'94 S-10 4.3 Stumbles on acceleration, rough running

[old_master]

Using the pump to drain it will take FOREVER. The return fuel line directs unused fuel into a well that the fuel pump motor is mounted in. It's part of the fuel pump module, and the circulated fuel helps keep the motor cool. The fuel pump picks up fuel from inside the well. The pump will pump the fuel until the well runs dry, then you have to wait until it seeps back in to fill the well. If you've ever removed one of these pumps, if you tip it after you remove the module, it dumps fuel all over, that's the fuel that was in the well.

I'd say you're on the right track with diesel fuel causing the problem.... probably plugged up the injectors.

[old_master]

94 & 96 ignition modules are not interchangeable.

[bigj-dog]

Ok, that's what I was wondering about the pump. I have just ran the pump to empty the tank, smacking it with a hammer to restart the pump after it seizes, when I've changed the failing pumps in similar trucks before. It didn't seem that they emptied that slowly, but I guess I never paid close attention either. I'll just drop the tank the way it is and use my regular pump to empty it then. How about those injectors? Will they clear themselves, or do I need to do something else?

And you're right about the ign modules, I thought about it some more, I took it out of a '89 K1500 5.7. My bad.

[old_master]

Another thing with the pump, you need to "hot wire" it to keep it running, which poses a problem with cooling it. The PCM will shut it off if it, (the PCM) doesn't receive an ignition pulse.

The injector might clear out by itself, if it will pass gasoline through it. If it's totally plugged, probably not. The 94 "W" engine only has one injector, but it has 6 poppet valves, and those are most likely what's plugging up. Same deal with them, as long as they're passing even a little bit of gasoline, they might clear out.

89 & 94 ignition modules ARE interchangeable

[bigj-dog]

Great info. I appreciate it. When it did run, it seemed to run ok, it just wouldn't speed up/accelerate under any power smoothly. And now it will run, it just has to be revved up to 2k or so, and as it drops to idle, it'll try to die (and succeed most of the time), so it can't be all the way plugged, but have the ambient temp about 0 probably doesn't help the injector pass diesel.
I ususally either hot wire the fuel pump relay, or find the test lead, but in this case, i think I'll just drop the tank to save time. I'll update when she's running, hopefully anyway.

[old_master]

Yup, diesel fuel tends to get kinda thick at zero You just might get lucky on this one.

[bigj-dog]

My buddy might get lucky. I'm already unlucky for chasing around demons without knowing that someone poured hardly refined oil into a gas tank.

[old_master]

Any idea how much they put in there?

[bigj-dog]

Supposedly 3 to 4 gallons. And I guess my friend just kept filling it up whenever he could to try and dilute the diesel. It did run fine for the most part for a couple weeks, just kinda intermittenly acting up, but it's gotten progressively worse now.

[old_master]

Probably be a good idea to replace the fuel filter after you get the diesel fuel out of the tank.

[bigj-dog]

Finally got the fuel drained, it didn't look horrible, or smell too much like diesel. Installed a new fuel filter, and primed the pump and it started right up (it was 25 this morning though, not -10 like last time). It idled perfectly for 10 or fifteen minutes, so I backed it down and into a parking space, barely touching the throttle, and everything went fine, until.... I put it in park and then decided to rev it up a little bit to see if it'd miss/stumble then. Which it did, and then killed. It barely accelerated, almost immediately it died when I gave it more throttle. I got it to start a couple more times, but only ran for a couple seconds. I wandered off for a few minutes and tried to start it again and then it wouldn't even fire. I know it has fuel pressure, and I can hear the pump cycle for a second or so before I try to start it each time. Previously I had checked spark when it wouldn't start and it always had that too. I know when I get home it'll fire right back up and idle fine. Any of that ring any new bells? I'm wondering if there's something wrong with the injector/spider assembly? I know these are problematic, but I'd like to be able to prove the problem before I spend more of his money on hope.

[old_master]

Check fuel pressure and leakdown. 92 to 94 CMFI systems: Key ON, engine OFF, fuel pump running: pressure must be 55psi to 61psi and must remain above 50psi for 3 to 5 minutes after the fuel pump shuts off.

[Rick Norwood]

Could the injectors be partially plugged? Open enough to idle but not enough to run?

[viggy58]

have you changed the plugs? they could have been fouled out. also, make sure you are getting good spark. almost sounds like a cracked distributor, which allows moisture in weakening the spark.

I dunno how easy it is to get the spider out on a 94, but if it is easy enough, i'd pull it, maybe replace the regulator, and give the poppets a good spray down with carb cleaner.

i drive a diesel for work regularly, and i know that when diesel gels, it clumps up. for example last winter, my fuel gelled, but enough was able to pass for the truck to idle, however under load it stumbled and stalled.

[bigj-dog]

I can't believe I forgot to finish this thread when I found the problem. It ended up being the electrical connector on the fuel injector. Unbeknownst to me, one of the tabs was broken off prior to me looking at it, and the heat cycles made it brittle, so it'd lose contact sometimes. I was able to buy a replacement pigtail and soldered it in, now it's been fine. What a waste of time. At least there's a lot of good information in this thread for other people with problematic 4.3s.