94 stumbles and dies once warm.

[TexasRebel]

perhaps look to see if the stalling/bucking starts when it goes into closed loop.

It seems to be. I can cold start it right now and it'll be fine for the first 3-5 minute. It'll rev and take gear like normal, but when it gets close to operating temp, it'll only idle. If I give it gas very slowly, sometimes it'll act right, but most of the time if it goes above like 1200rpm, it'll buck and try to fall on its face.

 

[tsr2185]

It's not throwing any codes when I plug into the obd1 port so I'm not sure.

my father in laws s10 had a cracked exhaust manifold. It would run like S^%$ when in closed loop and O2 sensor wasn't reading right.

it also didnt throw any codes

 

[Sabinoerc]

It seems to be. I can cold start it right now and it'll be fine for the first 3-5 minute. It'll rev and take gear like normal, but when it gets close to operating temp, it'll only idle. If I give it gas very slowly, sometimes it'll act right, but most of the time if it goes above like 1200rpm, it'll buck and try to fall on its face.

did you watch the closed loop flag or assuming it’s in the timeframe where it would go into closed loop?

if it should be in closed loop and it’s not - there is some criteria not being met like not up to temp in time, the O2 sensor is slow to respond to commanded changes in trim. There are a bunch of things which I can’t remember but you can probably find in a google. If you are data logging you can sort thru data looking for that stuff.

if it gets into closed loop and then goes bonkers, there is some feedback which is screwy. It picks a cell based on things like tps, rpm, etc and adjusts trims/fuel Based on the cell map and O2, etc. I don’t understand it well but there are people who tune these things who do. Again, if you have a data log you can look what’s going crazy and narrow down things.

if you don’t have data logging capability, memory is there is a pin in the aldl connector which will flash the cel differently when in closed loop. You can at least know which case you are in.

I had a knock sensor go bad once which screwed up drivability. I don’t know if pcm looks at knock counts on cold start or only in closed loop, up to temp, etc. maybe someone here knows.

good luck - at this point, I think hard to diagnose if you cant look at the data stream.

 

[TexasRebel]

Just changed o2 sensor. Problem still persists. I can only idle it up if I do it very slowly. As in one toe just barely easing into the pedal.

 

[TexasRebel]

did you watch the closed loop flag or assuming it’s in the timeframe where it would go into closed loop?

if it should be in closed loop and it’s not - there is some criteria not being met like not up to temp in time, the O2 sensor is slow to respond to commanded changes in trim. There are a bunch of things which I can’t remember but you can probably find in a google. If you are data logging you can sort thru data looking for that stuff.

if it gets into closed loop and then goes bonkers, there is some feedback which is screwy. It picks a cell based on things like tps, rpm, etc and adjusts trims/fuel Based on the cell map and O2, etc. I don’t understand it well but there are people who tune these things who do. Again, if you have a data log you can look what’s going crazy and narrow down things.

if you don’t have data logging capability, memory is there is a pin in the aldl connector which will flash the cel differently when in closed loop. You can at least know which case you are in.

I had a knock sensor go bad once which screwed up drivability. I don’t know if pcm looks at knock counts on cold start or only in closed loop, up to temp, etc. maybe someone here knows.

good luck - at this point, I think hard to diagnose if you cant look at the data stream.

I do not have a way to data log it. I wish I did so I could actually see what it's doing. It's still not throwing any codes. Haven't replaced knock sensor yet. Just did the o2 and it didn't change anything. Only things not changed are tps, knock, and ecm. Everything else is brand new and tested good.

 

[Sabinoerc]

I do not have a way to data log it. I wish I did so I could actually see what it's doing. It's still not throwing any codes. Haven't replaced knock sensor yet. Just did the o2 and it didn't change anything. Only things not changed are tps, knock, and ecm. Everything else is brand new and tested good.

Google “gm aldl field service mode”
I did this once to see whether closed loop or no before I got set up with data logging. Read the various posts and decide for yourself.
If you are handy with a laptop TunerPro is free/donate but you have to buy a cable from someone

 

[89obsSB]

1994 GMC C1500. 5.7/4l60e

Got the truck when it wasn't running. Dropped the tank and replaced the sending unit, pump and filter. Truck fired right up, let it idle for about 20 minutes and it drove perfectly fine on the 5ish miles I drove it around the neighborhood. Parked it and went to start the next day, it ran good for a couple minutes then started bucking and trying to stall. Would die unless I gave it a little pedal. Been replacing all of the normal parts that cause these symptoms and still have the issue of it not taking any pedal or changing into any gear without sputtering out once it's warmed up. Some more insight from people that know these TBI engines better than I do would be great.

Replaced: Fuel pump, fuel sending unit, fuel filter, fuel pump relay, coolant temp sensor, coil, plugs, wires, iac, and icm. I also rebuilt throttle body and fuel pressure regulator.

If it was me I’d check fuel pressure. Tbi don’t have a schrader valve so you’d have to pull the line and put the adapter in there. But in my opinion that’s where I would check first. I know you said pump is new but that doesn’t mean a thing. I’ve gotten defective parts plenty of times.

 

[8thNote]

I had a 94 with 5.7 TBI that was doing the same thing. I replaced plugs, wires, rotor, cap, and icm. None of it helped. Then I happened to read, on some obscure and now forgotten corner of the web, that an overheating icm can cause this exact issue (runs fine when you first start it up, but bucks and dies when you give it throttle at normal operating temps). So I went to BestBuy and bought a tube of thermal paste, which is used to dissipate heat between components of high end computers. I removed my icm, coated the back side of it with the thermal paste, and reinstalled. I never had another issue again.

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[weaponoffreedom]

If it was me I’d check fuel pressure. Tbi don’t have a schrader valve so you’d have to pull the line and put the adapter in there. But in my opinion that’s where I would check first. I know you said pump is new but that doesn’t mean a thing. I’ve gotten defective parts plenty of times.

I second that, unfortunately for OP, he has replaced many thing looking for the right thing. Hard to say. And 100% new parts dont mean they are good, and even if they test out good, they still might not work. Had that happen to me on my daughters tahoe, and the knock sensors.

@TexasRebel ......

I suppose this is a long shot.....but if it seems to stumble when warm, I wonder if it is a vacuum leak. Old school way of checking this was start the engine, spray carb cleaner anywhere air could get in the engine other than the TB. Its seems it would fit your issue. Or perhaps a clogged cat. Hope you figure it out.

 

[TexasRebel]

Google “gm aldl field service mode”
I did this once to see whether closed loop or no before I got set up with data logging. Read the various posts and decide for yourself.
If you are handy with a laptop TunerPro is free/donate but you have to buy a cable from someone

Looked into the field service mode. I'll give it a try tomorrow. Took the ecm out to see if there was any corrosion, popped caps, or broken circuits but it all looks normal. Smells like it's been fairly hot a few times but that's expected down here where it's been 100+ for the past few weeks.