Engine 2000 Mustang Idle issue

[Shift1]

So my car had a high idle issue for a while so I replaced the IAC with a new one. No codes. But when I turn on the car the same thing happens. When I lightly tap the throttle it jumps up to 1500 to 2000 for a couple secs then goes back down. The real issue is I start in first and it jumps up to 2k (I usually shift at 2k) Then When I shift to 2nd from 2k it jumps to 3500. At this point I think I need to shift again so I go to 3rd and it jumps to 4300.

 

[wmburns]

What you really need to know is what the PCM "thinks" the TP-MODE PID is. The allowed values are "closed throttle", "part throttle", or "WOT".

Or trouble shoot the TPS to measure the voltage at closed throttle. What you are looking for is IF the TPS voltage is higher when the idle is higher when this problem happens.This will require you to measure and take note of the TPS voltage value at start up and again when the high idle problem occurs.

Myself personally would go with the ODB2 scanner route.

Here's some information on how to trouble shoot.

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/