I have a 2000 f150 bi fuel with 75000 miles....always been a very good running truck. Recently, the idle started to lope when the heater/AC blower was running...ran fine otherwise...I thought I might have a vacuum leak, but didnt do anything about it, since when running the heater, I discovered it idled fine on certain blower settings, but not on other setting...it was pretty silly...if I forgot and it started loping at a stop light I'd reach over and switch the blower to dash only and it would sit there and idle perfectly...well...not perfectly...but acceptable...anyway, now its deteriorated to where even while driving at speed if VERY light throttle is applied to maintain speed it stutters and runs rough...but apply more throttle and the rough running disappears and it runs perfectly(relatively speaking)....so it's just at that very low throttle setting that it runs rough....idle air control valve maybe?...anyone have any ideas?...Truck still has gobs of power and runs smooth at all throttle settings except at idle or just BARELY touching the pedal at speed...a little more pedal and it smooths out and accelerates away very normally...can anyone help?....

 

Has the CEL (check engine light) come on? Are there any DTC's (diagnostic trouble codes) stored in the PCM?

Yes, it could be the IAC and it's not that difficult to clean and/or replace.