hello,im a bit stumped on this issue im having
1991 firebird auto trans. the vehicle will cold start and run fine until the engine is hot when at a redlight the vehicle will have a surging idle this happens with ac on and off sometimes will stall, will die if i restart with out adding throttle after restart vehicle will run fine in neutral or park when hot
replaced plugs ac delco
wires American motor products
cap and rotor American motor products
fuel filter ac Delco
replaced tbi injector pod with one i rebuilt from the junk yard
there is a hiss when i take off the fuel cap

 

Leave the gas cap loose, does it still do it?

 

went and replaced iac from spare tbi.did the iac procedure. Will did not stay running after 4th restart it idles but stumbling in drive is still happening and i removed gas cap during this

 

UPDATE ran engine until symptoms came back
while engine was running in park i unplugged the iac,issue went away no longer has surge.rpm sits at 700 in gear

 

That mostly means you took away the computers ability to adjust the idle. I'd systematically look for a vacuum leak for starters. Spraying carb cleaner around usually doesn't yield anything. I'd also look at ignition components carefully.

 

check your base gasket under tb