Purchased a 96 4.0 4wd for my daughter, and there is an issue with it dying at idle.
Threw codes p1504 and 1507 indicating an issue with the IAC.
After visual inspection, I determined that the IAC was disconnected from it's electrical supply ..... when I connected it the engine revved to almost 4000. I disconnected, and will remove the valve and clean it, and replace if neccessary.
MY QUESTION: Would the high revving be due to the previous owner manually increasing the throttle to hold idle? and Have any experts seen this symptom and am I on the correct path to repair?

 

I know you all have been waiting with bated breath to see this thread continue.....
Purchased a new IAC sensor and hooked it up, started the truck and it revved to about 3000 RPM and then slowly backed off to a fluctuating idle ranging 500 to 1000 RPMs in a steady pattern of about every second or so.
I disconnected the TPC sensor and the fluctuating was aleved, but not completely.
I returned the New IAC which was not altering the symptoms at all, and exchanged for a new TPC, which also had no improvement to the problem.

When both sensors are connected the high revving backs to a fluctuant one, and if you tap the accelerator pedal just enough to move the throttle, it revvs back up to 3000 RPM and then backs down steadily to the fluctuation idle.

As of now the IAC is disconnected the new TPC is connected and the idle is steady at 800rpm, which i increased with the throttle screw to avoid stalls which were the issue that began this project. Occasionally the car would stall when first started, but seemed to just sputter a bit at idle once warm.

Other codes being thrown are p0153 (slow O2 sensor), p0420 which I hope is triggered by this, as it points to the CatConv. and a 756,732 transmission codes which indicate a solenoid and I'm thinking the "valve gasket" issue.

Not sure if these other codes effect the 1504,1507 codes, but I felt to include the information just in case.


Other possibilities....
Leaky valve cover gasket? I will spray around to listen for rpm surge next time the truck is available to me.
Leaky vacuum?.... couldn't find any.... always looking and listening.
Dirty throttle body... will clean next time as well, but it didn't appear to stick, and returned fully.
Bad PCM? doubted at this point, but I don't know a checking method.

I know that you can test the voltage or resistance on these sensors... Can this be done with a multi-meter, if so how?

MAF sensor?

all help appreciated.