High Idle - Multimeter Readings - Pulling My Hair Out

[ParkerRocker]

I am stumped. I hope somebody can help me out.
1992 Explorer Sport, 4.0L, 5 Speed

When I start it cold, it idles about 2500 RPM until it warms up, then sits at about 1000. The last two days, it has been jumping randomly between 2000 and 3000 RPM, which I can't even begin to explain. When I am in neutral between gears, anything above 15 MPH, it shoots to about 3000 RPM. No CEL, unless it stutters when I'm in reverse, but that's only for a moment before it turns off. Been a while since I pulled codes, will do it again soon.

Replaced IAC & Gasket, TPS, Air Temperature Sensor and Coolant Temperature sensor. I have verified the voltages and resistance on the IAC Valve, MAF Sensor, and TPS. All are within acceptable ranges. Also checked PCV Valve, which flows freely. And cannot find a vacuum leak anywhere.

I don't really know where to go from here. I am thinking about picking up a throttle body from a junkyard because I may have cleaned mine with a corrosive compound before I knew about the coating on the TB (can't recall what I used exactly). Would that cause this issue?

Also, is the Vehicle Speed Sensor used at all to control idle? if so, how? And I did adjust my idle pin (before I knew that I shouldn't have).

 

[Roadrunner777]

There are so many issues possible here. I would replace the TBH to eliminate your cleaning and idle adjustment issues. Go from there.

 

[awood]

I had a different (but just as weird) issue with mine recently. Double and Triple check everything. Pull codes, both KOEO and KOER. Especially KOER. Post the codes, if any, and see if there's any more help here on the forum. Get a hand vacuum pump and check all the lines. Make sure the line under the throttle body is hooked up and in good condition. It's hard to see until you remove the TB. Remember, there's a vacuum line running from the vacuum tree down to the transmission. Look online to find how to read TPS and MAF voltages, and ACT and ECT resistances with a DMM. It's rare, but you could have put a bad part on.

As a last resort, consider replacing the computer. That was my fix, but remember the chances of it being something else are greater.

I don't know about the TB coating. I may stand corrected here, but I think the coating just helps prevent fouling over long term. And as Roadrunner says, fix your idle pin first.