Ok, now that winter has hit the Denver area badly these last few weeks I have been trying to figure out what is going on with my idle. This problem started a year ago and has been put off until now. The truck is a 95 F150 4x4 with a 351w and a E40D trans. Now, what it was doing is when I would start it the engine would run 2500-3000rpms and then once it would warm up some or after driving it awhile it would idle at 1500-2000rpms all the time. This became a real pain as it would hit hard going into gear and is a pain at stop lights as the truck wants to go. I have pulled the IAC and the entire throttle body off and cleaned everything and that did nothing (the IAC wasn't even really dirty).
What I did was just unplug the IAC and the truck idles perfect and as soon as I plug it back in the idle shoots up. I have messed with the IAC motor and it is working like it should but what gets me is the fact that it has been unplugged for the last year and I am not getting any codes what so ever out of the computer. The computer "thinks" everything is fine with it unplugged. I have even tested it with it unplugged and I get no codes. I just have to keep my foot in it to warm it up or I throw a brick on the gas pedal to raise the idle and let it warm up. If I don't, it will die. I am wondering if the ECU is toast?
Anyone else had/have a problem like this? What cured it? Thanks.

 

you say everything iseems to be working fine. Describe what checks you have done


Jim

 

the iac has a diode incorporated into it, if it goes it'll make you nuts.

 

I vote for the IAC. I have fought these on many Fords -- they are unpredictable when they go. Sometimes no idle, sometimes high idle, sometimes it will vary. They're basically the same on so many Fords, I'd hit the junkyard and pick up a couple of them. Try a different one or two -- might get lucky and find a good one or at least know that it's the problem.

I've heard parts stores have replacements but they don't work well, Ford is your best bet if you want a new one after this.

Adam

 

Another thought, you say it idles perfect with the iac unhooked-well it shouldn't; it should idle fairly low, about 450-500rpm. Things that come to mind- some one turned up the throttle stop screw, timing is set higher than specified, or vacuum leak. Check these and let us know.

 

There is a diode around (bypassing) the coil inside the IAC to stop oscillations. This will not show up with a VOM.

There is also a diode inside the EGR solenoid.

I do not see one in the wiring of these circuits looking at the diagrams.

1995 460 IAC:
http://i54.photobucket.com/albums/g9...IACdiagram.gif

 

There's a diode in the wiring leading to the IAC on my truck, but not in the IAC itself.

 

Are you saying that you ran the KOER self-test and you did not get any codes with the AIC unplugged?
Or you saying you just ran the KOEO test?
If it was the latter then I do not think you would get a code as the only way the PCM can check the IAC is in the KOER test as it uses information from the PIP to see if the engine is responding to RPM changes.

With the engine not running and the key on do the fuel pumps run all the time?
Do you have right at 5VDC reference voltage?
If yes to the KOER, yes to the fuel pumps and no to the 5 VDC then yes you may have a bad PCM.

 

Yes, I can run the KOEO self test with the IAC unplugged and/or plugged in and it doesn't make a difference, no codes. I have measured the resistance in the IAC and my service manual says it needs to have 7-12 Ohms resistance and if it is not within that range to replace the IAC. Mine measured out at 9.5 Ohms.

Now, if I do a KOER it still won't give any codes with the IAC plugged in or unplugged. I did get a code 40 once which is a cylinder #4 failed cylinder balance test (which internally the engine is fine). I have also gotten a code 122 once and that is a TPS circuit has intermittently failed below minimum 0.6 volts. In the last few weeks of driving it every where and checking it I am getting nothing. It idles perfect with it unplugged at around 600rpms. I have checked the timing several times and it is withing spec. The fuel pumps have also been replaced in the last 20k miles. As I said, it doesn't want to idle (just idles too low and eventually dies) when it is cold until it reaches normal operating temperature (or close to), but with the IAC unplugged it can't make it idle fast for cold starts/warm-up. Out of the 4 years I have had this truck this is the first problem it has ever given me and it doesn't bother me that bad but the fact that I know its there and exists makes me want to fix it.

*EDIT* Since this has happened there has been no change in how it runs, no spittering or sputtering, runs like the day I bought it.

 

what you need to do is check to make sure the IAC is functioning properly. You need to hit it with 12v and see if it's moving. You should be able to see it move about 1/2 inch



Jim

 

OK I will ask again.
With the engine not running and the key on do the fuel pumps run all the time?
Do you have right at 5VDC reference voltage?

 

No, the fuel pumps don't run all the time except when the engine is running.

So today I went and got a new TPS sensor from the parts store for $31. Put in the new TPS sensor and plugged in the IAC and the truck runs perfect now. Starts up and has high idle of a little over 1000rpms and idles down to 600rpms after a minute or so.

Don't know why or how it didn't get a code and I even used a Snap-on scanner this last weekend and checked it for codes and nothing was there. I decided to try to TPS being I knew I had a code for it once before but not recently. I didn't think anything was wrong with the IAC.

Thanks everyone for your help.

 

[QUOTE=subford]There is a diode around (bypassing) the coil inside the IAC to stop oscillations. This will not show up with a VOM.

There is also a diode inside the EGR solenoid.

I do not see one in the wiring of these circuits looking at the diagrams.

1995 460 IAC:
http://i54.photobucket.com/albums/g91/subford/Electrical/EEC%20PCM/IACdiagram.gif[/QUOTE]
you don't see the diode in the circuit your showing me??beside the coil??

 

ford moved the diode from the harness to the iac 88-89 model years

 

[QUOTE=Ben-jamin]I do not understand this post, I said there was one around the coil of the IAC. I said I do not see one outside of it.