assemblies no change in results, continuity and resistance test of injector harness, no faults found.
ECU was insp and service prior to new engine replacement at obd diag, out of LB, Ca. stated no faults found but got on the phone with tech/owner and my mechanic both agreed send back in for re-evaluation regarding injector circuit grounding internally.
Any one experience same symptoms, where else could i look in the interim?

I've seen ECU issues where all the gated circuits were 'on'.

So the injectors just acted like solenoid switches...just opening. The check engine light was on...but that was a sympt​​​om of being one of the 'on' circuits.

Engine would have a blower like idle hunt.
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Maybe the transistor controlling #3 injector is stuck on the 'on' position.

The MEFI 3 ECM uses batch firing. It fires 4 injectors one time then the other 4, and repeats

12V is applied to all injectors when the MPR is energized. The ECM applies a ground to J1-1 and J1-17 for each batch firing. So the ECM has to apply a ground to turn injectors ON. From what your saying my bet is that the injector driver in the ECM is bad.

This pic is for a 555 and shows the batch firing better. I can post the MEFI 3 but it does not show each injector as well and also note the wire colors are different


https://cimg9.ibsrv.net/gimg/www.off...a0d0f2f5ce.jpg

excellent info i will pass onto my mechanic and copy the company lab testing ecu for good measure, thx.

Update, Bob at OBD said driver side injector driver is damaged, aske what happen, I stated went from box to boat so no clue, went on to say inj driver circuit is grounded out, irreparable.
Spoke with owner at a place in Herndon Virginia, Maic Salazar Diag., that can actually REPAIR circuits not just test and re-flash or modify tunes and assures me it is salvageable for 1/3 cost of new.
Tech Guy in Virginia has videos on utoob of actual REPAIRS on a dead MEFI 3 marine ECU will send once get mine back from OBD2 and post results.

AllDodge can now take a bow - nice call / deductions

Sometimes circuits are grounded out when the transistors fail. They need to be removed from the circuit to test.

There is a big market market repair...many of the issues are 'the same' for certain models. Often the ECU only uses a few of the transistor drivers, while the same part is dormant in that model. Guys will pull out the good transistor and put it into the failed slot...and it's back up and running.

Just chiming in because i recently learned about my mefi 4b that even though there are two separate driving wires out to the injectors they are triggered at the same time so the batch firing is actually all 8 at once on an 8.1 mefi 4b