Stupid 5.7 stumbling backfire saga

[Deepwtr]

Ok, I just can’t win and I’m running out of parts that can be changed.

Background, bought the boat 1997 VIP with freshly overhauled engine. It runs strong with one exception. It backfires and stumbles around every so often. The tachometer didn’t work so as was pointed out to me I was chasing down fuel only.

However, I bought a new tachometer and it maybe shed a little new information. Whenever it stumbles and backfires the tach is going crazy ready much higher rpm than the engine was actually going. So, it seemed like the ECU was getting bad data from the pickup coil or the ignition control module.

So, I went ahead and put all that in new. New pickup coil, new ignition control module, new rotor and new cap.

Going out today, it runs great for about 10 minutes. Then it started doing it again with the tach showing the same thing.

The thing that is stumping me on this is the tach going crazy when it stumbles around and backfires. The second it runs good the tach also shows good rpm.

Is there something I’m missing that could be causing this? I have changed the fuel filter. Stumped......

 

[Bondo]

Ayuh,..... A shot in the dark,.......

Clean all the ground connections from the battery, to the various block grounds,......

 

[Deepwtr]

Well, I forgot about something that could be at play here. My oil pressure gauge wasn’t working and I eventually found a ground wire coming out of the wiring harness stuck on the oil pressure sender.

I just taped it up out of the way. I wonder if I should run that down to the block ground? I looked around and didn’t see anywhere that it makes sense to go on the distributor itself.

Don’t know if that is my problem here or not, but I’m convinced it is supposed to go to something. I’m assuming there should be an engine block ground somewhere. For this style of engine how many grounds should be there?

 

[Deepwtr]

Ok, I did some research overnight. The first thing I always see is they show one of the screws of the ignition control module going to ground. How exactly does that happen? There isn’t a dedicated ground wire and with the gasket the post certainly doesn’t make contact with the block.

Also, the diagrams and ECU pin descriptions always show the ECU grounds to be black with white stripe wires. The pure black grounds are sensor grounds. I don’t know if that sheds any light or not.

This is is hard because the only way to make the situation happen is to actually go out to the water and run a while. Can’t recreate this in the yard that’s for sure.

 

[slyrydr]

i've had the exact same symptoms with my 4.3 efi....solution, though not perfect, does work.....take the two wires that come out of the ignition interrupter switch and bridge them together, which takes it out of the ignition equation as far as the running of the engine goes....you lose the shift interrupter for getting it out of gear, but the engine runs well....then go to work on the rigging of the shift/control cables to make them work correctly and not trigger the shift interrupter when throttle is moved into a higher rpm setting...and yes, heat plays a factor, mine would normally send the tach into a schizophrenic episode after about 30 minutes of running time, which would also cause the engine to pop/backfire...anyway, it's a temp fix to help isolate the problem to rigging or something else, but it worked for me..

 

[Deepwtr]

slyrydr, what type of ignition does your boat have? That is an interesting thought, and one easy to test. I can have something setup to take with me to the water and if it starts doing something like that then hook the jumper up and see what happens.

I’m also going to clean all my grounds, I’m going to ground my one black wire that isn’t hooked to anything. I’m going to have a block ground wire that I can run to one of the distributor screws, and have another new ICM with me.

Once I start having the issues I can test one by one and see what happens.

 

[DouglasW]

Shot in the dark here, but since you have the Alpha drive, take a look at the shirt interrupt switch. Wiggle it a bit while checking continuity with an Ohmmeter setting on a multimeter.

 

[Deepwtr]

Update, it was indeed the shift interrupt switch. I tested with the ohm meter and it was all over the place. For short notice testing I pulled it off and while depressing the button down tried to squirt some PB blaster in there. Then I worked the switch a lot and eventually turned it upside down (button pointed down) and worked it some more to get the liquid back out. It worked great on the water, ran like a champ!

I will get a new switch ordered, but at least I finally tracked it down and solved it.