Yeah, my 87 suburban had the ignition control module slowly fail. It sucked. It was completely random when it didn't want to work. Well, if you already changed that out it's likely not the case then. Did you make certain to put plenty of dielectric grease on it? That timing wire you disconnected sounds like the next guilty culprit. Make a new one if you are so inlcined. I have seen those short to ground. The reason your truck runs when that wire is disconnected is the ECM then ignores that input and runs in failsafe mode. with it connected and if it is shorted to ground the timing module will give faulty info. Chase that wire down. It's defintely some sort of timing issue. I would not think about sensors right now. THe big clue was when you disconnected the timing wire it ran smooth. Chase the timing circuits.

 

well this is what I know at this point, there are no codes stored in the pcm, the map sensor is working fine. has 5v & gnd and puts out a signal relating to the vacuum at the port. the tps is also working as it should, putting out a signal that relates to the throttle position. the o2 sensor on the other hand, cant make heads or tails of it as the readings are jumping all over the place. not sure if it's because of the motor not running right or if it's a faulty sensor. So as I'd hate to do it I'm going to go pick up another o2 sensor and see how it does. I dont like shotgunning repairs so I test & verify before yanking parts (if I can with the tools that I have).

anyways, I'll post back once the new sensor is installed...

btw, drove it around the block in "open loop mode" and it runs pretty good.

 

Rip it out and put a carb motor in it I really hate the early gm TPI's, they are such a pita to work on.

 

If that's the case, why bother doing anything else? Why not just leave it that way?

 

poor fuel mileage

 

well the $25 o2 sensor didnt fix it either... so now jr's got some spare parts to keep. I am pretty f'n pissed at this point, there is no reason for it not to run right. MAP, TPS, O2, IAC, and CT are all working properly, no codes set and it wont idle. the only thing left are the oil pressure & knock sensors... I have never seen a knock sensor go bad and oil pressure wouldnt do this to the motor.

Fuel pressure is good, fuel injectors have to be good if it runs good in open loop mode, fuel pressure regulator also has to be good in open loop mode. the only thing left is the ECM...

 

Steve
Your story brings back some memories for me. I have a 93 Chev with the 350 TBI and went through what sounds like the same issue as you are describing. Did the Fuel filter, MAP sensor, Inginition Control Modual......all nothin'. So the last thing to go in was a brand new distributor, wires, and plugs. That fixed it for me.

now, I dont' know what open loop mode is, i'm guessing if it was the distributor, it would still run like crap in open mode. Yes? No?

Few years after that, I put a superchip in from another kids truck, same year, motor, everything, except the computer code was different. After about a week, the truck started showing the same symptoms as above. Took the chip out and all was well.

Not sure if I answered anything for you, but hopefully it helps.

 

I would think that if the distributor was the issue, it would run like crap regardless... wire connected or not. the open loop mode is where the PCM (computer) ignores all the inputs (sensors) and runs the motor a preset way. One thing learned here is if the motor ever runs like crap on him he'll at least have a way to put it in limp mode of sorts to get it home... (disconnect the timing wire).

 

When it comes to light duty trucks I am big chevy fan.

 

Well I'm sorry I missed your post.
I've been busy w/ work.
I've been playin w/ chevys for years.
I would go after the distributor and do a rebuild.
I would put in a new magnetic pickup coil and ignition module.
The biggest prob that those year chevys have are the distributor and map .
Every so often the fuel pump.
Ya already got 2 of them. So go the next step and take care of the distributor.
Also on another note chevy engines are also picky about plugs soAC Delco are the best to run.