94 K2500 suburban issues

[greatwhitez71]

I'll try to organize this the best I can and get the details out there without a long story,

In the early summer I bought a 1994 k2500 suburban that would be my beach rig this year.

7.4l with 4l80e, 4x4
If I recall mileage is around 250,000 or so. Has had several owners but is overall in good shape.
Went and test drove it and all checked out well, drove it 3 hrs home and never had any issues.
had it inspected and drove around town here and there until I was sure on how reliable it would be.
Changed the oil and replaced the two oil cooler lines since they showed some leaking.

Later that weekend Drove it 25 miles or so to moms hows for dinner, when I got in the truck to head home it started as usual. Part way home it started to backfire and seemed very lagging. I made an early turn off to get off the highway and it died. It did restart and I made it home, had to stay on the gas heavy.

Fast forward I changed out the Map sensor, TPI, rebuilt the throttle body, coolant sensor and AIC as well. the injectors seemed to be spraying fine.
Still no help, if anything it ran worse.

Took it to a friend from highschool that owns a mechanic shop and has alot of GM experience.

Fast forward and he has put in the correct injectors, checked the fuel pressure to make sure it isnt the pump. Installed another AIC as well as a few of the other sensors with gm parts rather than off the shelf parts.

now, its running much much better than before but occasionally and with no real pattern it starts to belch and runs horrible, it may drive great for several miles or even a day in and out then it runs bad again without warning, after a little throttling it usually lines out. He says when its doing this the injector mist is going from the fine spray that it should be to heavy droplets. Currently he doesnt have the OBD1 stuff to hook up to the truck while its running when the problem shows itself.

He says he has seen an issue like this before and they swapped out the ECM and the truck worked fine... i swung by and checked today and the ECM in the burban is already a reman.. is it possible this is my issue?

 

[mrk5]

What you describe is what I've experienced with fuel issues. The first time when the rubber line between the pump and sending unit in the tank was almost wasted away. It would only happen when the tank was low, like less than half. Was frustrating at first because the truck ran so great when I test drove it. Then all of a sudden it's running like crap. Luckily I noticed it was linked to fuel level.

The other time it ran like that was the 12hr trail run busting thru snow. I ran out of fuel. It amazed how badly it would run with almost no fuel but did keep running. Eventually it quit completely.

So I would wonder if there's something wrong with the fuel pump. Be interesting to see what fuel pressure is when you're having poor performance.

 

[greatwhitez71]

I agree. I did ask what the fuel pressure was knowing that for the 94 and 95 model 7.4 it was higher than years prior and they said it was fine. After all the sensors being replaced with good quality parts I've been researching some more. A lot of threads point to vaulty ignition module, bad grounds or like you said, pump issues. When I mentioned this to the mechanics they said those pumps either worked or they didn't, wasn't any inbetween.

I'm starting to feel like is I want to gain any progress on this thing I need to go just pick it up and stick to doing my own troubleshooting. He has it running good, it's just the intermittent problem that's eating them up.

It's nice being able to take a truck to someone you know and tell them to work on it as they can and they will keep the labor down on you, but after a few months with no solution I feel like they're dragging their feet now.

 

[greatwhitez71]

The only thing making me feel like the pump and hoses are fine is that they said when it randomly starts to act up they can rev it a bit or just keep trying to drive and it will line out and go away. And they said the injectors would bead up heavy when the issue started also (they were able to catch it happening while they had the intake off and watching the injector spray)

 

[sreidmx]

I point the finger at fuel pump slowly dying and then the distributior, I'm not sure why but it seems in my experience a bad distributor caused me a lot of headaches, the ecu gets the injector firing pulse from the distributor, they're essentially tied together.. obviously the ecu can control the timing but still one will not work without the other in this system.
I also had a bad coil act like a degrading fuel system, idle at cold was bad but cleaned up when warm and then if hot days came around boom acting like crazy again.. tried injectors nothing rebuild throttle body etc, bought all the tools to troubleshoot a fuel problem which ended up being ignition.. sure as hell didn't act like it... so maybe look there too.

As far as obd1 diagnostics, it helps but it did not point me in the right direction, the tbi system is not smart enough to know if the injector or plug fired so the feedback is minimal and really is good for finding out your rich or lean or something is no coming on properly.

 

[greatwhitez71]

if I drop the dang tank to check the fuel pump there's a good chance I'll go ahead and replace the fuel pump setup as a whole.

Same thing with the distributor... it seems like I recall the previous owner saying it had a "tune up" in that area but no telling what quality of parts were used.

 

[az_762_nato]

I've had the module act like that sometimes when hot when it starts going out, other times it just shuts down until it cools

 

[greatwhitez71]

Talked with my mechanic buddy. I hate being the guy that asks "did you check blah blah blah" but i did it anyhow, he said they had run through the grounds and that they also checked out all the ignition/distributor stuff.

The truck has a remaned ECM in it so I went ahead and located a pull out ECM online and have it headed this way to see what comes of it. Worse case scenerio I guess ill have a spare if ever needed.