So many new parts, yet '89 350 TBI occasionally acts up, stumbling, lurching, missing

[Dead Reckon]

Long story short, I traded a '84 Dodge D150 with a 318 / 727 auto to my cousin for his '89 GMC C1500, 350 / 700R4 auto, we where both bored of our trucks, he wanted something slower, I wanted something faster. The '89 GMC was running like complete crap, wouldn't go up a hill, I replaced the speed sensor and it was fine for a while.

About a month after replacing the speed sensor, the truck started to lurch and lunge at idle, I'd be sitting in a drive-thru and the truck acted like a wild dog on a leash trying to kill the Hyundai in front of it. This seemed to go away when I manually set the idle a bit higher, though I'm certain the idle isn't set right at all, it still occasionally does this though nowhere near as badly as when it was idling low.

Occasionally it will also randomly stumble at any speed / RPM, I've had it happen at idle, under light, moderate, and heavy acceleration. It always happens when changing the amount of throttle input, doesn't matter if it's a minor change, or flat to the floor, it'll stumble at random. I've had it happen on the highway doing 78MPH, and while idling through a parking lot so slow the speed barely registers on the speedometer. This persistent stumble / lunging behavior is quite annoying, and I'd like to solve it.

So, lemme tell you what I know about the truck, the engine was rebuilt back around 2010, and since then the following parts have been replaced (More than these, though these are the big ones I'd think)

TBI (New not remanned)
ECU (New not remanned)
Speed sensor
IAC
TPS
MAP
EGR / EGR Solenoid
Distributor (New not remanned), plugs, wires (It's due for new plugs and wires I think, about 30-40k on them)
Fuel pump + filter
Few other parts as well, I also have an aftermarket 14" air cleaner with a 3" tall air filter, the lunging happened before installing this, and hasn't been affected by it.
Also installed a 180f thermostat, the thermostat was sticking badly
Several other sensors I've forgotten the name of.

I know the O2 sensor is bad and occasionally causes the truck to run rich (Closed loop I think its called?), gonna fix that with a 3 wire O2 sensor (heated) since I'm soon to have a much more open than stock exhaust. There is some patched wiring in the truck, one of the injector wires has a crimp connector inside of the air cleaner, I think someone pinched the wire off with the air cleaner and repaired it, I'd like to replace that leg of the harness but it's probably more trouble than it's worth.

Lastly, it also occasionally has a miss to it, sometimes the truck runs amazing (Apart from a oil leak and the smell of that hitting the exhaust Y pipe), other times it runs with a slight miss when the SES light is on from the O2 sensor, other times it runs like complete crap for a while and resumes running like slight crap with the SES light on. I'm really not sure where to start here, this is a bit out of my depth, I'm use to working on carbureted engines.

 

[Hipster]

Crimp connectors can cause some issues as they add resistance to the line and can cause issues with the ecm reading proper voltages and such and don't always crimp as tightly as they should leading to some erratic issues.

I would start with the injector pigtail. Get rid of the connector and solder and shrink wrap it if there's enough to work with. If not source a pigtail. They used to be available from GM??Scabbing in extensions with multiple joints is not the way to go. Put the o2 sensor that supposed to be it back in there for now. Get it running correctly before modding anything and adding more variables to the mix. Check/clean engine, chassis, frame grounds. Look at any other patched wiring, wiggle test the wiring etc.

 

[DerekTheGreat]

What Hipster said.
Also, get a scanner and hook it up to verify the TPS is working correctly. It should put out like 0.5-0.65V at idle and 5V at full throttle if I recall correctly and there shouldn't be any voltage spikes when slowly & steadily going from idle to FF. Have the idle set correctly will help too. Our old '88 454 TBI was erratic almost like yours and much of that cleared up when we set the idle correctly. It further helped to get the TPS dialed in and swap out the 02 sensor. Oh, and check for vacuum leaks, especially between the MAP sensor and TBI unit and that it is routed correctly. Ours was not and so I put it back the way the factory intended. It still on very distant occasions had a stumble we could never trace. Would only do that like once a month so we still considered the beast tamed.

 

[Tom P]

Replace the plug wires and O2 sensor and see where that gets you. Closed loop just means that the computer is no longer in "engine warm up mode or Open Loop" so it backs off the fuel it puts to the engine. Staying in open loop when your truck is warmed up would cause issues you're describing though but is usually only caused by stuck open thermostats. Your O2 sensors have little or nothing to do with determining whether or not your engine is in Open or Closed Loop. If your plug wires are even remotely suspect, replace them. If one wire spent a little too much time next to an EGR pipe or exhaust manifold it could make cause a jacket failure and nothing will drive you more crazy than a grounding spark plug wire (I know because I fought one for a couple months.)

 

[eyesi]

if you modify the closed throttle position forget about resolve your issue, iac control relies on throttle fully closed, and the bolt that hold throttle is a stop for not letting overclose and stuck, all fixes you can try relies on that fact, also, each time you make a repair or modify in those models you need clear ecm adaptative strategy memory, by disconnecting battery negative some moment.
Desired Idle rpm relies on a well tunned engine, so not subestimate basic checks, like spark plugs condition and , very important, regulated fuel pressure check, and, AND overhead fuel pump pressure check, there you must expect 18+ psi. Regulated 9 to 13 psi.