My F-150 has been acting up as of late. It starts and drives fine, but anytime I press the clutch or slip it into neutral, the rpms go (or stay) up, usually in the 1500 - 2000 range, instead of dropping back to normal ~1000rpm idle. This only happens when I'm moving. As soon as I stop (once I get down to about 3mph) it drops back down to normal idle speed.
Yesterday on my way home from work I stopped and unplugged the Idle Air Control valve, wondering if that was the problem. It ran perfectly after that. When I started it this morning, (IAC still unplugged) it sputtered and barely stayed running, rpms fluctuating between 500 and 1000. Plugged IAC back in, and it ran better, but the old idle problem was back. Is my IAC valve the culprit? I'd like to know for sure before I spend $60 on a new one.
Help a guy out! This has been baffling me for weeks!
Thanks!
1999 F-150 XLT 4.2L 5 speed

 

Have you checked your PCV and PCV elbow? Almost sounds like you have a vacuum leak.

 

This issue has come up many times over the years with no positive cause identified.
I just spent hours looking over the service docs and came up with a possibility.
Your hint about vehicle speed at which is clears indicates a speed sensor is involved.
Since you seem to indicate the issue was not always present suggests a worn part or adjustment.
A manual trans speed sensor control circuit is different than an automatic.
At deceleration the speed sensor is still signaling the computer the vehicle transmission output shaft is still rotating. It has no other way to know this.
The only way to stop this is with a clutch position switch.
It has 6 contacts that are operated as you push the clutch pedal in.
This switch is located on the clutch push rod to clutch master cylinder.
Check the connector seating and meter the contacts for action.
There may be wear such that contacts might not open or close as needed.
I could find no further info in the Ford Docs I have.
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I suspect this switch is not signaling the computer to shut down the IAC keeping the engine speed high until the speed sensor stops and has no output, then your idle come down.
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Now, only an analysis so could be all wet on it but sure looks like it might be the answer.
Let us know if it is, as you will have provided a lot of owners the answer.
Good luck.

 

With standard shift the PCM is probably using the IAC like a carb dashpot back in the day.
When the TPS indicates a closed throttle while the vehicle is in motion, the PCM will hold the IAC open to prevent sudden drop in intake manifold pressure (if you used to carbs, think sudden spike in intake vacuum). A sudden drop in pressure will suck all kinds of hydrocarbons out of the intake tract, as well as mass purge of crankcase fumes through the PCV system. This will result in an emissions spike.

Not sure why the OP is holding the clutch in or shifting into neutral at speed, but the PCM is trying to minimize a spike in emissions. At some point the PCM should give up and bring the rpms down via the IAC. So I suspect @bluegrass in on to something re: speed sensor issue as well as @z31feakify re: the PCV valve. A gooked up PCV valve pintal may stick within the PCV valve body and keep feeding air, bypassing the throttle body.

It would be interesting to graph both the idle speed pid and the intended idle speed pid along with the tps voltage and the OSS sensor to see what is actually happening.

 

Thanks, Bluegrass 7.
Do you have a diagram/procedure for meter testing the clutch switch? I know at least part of it is working; it won't start without the clutch pedal pressed in.
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The service Docs I have do not address this.
No wiring specific to the manual trans application, only the automatic that does not use any switching.
There may be other service Doc that do but I do not have them.
The clutch switch has a 6 terminal plug. The switch is clamped on the push rod that has a step that forces contact operation as it slides on the shaft.
As said, I only did an analysis based on what I have to work with and knowing the speed sensing is involved on Decell to signal to the PCM.
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Back years ago when owners converted Mustangs from auto the manual, they ran into the same situation of high idle because this was not addressed and very little factory info is offered about it, so I already has sensitivity to the problem.
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Another case in point is the bottom intake Plenum tuning controller operation on the 4.6 is motors is not addressed but did finally find some info about it.
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I hopefully offered enough info on how the system needs to work but your still on your own to try and see if it is of any value.

Over 16 years and 279000 miles, my truck working as a rolling lab, I learned a lot that has been presented on the boards as I had experienced them and done the diagnostics and repairs.
Good luck.

 

On an automatic, during a Decell, the IAC is closed down and fuel is cut back/off by design to both save fuel and reduce emissions.
As the engine RPM and road speed get down to a programed point, the IAC is opened and fuel turned on to recover idle so no stalling occurs. It's so smooth the driver never feels it.... unless there is a fault.
So far I have not found a code that addresses this malfunction.
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As a side point, the computer does 'house keeping' to update it's moving table values when it has the least to keep track of controlling the engine. Some of those tables are the CHT, IAT values, Long term fuel tables, memory count 'hits' such as misfires stored in mode 6, and many others.
Good luck.

 

Update: I went ahead and replaced the IAC valve, and no change. I guess that part will be returned. I also replaced the pcv hose. It wasn't leaking that I could tell, but it was pretty dry-rotted. I did notice that if I slip it out of gear while moving, it revs high, but when I press the clutch, it drops back to normal. Also tried unplugging the speed sensor, after reading Bluegrass 7's comment. Still no change.

 

How about the PCV valve?
Is it free and clear inside?

 

Turned out to be the PCV Valve. 5 bucks, 10 minutes, tops! Thanks for all the suggestions!

 

Something I would never expect to be directly related to the high idle issue precipitated with a clutch action.
Would you do the following to better verify the relationship?
Replace the original PCV valve and any original parts and see if the issue returns?
If yes, replace with your new parts and recon duct the test interval.
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I would like to do a theoretical analysis of this inter reaction if it does indeed prove out.
Reason: the clutch operation has no direct relation to the PCV valve and crank case vent operation except from pure RPM changes from a speed sensor, clutch electrical signals, and PCM reaction and changes in intake tract vacuum levels under forced deceleration/acceleration conditions..
There has been a lot of these issues reported over the years with no satisfactory explanations proven.
If you would, it will be a large step forward to answer the issue for cause.
Good luck.