98 2.5 with 5 speed trans. every now and then (not all the time) my truck will raise the idle by itself without me touching the throttle, only after i am cruising and take it out of 5th gear or so to slow down or stop and throw it in neutral; the rpm's will stay at the cruising rpm (2000 or so) and then start to ride, i've seen 2500 or 3000. then when i stop it all the sudden drops to normal idle rpm. just started doin this yesterday so im not familiar with all the issues yet, just a little ackward. it's acting as though your starting the truck up and getting the initial high idle, except it isn't dropping back down, and sometimes it's going higher.

where do i start with this ghost idle?

 

Possible Idle Air Controller sticking but that should continue to hold RPM even after the truck comes to a stop.

More likely, it is a problem with the Throttle Position Sensor voltage. TPS voltage to the PCM typically drops to around 1 volt with the throttle closed. If it stays above 1.27V at closed throttle, the PCM will think it is in pre-position mode and keep the IAC at too high of a duty cycle.

EDIT: corrected voltage value.

 

i can measure this with a dvom or scanner i assume? if the readings are faulty; replace tps?

 

I have the same issue with my truck and have an Idle Air Control Overspeed Error. I have replaced the IAC 3 times and has not fixed it.

 

If it is the TPS sensor having a voltage issue, what do you do about it Bob? Replace it?

 

What I would do:

• Verify that the throttle plate is closing fully.
• Measure TPS bn/wh to gy/rd with key on. Should be approximately 5V.
• Measure TPS gy/rd to chassis ground with key on. Should be approx 0V.
• Measure TPS gy/wh to gy/rd with key on. With throttle closed, should normally be approx 1V (acceptable range is 0.53~1.27V) Open and close the throttle a few times to be sure that the voltage returns to that range every time the throttle is fully closed.