Hi all.
Im driving a 1998 Ranger. 4x4 4.0L, V6. 5spd manual. Has about 250,000 miles on it.

I'm getting a high or hanging idle. primarily when coming to a stop at a light.
The truck will hang at about 1500 before slowly dropping down to about 900. There could be a vacuum leak, but im not finding it.
I am wondering if it has anything to do with the TPS sensor. It is newer, but when started checking things out, I noticed that when installing the TPS it is slightly offset. I mean that when I line up the sensor to the part in the throttle body, I have to twist it slightly in order for the screw holes to line up.
Everything on the truck runs good as far as I can tell.
I guess it could be the throttle cable possibly. When at the stop light, and it is hanging high, I can tap the pedal and it will drop pretty quickly.




Its got a newer MAF sensor
IAC valve
brand new PCV valve
newer DPFE sensor
New Thermostat
Brand new intake gaskets and valve cover gaskets done in a shop

 

On the manual trans models the warm engine idle RPMs should drop to about 1,100-1,200 when speedometer is above 5MPH, then drop to 650-700 under 5MPH
This is an emissions requirement

I would test for vacuum leak first, its easy to test for it, not to find it, lol

After engine is fully warmed up let it idle, open the hood, pull throttle closed, just to see if its sticking, or its spring is not pulling it closed all the way
Unplug the IAC Valves 2 wire connector
IAC Valve will close all the way and RPMs should drop to 500, barely running, engine may even stall, either is GOOD, no vacuum leaks
If RPMs do not drop then there is a leak(but read about TPS below first)


TPS should send computer under 1 volt on its center wire when throttle is closed, .69-.99volts is spec
You can test this with key on engine off
Use a sewing needle to pierce the center wire and test its voltage
Under 1 volt throttle closed
4.5-4.9volts throttle Wide Open(WOT)

TPS often has slotted holes to adjust its voltage, if not you may be able to make holes slotted with a drill, you usually don't need much
TPS doesn't set idle RPM, computer just uses TPS as the "accelerator pump" same as was on a carburetor
When driver presses down on gas pedal the throttle plate opens instantly, but the air flow sensor(MAF) won't detect that "instantly" there will be a delay, so you would have an engine stumble on acceleration.
The TPS give the computer "instant" heads up to add more fuel "right now", so stumble is prevented.

The new TPS may even be holding the throttle plate open a bit, it shouldn't of course
So test TPS and maybe remove it if idle RPMs do not drop with IAC Valve unplugged, because it could be new TPS is holding throttle open, so not a vacuum leak