I would have to agree - if my 94 has sat for a few days I need to turn the key on and off 3 times to get the fuel pressure up to get it to start, and after doing this it starts immediately, check the fuel pump as stated above

 

If you are this serious, I am surprised you have not put a scanner on it.

As most have said, seems to be fueling if you have eliminated all electrical issues.

 

Yes, it's obvious that it's a fueling issue, but explain to me how a fuelpump, plugged cats, electrical grounds, etc only effect stumbling at low rpm when the engine warms up and goes into closed loop? seems to me, acording to the OP that they all work fine when the engine is not yet warmed up enough to go closed loop OR over 2K when in closed loop??

What you need to do my friend is to find out how the pcm gives fuel in open loop and what sensors if any are used, and then how the pcm gives fuel in closed loop ESPECIALLY DURING THE TRANSITION FROM CLOSED OR NEAR 0%TPS-- TO WOT OR 100%TPS and what sensors are used then. You might have a TPS sensor that is kinda flaky as I don't recall you saying you replaced that did you?
Hope this helps

 

Here's some good reading for you

http://www.noid.org/~lj/

On the left side towards the bottom you 'll find "PCM tutorial" click on that and during the first few paragraphs, you'll read the difference between engine operaton and fuel delivery in open loop and closed loop. You'll see what sensors are used during each. Farther down you'll see how PE mode or WOT fuel delivery is calculated and executed.
This read may help you to decide where to look for your problem.

 

If you have a laptop for $65 you can get a cable and the software is free. This way you can run a datalog and send it to someone here and see whats going on.

 

What about the MAF? If you think it may be a closed loop thing, unplug the MAF and run around in open loop and see if still stumbles. If it doesn't stumble, it may be the MAF or some other sensors screwing up. Worth a shot.

 

Thanks again. As for the responses so far:

Battery has been checked.

Fuel pressure has not been checked... seems to be adequate for heavy throttle high-RPM, so wouldn't that indicate enough pressure?

Scanner... somehow just never occurred to me. It's easy to forget the obvious stuff, that's why this message board is so helpful.

Thanks for the PCM tutorial link.

Tried running with MAF disconnected and other than raising my idle speed too high (maybe 1000 RPM), no effect.

Throttle position sensor has not been checked.



I have a laptop and I believe I even already have the right cable - it came with my Hypertech (under dash connector to RS-232-type serial). Can someone kindly point me to a link where I can get the software?

Thanks.

 

Software is TunerCat I believe, I'm not familiar with it. Running with the MAF unplugged is changing to Speed Density mode so there wouldn't be any change as your issue seems to be Open Loop/Closed Loop related.

Open loop uses a set table to provide fueling using various sensors, MAP, TPS, MAF, etc., but not including the O2 sensors. Closed loop begins using the O2 sensors to instantly add/subtract fuel based on what the O2's are reading.

 

http://www.ttspowersystems.com/

Download Datamaster for your car. Run it. First 20 runs are free. After that you have to purchase the program if you want to use it. Before you start the car turn the key forward and initiate the program with a good link. Push the gas peddle all the way to the floor to get the TPS at WOT volts reading. Start the car. Run the car at Idle/Park for a while(2 to 3 min. warm up) to get an eval on the IAC, then drive it around for a while at normal driving. Get a couple two or three good WOT runs all the way through the rpm band not necessarily all the gears if you get my drift (80mph is fast enough from a slow roll to get two well defined graphs per run). Save and send the file to someone who can help you out. If I'm not busy I'll check it out for ya.

Tunercat is a tuning based program so until you evaluate what's going on you won't need it.

 

I didn't notice what year T/A-Bob's car is. Datamaster says it is for 93, 94 & 95 F-Body cars. If it's a 96 or 97 will it still work?

 

I would still check the fuel pressure.

 

Thanks. Very much appreciated.

Regarding other posts...

My car is a 94 so I should be OK.

Fuel pressure has been mentioned enough times that I plan on testing it.