1991 Chevy Lumina Idle Problem

Lots of things can make the idle change. In your case, with all the work you've done so far, you cancelled out vacuum, iac, tps. Now test your map, o2 and iat. If all else fails, you may want to get the car scanned. It could very well be the computer itself. Lums like to breathe and are also known for the timing to go off. Considering the money it takes to go thru and test every possible thing that can cause the issue, get it scanned. That alone will better your chances of finding the culprit without spending a ton of money.
Oh, you may also want to clean the egr valve and hose.

Thanks for the info. Forgot to mention that I did clean the EGR and I have checked the MAP.I don't think it can be the O2 because it does it cold also and it is not in closed loop and the computer doesn't accept info from the sensors until close loop. It shoud be operating in cold run mode from the temp sensor (also replaced). This can happen hot or cold, and when I get it to idle correctly, I can drive the car and it is fine at idle until I turn it off for a few minutes and then the problem is there again. The vehicle has been to 2 garages and scanned and they couldn't find the problem, but I live in a small town and not sure of there knowledge. I have been a mechanic for 35 years, but sold my business 5 years ago and retired. Don't have the equipment now. Update. Still checking. When started the vehicle dies when returned to idle, but if you disconnect the TPS it will idle. I checked the voltage reference and it is 5 volts.(Still no scanner) I continued to check and have found that the IAC does not work even though it is new.I have 3 iac's and one of them is about 1/2 retracted. When I install that one it will idle but is sucking a lot of air through the throttle body(expected), but still does not move. One of the garages said that they installed anew ECM because they found that sometimes the computer was not controlling the IAC, but the new ECM made no difference. ANY SUGGESTIONS?/

Gotta love these lums. You've got a stumper. I bet we're overlooking something simple such as wiring or vac lines.
I hate to tell a natured mechanic to check his vacuum lines but as you know, the smallest crack can do weird things to this car. I replaced all of my vac lines with silicon hosing.
Then again, I also changed all my sensors, vac lines, ecm, wiring, etc etc just to cure the same issue.
218,000 miles and purrs like a rabid lion who hasnt eaten in three weeks.

Thank for the reply.I found the problem about 3 days ago. Using a DVOM I found that there was no power (sometimes) to the IAC. Traced the Green wire with black tracer back to lthe computer (no wiring diagram) could pull on the wire connector at the computer and watch the power come and go. Pulled the connector, ECM had power at the pin. With a safety pin I closed the wiring connector at the harness and made a good connection and everything is great. THANK FOR ALL YOUR HELP.

Yer not going to believe this, I got the ses light today while car was idling. Just hooked my reader up to it and guess what? Yep, code 35. So as you get yours fixed, my iac went. Hmmm, think ill check the wiring first lol.

Always retorque a new manifold after a few days to insure gasket final seating. Air in gaps will cause IAC plunger to work itself to destruction and be unable to set with a hand held adjuster eventually. That's when you know it is gone and need a new one for sure. Hope this helps