I have a 2002 F150 XLT 4.6 liter 8 cylinder with 94000 miles on it. For the past 6 months I have been experiencing what I call a strange idle problem. When I first start the engine, it idles as normal at approx. 1200 rpms. When I put it in gear, it drives normal up to about one mile. When I slow down and come to my first stop after starting the engine, the engine idle drops back really hard to about 600-700 rpms. The strange part is that it drops so hard that a lot of times it will stall. At that point, I simple restart the engine and drive as normal for the rest of the trip. Everything seems to run normal at that point until my next cold start. I changed the IAC a couple of years ago with an aftermarket from Advanced Auto. Other than this, I have never had any work done on this engine. I know it's due for a tune up. I am wondering if a normal tune up will solve this problem. At this point I have no check engine lights. Thank you for any help I can get.

 

I would first suspect the after market IAC.
Replace with a Ford OEM part number to be sure you have the correct calibration.
Otherwise an investigation would be in order with a Scanner looking at live data each time for something out of normal operation limits providing you know how to use the Scanner for more than just codes..
After all the IAC does control idle at all times.
It is not supposed to allow the engine to stall anytime in the idle range up to the +/- 1500 rpm range.
This includes vehicle slow downs, braking, closed throttle etc.
Good luck.

 

Before spending more money on parts, I would just disconnect the ground at the battery for a couple minutes and reconnect. Then it (computer) will have to relearn. The early 2000s rudimentary computers sometimes lose their way( go nuts) and need this reset. Also, if you are wondering if you need a tune up, it is best to put a scanner on that has the Mode 6 test 53 to see if you have some missing cylinders. These era Fords will not set a code until the thing is just about litterally shaking itself apart from a dead cylinder.

 

Thank you acogoff for your reply. This sounds like a good idea. Would a parts place such as AutoZone have a scanner to do this, or would I have to go to a garage and have them scan it?

 

Probably a garage. That said, you can buy an Autel AL519 for about $60 that will perform the Mode 6 test 53, but it takes some experience to easily interpret it's read out, but I was able to figured it out with a bit of practice and I am sure you can also.