I have 91 4l ranger longbed 5spd. I have had for many years a high idle 1500. When shifting tom rise to 2k then slowly drop. When I snap throttle tom rise then hang for second then drop to 1500. After driving it tends to drop to 1300. The Eng runs smooth and just perfect otherwise. I have verified all sensor wires to Ecu. I have changes out all sensors, I-ac motor. Verified tps. Searched for vac leaks. I attempt to choke intake by hand and it stumbles and wants to stall. Some say it?s a leaky intake gasket but I?m unable to find any leaks. I?ve use propane as sniffer, and carb cleaner to see if it gets sucked in. All no results. I?ve given up till I get another idea. Anyone?

 

Would a '91 4.0l have an IAC Valve?
Let your warm engine idle and then unplug it. The engine should stumble or even die.
If bad, replace with a Motorcraft or Hitachi ONLY.