Well, I figured I'd beat this topic to death just a little more. Long story short, my '91 F-150 has had an idle "issue" for some time now. Two years ago I had it brought in to a local shop and the guy replaced the TPS and that never did solve the problem. I then replaced the IAC and that seemed to help for about a year. This past winter the high idle returned. I then replaced the IAC again and that only seemed to help a little. Problem is, it doesn't do it all the time. When the pickup is cold, it starts up fine, idles fine and so forth. But every once in a while, once the pickup is warm, I can be driving along and when I slow down the idle stays up anywhere from 1500 to 2000 RPM. It's been real hard to pinpoint what is going on. I've tested the TPS and it seemed ok.
So here is my question. Where does the IAC get its information from? The reason I ask is, when the idle goes way up, I can unplug the IAC and the idle will come down to around 500 RPM and it runs fine. My next step is to figure out where the IAC is getting the "bad" info. Hopefully that will help solve the problem.
Thanks,
JEV

 

90 F-150 351W I had a prob getting mine to idle at all.....i changed the MAP sensor then i had the high idle prob like around 3000,,, then i changed out the IAC for the one that was on my 302 b4 i got the 351, still nothing....finally changed the TPS,, and finally seems to be working alright.....are all your gaskets good?

 

Yep, I'm pretty sure the gaskets are all good. I've checked them all over for leaks and I checked all vacuum lines for leaks. As I stated before, when it starts to run at a high idle, I can unplug the IAC and she idles right down. This makes me believe it isn't a vacuum leak but a problem with whatever is controlling the IAC or something similar. I'm still stumped.......

JEV

 

Okay, the only thing controlling the IAC is input from the TPS to the computer. I'm betting you bought a cheap TPS (probably a wells brand). I have had nothing but problems with wells products, as has my father. So when it comes to that stuff we get the more expensive parts at napa. If they have 2 and one it more money get it rather than the cheap one.. You won't be sorry. If that fails then try a dealer part.

 

It makes sense that the TPS is bad. It tested out fine but like I said, the problem doesn't happen all the time. So, it could be going bad. I'll swing by Napa during my lunch break today and pick up a new one......
Thanks,
JEV

 

I've been having the exact same problem with my '95 F-150 (351 Windsor/E4OD/4WD). I've replaced the IAC, TPS, CTS, MAP and still the problem persists. Took it to my FoMoCo dealer & they put code reader on it and breakout box - said everything was fine. I think they thought the only problem was a loose nut behind the steering wheel. Of course it is an intermittent fault and did not appear for the mechanic. But it started doing it as I was driving away from the garage. I have my own code reader & in checking it multiple times, I always get 111 for KOEO, KOER and no memory codes. The computer thinks everything is fine.

Ah Ha! I've come to the conclusion that it must be either (1) the wiring harness or (2) the EEC-IV PCM itself. If it's the harness, that's it. Not worth replacing the entire harness and I doubt if I could ever chase it down to the bum wire/connection. I have just today replaced the PCM with a Ford Reman ($298 + $100 core, yikes!) PCM. Cardone & NEC no longer supply reasonably priced reman PCMs for my vehicle. On the first drive after installing the new PCM, it did just fine, no high idle. But I'll have to drive it a while before I can tell if this makes any diff.

I'll let you know. Oh by the way, mine did one other wierd thing in addition to high idle & I'm wondering if yours did same. While on steady cruise with cruise control on, the converter occasionally dropped out of lockup repeatedly. Lock/Unlock. Lock/unlock/lock. Again ornly intermittently and only rarely. Very strange. Your's do that?

 

Have you checked the gaskets at the intake and throttle body? If they are bad or those components aren't torqued to the proper specs there will be a vacuum leak, and a high idle condtion. If the TB is loose it could be sealing at times but not at others. Check your bolts before you rule it an electronics problem. The computers VERY RARELY go bad.

 

XR400 is on the right track with the wiring harness. My dad's '95 f150 4x4, 351, auto had an intermittent high idle. The problem is corrosion on some weather pack connections that are held together with a bolt/cap screw. These are located below and maybe a little to the passenger side of the brake booster/master cylinder and also a more rectangle shaped one that connects the engine bay wiring harness to a harness that comes through the cab firewall below the brake booster/master cylinder.

Dad spent many hours tracking this problem down - tryed ECM, IAC, MAP, etc, etc, etc. All you need to do is take those weather pack connectors apart, clean them with electrical contact cleaner or similar product, apply some dielectric grease and put them back together. It solved Dad's problem - hopefully it will help you out rather than throwing money away tracking down this problem.

Eric

1978 F250 Crewcab, SWB

 

I had play in the shaft that the the butterflys mount on in my 351. I would push in on my clutch and it would idle way up. Like to drove me crazy trying to figure it out. Might check and see if you have some play in yours.

 

Eric

That's great information. Corroded pinouts on the harness connectors would explain a lot. I thought of that with regard to the PCM and pulled it out to clean the pins & reinstalled prior to purchasing a reman. Made no difference at all. But it didn't occur to me to clean the other harness connectors. Duh!

Sorry I didn't connect with you & your Pop before I purchased a non-returnable Ford PCM.

Glad to hear your Pop has a '95 with 351 & 4WD. He's obviously a man of discriminating taste who wants only the best!

Thanks

XR

 

Well, I put in the new TPS. I haven't had any idle fluctuations like I did before. That's the good news. Bad news is my base idle seems to have increased. For the most part, it wants to idle around 1000RPM. When I installed the TPS I adjusted it to read .95 volts at idle. I did check and I have enough play to get it down to .7 volts at idle. I may have to pull the throttle body off and adjust the TPS down some and see if that helps.
JEV

 

Did you disconnect the battery for the EEC to reset then learn the new TPS. It's worth a shot if not.

 

Well, I tore into the old girl again last night. I adjusted the TPS so the voltage read around .74 volts at idle. I unhooked the battery for a while, then started her up. She ran at a warmup idle for a while and when she came down it was still around a 1000RPM. So I had it running and I actually pushed on the linkake to the butterflys on the throttle body to see if they were closing completely. They must be sticking just a bit because as soon as I pushed it fully closed, she idled right down. I then added a spring to help the butterflys close and all seems good again. When I get time I may have to polish up the inside of the TB and check over the butterflys to make sure there is no wear.....
JEV

 

I too have this weired idle problem on 89F150xl 4x4 302 4sp.
Idles ok at park but once I start driving it idle stays up from 1500 to 2000 rpm range without any gas. and when I come to stop idle stays at that range until I rub it up real fast and it comes down.
I've replaced the IAC, TPS, CTS, MAP and still the problem persists.
Ford dealer charged me to replace all the parts and told me that it's just how it is, but it really isn't how it was as I remember it. I gotta check out butterflys to see if they are closing like they should be.

 

If the TB butterflies arent closing all the way, you might want to check the linkage cable for any places it might get stuck. Also, lube all the works on the cable.

The IAC unpluggin idle is fine at 500RPM....that means your IAC isnt stuck open.

1500-2000RPM is not 'how it is'. At idle, it should be turning ~650-750RPM.