I have been diagnosing a demon problem with my truck not running right when it warms up. While testing the IAC I have found what I think is odd behavior.

TPS Sensor was at .92 VDC (should be .95, im sure this is good enough)
When I unplugged the IAC, there was NO change in RPM at all. To get it to run rough with the IAC unplugged I had to back the throttle screw completely out, and it still would not die when I unplug the IAC. This put the TPS at .8V. I have smoke tested the truck 3 times and the MAP showed 21-22 inHG of vacuum, I am confident there is not a vacuum leak.

IAC is supposed to have 10.5V at idle, I had 9V, sometimes as low as 8.

While revving the truck up at a standstill it would maintain 9V.
While driving it would hover around 3-5V at WOT.
While cruising on the highway it would hover around 6-7V.

When i had the throttle screw completely out (TPS at .8) I drove it for a day with no change in performace. I put the TPS back to .95 by screwing the throttle screw in to where it was and it has 12VDC at idle. After a couple revs it would move to around 9-10V.

CAn anyone help explain this or provide insight into how the IAC should behave?

Another thread with a different subject on the same problem.
https://www.ford-trucks.com/forums/1...e-bad-pcm.html

 

Bump, anyone?

 

Try another IAC valve? fairly easy to pop off at the junkyard.

What about the brake booster leaking? I don't know if a leaking one would show a symptom like this...

 

The IAC has been replaced.

Not sure if a brake booster could cause such an on/off problem like this?