I am new to this site so bare with me guys..... I recently purchased a 1988 F250 5.0 AOD. The previous owner was an engine machinist and built motors for a living... Anyways...
When I picked up the truck (two days ago), the idle was still set very high,(as he told me he had not yet adjusted it). In my past experiences adjusting idle was very easy and just entailed adjusting the stop screw for the throttle plate on top of the throttle body.
However I tried that on this 5.0 and no change. How can I drop my idle from the current very high, approx. 1500-1700 upon start, to the normal 700-800 (once warmed up) normal idle....

Again bare with me folks, I am new to this site and and new to the ford world also...

Any help would be much appreciated.

 

That stop screw does not adjust the idle! You most likely have avacuum leak, but pull the codes to get an idea what is going on. If possible, return the screw to it's original position.

 

Idle speed is computer controlled to 650-700rpm after warmup, and that control is via the IAC valve and based on feedback from the TPS sensor.
First check that the throttle blades are actually closing and nothing is preventing them from doing so like a sticking throttle cable or linkage, or a poorly assembled throttlebody... if the throttle plates were removed for cleaning they may have been but back on incorrectly.
Then unplug the IAC and TPS one at a time while the engine is running to see if it makes any difference, if not you have a large air leak somewhere. If it does then one of those devices is bad or improperly installed.