i have a 91' f-150 that i have recently replaced engine and tranny.
the engine i built to stock specifications with the exception of an rv cam, and the aod tranny was built also to stock specs. for some reason my idle has been irregular. truck starts and idles perfect but after about 3 or 4 minutes when the choke kicks out, it starts a surging that lasts until you turn the truck off. the ect is new, the iac is new, no vacuum leaks, ignition timing is set to 10* btdc. this is a very frustrating issue, truck sruges all the time under 1800 rpms. ive heard rumor of a conversion to maf instead of speed density but this is an expensive task. i hope there is another alternative. the engine surges between 1000 - 300 rpms and almost kills truck. its bad enough that at an intersection the lights go dim then bright. it does this when the engine is cold and hot, all the time.
thanksfor taking the time to read this. i am completely frustrated!

 

i would have to guess the maf conversion is what you need. the speed density is not able to compensate for internal mods. you could always pick the parts up at a junk yard.

 

Recheck for vacuum leaks, perhaps listen thru a rubber hose for sucking air. If no leaks, will the engine run at all with the IAC unplugged?

 

It sure doesn't like to run with the iac unplugged. The temperature dropped to -40 degrees here. That could be part of the problem. It's supposed to warm up today. I'll try it again. Thanks so much.

 

With the IAC unplugged and the throttle stop screw adjusted properly the engine will have trouble idleing, probably around 400 RPM. This is normal and a good thing. It sounds like the IAC is having trouble doing the job its hired to do. Need to find the cause, hence looking for vacuum leaks is the first responce, not a silly one. Naturally we need to pull any codes from the computor, need a code reader. Been told Auto Zone does this free of charge, check into this and get back to us.

 

How do you ensure that the idle stop screw is where it is supposed to be? I haven't touched it but would like to double check it.