Ford Repair: Idle Surging Problem, coolant temp, vacuum leak

Question
Idle surging along with voltage surging on the dashboard voltmeter (it goes up and down with the idle surge).   Replaced the IAC today.  No help.   Cleaned the MAS, IAT and coolant temp sensors.   Checked the vacumn lines and looked OK.   Surge is worst when the truck is warm.  Seems to run OK at speed.  Any ideas?


90 Ford Ranger 2.9L v6

Thanks,
Lance

Answer
voltmeter going up and down with rpm change would be normal unless you are have charging system problems-i would probably be looking for vacuum leak/dirty throttle bore or cracked air inlet first-if the check engine light is on i would have it scanned-that is a starting point but without having the vehicle here that is the best i can do-if you don't hear a hissing noise(vacuum leak) then take it somewhere to be checked out there are to many things that can cause a rolling idle.

when you check for dirty throttle bore pull air inlet tube off and have someone push accelerator so you can check back of throttle plate also. if you need to clean it use cleaner like carb cleaner and use a small wooden handle brush to scrub it-i have used old toothbrushes but it is easy to break off the head of brush and it can go in engine. good luck.