Hi there, after spend several months doing research on How to fix my High Idle condition on my Explorer 1998, after doing a lot a repairs, replaced parts, it seems to me I found the problem....
I did all service required to fix any vacuum leak, new IAC, cleaning all connectors, new intake gaskets, no errors codes, no nothing just the engine sometimes gets the idle increased between 800 to 1500 RPM
then I did this...
1º Using a needle, measured sensed voltage at TPS while engine Idle and Hot. I found 1.092 Vdc
2º When the engine start increasing idle took readings again and found 1.116 Vdc !!! but the throttle plate was in its stop pin, so why the voltage is now higher???
Reading some old ford manuals (4.9L, feddback carby, 1986) I found the TPS voltage should be around 0.850 to 0.900 Vdc while idle. The old PCM ignore any voltage between 0.900 to 1.000 Vdc because the internal voltage reference regulator can supply voltages between 4.950 to 5.050Vdc depending on the PCM temperature.
It explain why sometimes the TPS and all other sensor can return minor voltage differences even registering the same engine condition, then I guessing...
Probably the PCM Internal Voltage reference regulator on my explorer was producing the voltage variation but the TPS voltage still been so high so I did...
3º Using pliers I did turn the Throttle Stop Pin back while measuring voltage at TPS signal wire and found the voltage was reduced from 1.116 to 1.060 Vdc and the Idle immediately return to normal 650 RPM !!!
4º After doing some more test and experimentation I found the PCM ignores any TPS voltage under 1.100 and the minimum TPS voltage I could obtain was 0.960Vdc after using several different TPS sensors.
It makes me think: when the PCM receives voltages over 1.100 Vdc "Think" the driver is light pressing gas pedal then it respond increasing Idle by commanding IAC to do so...!!!
5º I found also the TPS screws let a minor TPS sensor rotation also, just a few degrees but enough to increase the TPS voltage over the ignored limit of 1.100Vdc.
Problem Fixed, just adjust the TPS sensor to the lower possible voltage and if still not been enough, move slightly the Throttle Stop until the TPS voltage is reduce under 1.100 Vdc.
WARNING: Be sure the throttle plate still making stop over the Stop pin, not in the internal wall of the Throttle body or wall damage/wear will appear.