I installed a refurbished stock throttle body from AutoLine on the truck in my signature. New gaskets and new Motorcraft TPS. Got the unit installed night before last, and started it and drove it to warm it up this morning to check the set screw setting. Unplugged the IAC, started it up, made adjustments, plug IAC back in, and started the truck and went for a drive.

Problem. The IAC is not closing once the throttle moves, and hanging up anywhere from 1500 to 1000 rpm. I know it is the IAC hung open because if you unplug it it dies down to the mechanical setting. Ran codes, and get 111 KOEO, 111 in memory, and 311 & 411 during KOER. 311 = AIR delete. 411 was not too much of a surprise considering the problem. Tried a new Motorcraft IAC with no change. Pulled out the break out box to check TPS signal. Closed reading seems low at .8. Seems to progress normally as the throttle moves. Not sure what to look for in regard to the IAC. or if there are any other inputs I should look at. I gave up for the day at this point.

I don't really know where to go from here. It seems to me the computer is commanding the high idle and/or something is mechanically sticking. Or the computer is damaged.

I will confirm again for my own sanity that the mechanical part is not sticking. In the past I have always targeted 500/600 ish rpm for the base mechanical setting, and the computer usually seems to set it a little higher with the IAC. Is there something I should do different here? I figure the IAC can not lower the idle less than the mechanical base line, just higher.

I can try a different computer...

I can pull the TB and look for a mechanical problem...

Thoughts and suggestions appreciated.

 

Found a related thread, and will try unplugging the TPS as suggested in the thread.

https://www.ford-trucks.com/forums/1...tuck-open.html

 

Closed throttle TPS value should be between 0.6 to 1.0 VDC. Anything between those values is good.The mythical 0.99 VDC is an urban myth for these trucks. Like all EEC-IV systems the computer is constantly looking for the minimum closed throttle TPS value and adjusts accordingly. There in lies a potential problem....

If the TPS output is "noisy" the computer is going to receive false minimum values. As you drive the computer remembers that minimum value which can lead to a high idle after a short drive cycle. If you shut the truck off, key to the Off position, then restart and the idle RPM returns to normal you most likely have a bad TPS.

 

What I don't quite understand what adjustments did you make with the IAC unplugged?

 

I swapped the TPS for one that worked when it came off the truck and same symptom. It does seem to idle normally, until the first blip of the throttle and then it hangs up. I can get yet another TPS and swap again... can I bench test the units? the output seems to be fine by I am using a digital meter so I may not see "noise." should I find an analog meter to confirm bad signal?

I also tried to route the ignition wires away from the pigtail, but it is close quarters... they were touching, now they have a few inches of space.

 

Since it is a reman throttle body, I would assume that he set the base idle like you would do with a new BBK throttle body.

 

That was the idea...

 

Try shutting off the engine, then restart. If the idle returns to normal it really does point to a bad TPS.


You can use an analog meter if you have one. An oscilloscope would be better. Your symptom is very common with a bad TPS or something that is compromising the TPS signal. On my old 1992 F350 w/5.8L engine the problem was caused by green corrosion build up at the TPS connector.

 

When I check TPS output - positive probe in sensor input and negative probe in signal return yes?

I put in an aftermarket "Standard" TPS with no change in condition. I am starting to think there is a harness problem...

 

Correct

Could be that is the case here. I assume you tried shutting off the engine and restarting to see if the high idle went away. As stated before by doing this it clears the stored value for minimum TPS signal.

 

When I shut it down and restart, sometimes it hangs up immediately sometimes it behaves normally until you open the throttle and then it hangs open. If I wipe the memory then it behaves normally until you open the throttle.

I haven't found any continuity problems in the harness...

I checked the resistance of the aftermarket and the new motorcraft and it was within spec that I found. https://www.fullsizebronco.com/threa...stment.203080/

The voltage swings from .7 to 4.4 smoothly on both.

I noticed some folks and manuals show the TPS installed with the pigtail toward the driver side, and others toward the passenger side - doesn't seem that would make a difference after looking at it...

I mean at this point, I can put a different throttle body on it and see if anything changes. I can install the original computer and see if anything changes.

Feeling frustrated.

 

When it is idling high can you push on the throttle linkage and get the idle to come down? It sounds like the throttle body is binding up. At least that is my experience with a couple throttle bodies that I had.

 

found this on how to test IAC, 91-newer, has internal diode, blurry but maybe well enuff to see

 

new information!

Using a breaker box to check the TPS output one more time for ****s and giggles this morning, and when probes are inserted idle went down. Hot probe on pin 47 TPS input, cold probe on pin 46 signal return. With probes in place it behaves normally, verified with several duty cycles.

So can some one help me interpret this new info. Sounds like a harness problem, but where to look/how to isolate?

 

Perhaps the pins on the TPS connector are not making good contact or are contaminated.