2002 Ranger 4.0 4x4 160xxx well maintained.

I have the seemingly unsolvable high idle problem. Popped up after I installed the metal thermostat housing, during which I disconnected the battery. Took a couple days to really develop, I was on a road trip across a few states and it got worse and worse until it reached maturity lol.

Truckle idles around 600-750 when IAC unplugged, but when driving idle sticks wherever my RPM is at or even increases RPM. As an example, shift at 2500 rpm and motor will rev up to 3k on its own and stay there even if I coast in neutral. If I come to complete halt idle drops to 1200-1500 so still a bit high.

New IAC, cleaned throttle body, new ECT sensor, disconnected battery and waited several times, tried grounding positive cable to ensure caps bled, yes coolant is bled no air bubbles, took to shop scanned all sensors reading good but IAC unusually high duty cycle. Shop soft reset my computer and problem was fixed for about a week, then came back. I checked PCM connectors they look good and are tight. Wire brushed the PCM ground and checked resistance to battery ground, 2 ohms so good there. For now I have a block-off plate with 3/16 bypass holes which actually works pretty good but IT IS AGGRAVATING TO NOT BE ABLE TO FIND THE SOLUTION!

Since the truck idles fine when IAC unplugged and the IAC is new, I am deducing that the computer is getting bad information or generating bad IAC signal. Since the problem went away when my local shop did a soft reset of the computer, I am deducing that it is not a mechanical problem- no wires were rustled, parts loosened etc but it still got better from that soft reset for a bit. Therefore, I am thinking that the PCM itself has a problem (bad capacitor or ???) and am considering replacing it. Has anyone else tried replacing PCM to fix this legendary problem?

 

Just as a test after engine is warmed up manually open throttle and hold RPMs high then release throttle, make sure its snapping closed, make sure cruise cable isn't holding it open


Warm engine idle with IAC Valve unplugged should be under 600, barely running, there is an anti-diesel screw, looks like idle screw, on the throttle linkage
With engine warmed up and idling unplug the IAC and turn screw counter clockwise to lower idle.

I assume it was doing this BEFORE you replaced the IAC Valve, 3rd party IAC Valves were known to do this, return springs weren't strong enough to push valve closed

The PCM does have emissions software that holds RPMs for shifting but should NOT go up, also RPMs will stay around 1,100 until speedometer is under 5MPH, then it should drop to 725 warm engine

Check with Ford Dealer, give them your VIN and see if there is a software update for your symptom
I know there was for the 3.0l Rangers in early 2000's

 

I disconnected cruise control entirely, lubed throttle cables it not hanging up butterfly valve is closing entirely, yes it was happening before swapped iac, never happened before and I didnt adjust the idle screw at any point so dont think that would explain it also it idles properly with iac unplugged

I'll check with dealer tomorrow but from what I've read online they are not helpful with this issur

 

I HATE this feature.

 

its easy to override just put a block off plate underneath the iac
but it actually drives way better with this feature, shifting feels smoother on the syncros and there is less drive train clunk when you let the clutch out since the motor and tranny are pretty close to matched on speeds

 

Great feature for people who can't efficiently drive a manual, terrible for those who can.