It is very possible that the throttle plate is dirty and causing the throttle to hang up.
Could be you need a new fuel idle motor. Or you need new spark plugs
transmission wont shift from low to high very good
1990 gmc p/u 5.7 wont idle showing code 12
You have to change the mass flow sensor
Trany shot replace
possibly a vacuum leak, low fuel pressure, or the idle air control has failed,
Check the throttle position sensor or the idle control motor
I had a similar problem turns out the Alternator was bad.
could be a bad vacume hose. somtimes they can fall off and the engin will not run right.
sounds like pluged catalitic converter or egr valve
stopped up fuel filter or poor fuel delivery
Underneath the passenger side rear floor, under the vehicle. It is round, cylinder shape.