I'm pulling my hair out a bit over this mostly annoying thing, hoping some of you with more FI 460 experience can point me in the right direction.

The problem: My truck (94 460, 5 speed, F350, stock other than 3" exhaust and K&N filter) idles at 950-1000 RPM hot or cold, all the time.

I've cleaned the heck out the IAC valve, swapped on a known good IAC valve, checked the base timing, checked the fuel pressure, scoured all the vacuum lines looking for leaks, capped the vacuum tree and every other vacuum line I could find in hopes it would uncover a leak, sprayed carb cleaner all around where the intake meets the heads, and I swapped out the coolant temp sensor for a new one today. Still the same problem. I took the throttle body off and cleaned the butterflies and the whole inside that I could get to.

The truck starts, runs and drives like a scalded dog.... it just idles at 1000 RPM's. If I unplug the IAC, it drops instantly to a nice 500 RPM's or so as if it was meant to be there. This leads me to believe that the IAC is working as it's designed, since if the IAC was stuck I would expect the idle to stay high even after unplugging the IAC, but the computer is for some reason telling it to open even when it shouldn't be. The "base idle" screw on the throttle body is backed way out, but the throttle plates don't completely close. Should they?

What do I look at next? I've checked the codes and the only code I'm getting is a 172, which corresponds to an O2 sensor, that seems to only turn the CEL on occasionally when I first drive it in the morning. Once the O2 heats up it seems to go away. Could a lazy O2 sensor cause the truck to idle high like this?

How can I test to see that the voltage I'm getting out of the ECT is proper and not low (fooling the engine into thinking it's cold all the time)? What number should I be looking for on what wire?

I saw on here or another forum a "holy grail" ford part that was a plate for under the IAC with an adjustable screw to tune the idle down... I could make something like that easily but that seems like a hack.... I want to understand why the computer is telling the IAC to stay open far enough to make the truck idle at 1000 instead of 750 or some other reasonable number? Anyone have any insight/thoughts?

Oh yeah, and I love this smiley:

 

Bump... any got any wisdom here? Just for reference since I realized I didn't include it in the original post.... the timing is set to 12 degrees with the SPOUT disconnected, and the fuel pressure is 38 or so at idle and jumps up to 45'ish when I rev it.

I also ordered Charles Probst book on EEC-IV fuel injection. Probably good to have, maybe it has some wisdom on this problem.

 

I'd start with replacing the O2 sensor and verify that the TPS sensor voltage is below 1 volt at idle

 

Might be unrelated, but Mine did the same, and after doing the headers and exhaust with a new O2 sensor, it idles down now. Battery was also disconnected at that time, so lots of changes at once.

 

Interesting.... I will get a new O2 sensor in tomorrow if the parts store has one in stock. How long should the battery be disconnected to get the computer to "reset"?

 

A bit off-topic, but what headers are you running. Do they have an EGR port, and do they hook to the stock B-pipe or did you do something different there? I've got no cat, and 3" all the way back from the B-pipe, contemplating whether a set of headers would be worth it or not.

BTW, is this Pat from NW-Wheelers... with the BII and the Mustang?

 

5-10 minutes should do it.

 

Yes, Thorley's and I put it in my thread there in the tow rig section. Yes, they are worth it.

Thought the name was familiar.

 

Alright, went ahead and unhooked the battery right when I got home at 6:00 this evening. Put in a new Bosch O2 sensor, hooked battery back up and it fired right up and idled up to 1000 or so and stayed there... never dropped.

The base idle screw is now set so that it just Barely holds the plates right at the stop. If I let the truck warm up completely, and unplug the IAC it idles maybe just slightly lower than 500.

I poked at the TPS sensor harness. I'm getting 5.05 volts on the top wire, and 1.15 volts key on, engine off, and the same with the engine running at "idle" on the middle wire.

 

1.15 volts is right on the limit of being acceptable. I would try to loosen the TPS screws and see if you can wiggle it a bit to get the voltage down.

 

Since the TPS is non-adjustable, how do I get the voltage below 1 volt? I've read elsewhere that anything in the range of 0.49 to 1.19 volts is acceptable for the TPS output, and the computer uses that value as "0" when the key is first turned to ON... thus the non-adjustable TPS.

I swept the throttle with engine off key on and get up to about 4.7 volts at WOT.

I loosened the TPS screws and rotated it a bit and was able to get it down to 1.11 volts. I tweaked the base idle screw a bit just to see if it was in too far, it's not, so I put it back where it was.... voltage goes no lower than 1.11 even if I back it out further.

Same high 1000 rpm idle.

 

While your multimeter measured 1.15 volts, it's likely more accurate and precise than the simple system the computer uses to measure voltage. As the computer ages, it's quite probable that it really isn't measuring quite accurately anymore.

Do you get cold start-up high idle (~1200 RPM or so?), or does it go right to 1000 RPM no matter what?

For kicks, try unplugging the TPS sensor with it running, and see if the idle comes down.

 

I'll see what happens with unplugging the TPS tonight. It doesn't cold high idle at all, just goes straight to 1000 and sticks there basically.

 

Unhooked the TPS while it was running. Idle shot up to 1200 and CEL came on.

 

Was the engine cold, or warm at this point?

Looking through the tuning software I have for a '95 F-150 5.0 Computer, Ford has multiple functions inside the computer that can that can influence idle speed:

Startup: Adds extra RPM at startup until engine has been running for 60 seconds
ACT: Adds RPM based on air charge temperature
ECT: Adds RPM based on engine coolant temp (cold start high idle, etc...)
Voltage: If battery voltage is low, the computer can increase idle speed by 100 RPM or so.

Is your alternator putting out good power at idle?