Pontiac Repair: 1996 buick lesabre erratic idle, intake manifold gasket, intake plenum

Question
hey todd im kinda new to this site....but i hope you can help me, i know my way around engines pretty well, i just bought 96 lesabre ,79,000k miles, well the old man i got it from told me it sat for a year, i bought it because i saw it needed an intake plenum....so i did the upper/lower, everything is in the right places,i also put new plugs, wires,fuel filter,i broke the map sensor so i replaced it, then the iac just because it didnt look so good, and well thats about it...... the idle gets erratic sometimes after i drive it then throw it in park or neutral...like up n down but almost like it wants to stall and will sometimes stall....it even stalled in a parking lot when i let of the gas and then it shifted but then cut off.....note it starts right back up and then runs fine again......is there anything else i can check as i dont want to fork more money into it, i am married to it!

Answer
Hi Bill:

a couple of things to look for #1 since you replaced the lower intake manifold gasket and upper plenium I would start there. did you tighten the inside lower intake bolts properly the is one in the front and one in the rear that are under the upper plenium. I have seen that a few times. The next thing is the most common thing I see if you swaped over the pcv valve ther is an o-ring that get over looked ALL THE time if gues under the pcv and seals that to the upper plenium. if you forgot that thta would explain the idle issue you have. The last thing is you mentioned you replaced the IAC valve. Was it a "fits all" or did you get one at the dealer? if it's a "fits all" they may look the same but the pinel angels are not the same on all of them and that could cause your issue. How about the throttle plate and bore is that clean? Any service engie soon light on?  I'm betting it's one of those things. if it's not any of them try a hard re boot of the computer disconnect the battery cables from the battery and touchh them together for a minute it will drain all the computers memory out then reconnect the battery and turn the key to run do not crank it. wait 30 seconds turn it off wait 30 seconds off and then back to run and off again. The cmputer will cycle the IAC valve in and out and learn it's position then start the car and see what happens. Good luck :}