This forum is 2 for 2 over the past few days helping me get my old truck going, but I am finally to the issue that got the truck parked in the first place, so err we go!

The truck
89 f250 351w zf-5

The issue:
The truck has had surging idle issues that go as far back as when I was in high school around 2008.

I drove it for a few years and dealt with it. Once the truck became a farm truck, it got drove less and the problem got worse, to the point where it is now. It fires up and sticks at 1,800 rpm. And throttle input causes it to jump to around 2,200 rpm and stick there. Nothing will bring it down short of killing the truck or unplugging the IAC.

I am unable to even attempt the idle setting procedure be as it is so high

Unplugging the IAC causes the idle to immediately drop to 5-700 rpm.

I can kill the engine by covering the throttle body holes, so no vacuum leak.

I made the poor choice a couple years to just start throwing parts at it. I know it was the wrong thing to do, but I was desperate and ignorant to proper troubleshooting.
Here are all of the things I replaced. None had an impact.

Distributor and tfi module
plugs/wires
Motorcraft IAC and TPS sensor
Map sensor
Swapped computers for rebuilt. No change so returned the rebuild.
FPR
Ignition Switch
o2 sensor

The engine wiring has been swapped out for a Ron Francis. Big $$$

While or sat for the past two years, rodents got at the wiring harness, so the entire wiring harness, including in the cab has been swapped from a 1990. It blew my mind when I fired it up and the idle issue still existed.

Most of the old high idle threads suggest doing most of what I have already done. Where would you guys turn next?

 

So have you performed the KOEO self test?

 

Ahh, yes. I forgot to mention that part.
The only code I got from the KOEO test was code 18.
Loss of IDM input to processor/SPOUT circuit grounded

I checked both sides at the SPOUT connector, and my volt meter does not show signs of a short

 

This morning I went out and checked the wire running from the ECM

I have good continuity from the IAC to the ECM
OHM test shows no sign of being grounded

i wanted to do a KOER test, but it is now idling over 2500 rpm. I am not hip on leaving it idle like that while I wait for the test

Again, unplugging the IAC immediately drops rpms below 700 and the engine barely stays running.

something is causing the ECM to hold ground on the IAC instead of pulsing, but what could it be???

 

I also went ahead and checked the TPS again. Voltage looks great from closed to open throttle. .99-4.5
the 5v signal wire looks good as well

 

If there isn't a short in the wiring harness... which it sounds like there isn't, then the PCM is suspect.

 

Thanks for the reply.
I’ve tried twice now to pull the codes while running and when I turn the code reader on it ramps up to 3k rpm. I shut it off before it can pull any codes. I suppose I can go buy another ECM to swap in just to see and take it back if it doesn’t work.

 

Just to be clear about the wiring changes you made, everything associated with the engine was changed? You do not have a mix of wiring from two different trucks under the hood right?

 

that is correct. The only thing remaining from the 89 is the fuel pump and tail light wiring that runs along the frame rail. That has been modified to adapt the 90 harness to run the frame mounted high pressure pump.

 

A reman pcm from Oreillys did the trick! I know I tried swapping in a reman a couple years ago, but I have changed a few things since then.

I can finally get some momentum going with this build again!

I cannot tell you how many unresolved “high idle” threads I went through trying to solve this. For $110, a new pcm is definitely worth a try when all the normal issues have been addressed.

 

Glad you found the root cause and reported back!