Stumped re: ECM/fuel injectors

msballin

Member
May 30, 2008
259
Islip Terrace, Long Island
Boat Info
2000 340 Sundancer
Engines
Twin 7.4L Horizons w/ Hurth V-Drives
Good afternoon -

Subsequent to injectors being replaced w new, port engine presents with rough/vibration from idle to 2200 rpms then she's running like a top after engaging throttle to 34- 3600 rpms. Mechanic states system scan shows ECM is flooding one bank of injectors (3,5,2,8) with fuel as if it perceives engine is running at cruising speed. Swapped ECM to starboard engine, which is running flawlessly, and same symptoms present. Once starboard ECM is connected to port engine, all prior explained symptoms disappear. Deductive reasoning says let's send the ECM out for analysis expecting to either have it recalibrated or replaced with new. After bench test @ OBD Diagnostics in CA, no faults reported and all is fine. Mechanic (very seasoned and trustworthy...been with him for 13 yrs) is stumped and still feels it's ECM.

Any thoughts or input would be greatly appreciated. Thx in advance

Mike Ballin
 
Did you have a recent sensor fault or failure on that motor, IAC, temperature, etc? Sometimes when they fail, they fry a circuit in the Ecm.
 
Have you re-installed the ECM to see if it is magically fine now that they tested it?
They may have simply rest everything and it tests fine.

If it repeats the same symptoms then it's bad
 
Have you re-installed the ECM to see if it is magically fine now that they tested it?
They may have simply rest everything and it tests fine.

If it repeats the same symptoms then it's bad
Awaiting return of unit. However, I do concur and will purchase new. It's a $1700 gamble I'm willing to take.
 
The frustrating part of this is originally, the port engine was looked at by marina mechanic (no personal mechainics permitted on premises) for a different issue when upon initial look/see he observed a broken #7 injector which was suggested to be replaced. Upon replacement #5 was broken (I've been told if you look at them the wrong way they snap) so that one was replaced. Directly after this, the engine began to show symptoms as described in my initial post.

My question is, what could possibly cause an ECM failure as a result of potentially reinstalling injectors improperly? Thx for all of your prior responses and in advance for your continued input.
 
A bad ground can fry a ECM. If your injectors are that brittle replace them all.
 
Awaiting return of unit. However, I do concur and will purchase new. It's a $1700 gamble I'm willing to take.
My point is don't buy the ECM yet, see if it works now when it gets back.

As to the injectors yes replacing all is expensive, but so is labor. Since this is because they are brittle and physically failing If you are going to remove the fuel rails and usually the intake plenum to get to the injectors you may as well replace all.

Be sure the wiring harness for the injectors isn't just a brittle and cracking, injectors are controlled by the ground side. If one grounds out that injector will just keep dumping fuel.
 
My point is don't buy the ECM yet, see if it works now when it gets back.

As to the injectors yes replacing all is expensive, but so is labor. Since this is because they are brittle and physically failing If you are going to remove the fuel rails and usually the intake plenum to get to the injectors you may as well replace all.

Be sure the wiring harness for the injectors isn't just a brittle and cracking, injectors are controlled by the ground side. If one grounds out that injector will just keep dumping fuel.
Makes sense. As an FYI, all injectors were replaced with new and or ultrasonically cleaned ones while both engines underwent install of new OEM Merc head/valve assemblies, distributor assemblies, and full tune ups. Was not expecting this issue to reappear which was present prior to the rebuilds. It was at this time my mechanic discovered the injector firing issues while scanning for fault codes. Wiring harness appears to be fine as when swapping ECM from other engine, she ran flawlessly.
 

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