Let me start by saying that I know idle issues have already been discussed at length here. I’ve read all the threads and I’ve still got questions. In an attempt to fix the 3000 rpm idle at startup and hanging idle between shifts, etc., I have changed the IAC, TPS, cleaned the MAF, changed the PCV, and checked the EGR. I adjusted the throttle stop screw and got it to idle at 700 rpm with the IAC unplugged, but upon plugging the IAC in the idle went right back up even with copper cap restrictors put in the IAC inlet. The only suspect issue I can come up with are my TPS values. I am only getting .18V at idle and maxing out at 1.6V. Why would my TPS values be so far off? Even with Motorsport TPS? Gotta believe this is part of the problem. Any ideas?
^^^This is part of your problem. IF the idle is 700 RPM's with the IAC unplugged THEN there's no IAC adjustment left.
The idle needs to fall a substantial amount or the motor die outright when the IAC is unplugged. This goes double for a model year the uses the IAC without the black vent. What we are looking for is an IAC duty percent between 40-60% with the motor at idle. In your case the IAC duty % is ZERO at idle.
With regards to the TPS value. The KEY to correct TPS operation is for the TPS to produce the SAME value EACH time the throttle is closed. About the low max value my guess is this is left over from aborted attempts to "adjust" the TPS value. Restore to factory set up and focus only on the repeatability of the values. The TPS needs to move with the throttle plate through it's whole range of motion.
I agree with you that if the idle jumps up as soon as the TPS is plugged in that says that the PCM is not getting a TP-MODE=closed throttle (CT). Likely the TP-MODE is "part throttle"(PT). But consider the PCM "learns" the TPS minimum position at each start up. So if the TPS is disconnected at start up, then the PCM will not have an accurate TPS minimum value to start with.
Do you have an ODB2 scanner that can monitor operational PID's? If so that might make short work of this TPS trouble shooting. Here's some information on an affordable Windows based ODB2 scanner.
Wmburns, thanks for the reply. I had followed a popular procedure by which you use the throttle stop screw to train the computer at 700 rpms with the IAC unplugged, but there was no mention of backing off the screw after reconnecting the IAC. Not sure how much to back it off? Also, the Motorcraft TPS wasn’t altered. .18 is the signal value I get at idle with 5.00V at reference wire and .5ohm resistance on the signal ground wire. This is way off from the .75 - .99V range I am seeing on all these threads. Any help is appreciated. You seem to be the authority here. Thanks
Wmburns, thanks for the reply. I had followed a popular procedure by which you use the throttle stop screw to train the computer at 700 rpms with the IAC unplugged, but there was no mention of backing off the screw after reconnecting the IAC. Not sure how much to back it off?
The method followed would be used on an "older" Mustang that uses the TPS to measure how much the throttle is open. Not for a later model ODB2 era Mustang. The 1996+ uses the TPS only for closed throttle (CT), part throttle (P/T), or wide open throttle (WOT) determination.
Just wondering if you read the information provided in HOW the 1996+ idle strategy works? Understanding that the IAC works by "adding" bypass air it would be easy to see that IF the motor idled correctly with ZERO bypass air, that the IAC can't slow the motor below that point. Bypass air can only "add" air never "subtract".
So if the motor is idling at 700 RPM's with the IAC disconnected (zero duty cycle and zero bypass air), it's easy to see why the idle jumps up when the IAC returns to a default position at 50 percent.
Again, for the 1996+ idle strategy to stand a chance to work the idle must drop or the motor die outright when the IAC is disconnected. For any model year that uses the version of the IAC without the vent, the motor should die when the IAC is disconnected.