My truck idles rough, 4.2 1997 F150.

 

Hit the IAC with a wrench while it's idling and see if it clears up. Also listen for a high pitched noise and look at your vacuum lines to see if any are disconnected or broken. How long has it been since you changed your air and fuel filters?

 

Is it missing at idle?

 

It runs clear for a few minutes then starts idleing rough, does'nt stall out, step on gas and it clears upfor a minute than idles rough again.

 

Check Engine Light at all? I have the same issue.... I replaced the IAC valve, but it didn't fix the issue...

Not to mention, on warm days, it doesn't do this at all.. I haven't replaced it yet, but next up for me is the TPS(Throttle Position Sensor).

 

Both of you should spray carb cleaner around the intake gasket. When it's warm the gaskets swell just enough to seal but when it's cold they'll cause a vacuum leak. Obviously this will only work if the truck is running rough at the time.

 

another common problem is a cracked PCV hose, causing vac leak

make sure you buy OEM Motorcraft IAC, over the years I have learned the aftermarket ones are junk

 

I will admit that I either by Motorcraft or BWD products. For the most part, I've had pretty good luck with BWD products..but that's just my experience.

As for spraying around the intake and the gaskets... i actually did that during rough idle and the RPM's didn't change at all...just continued to stay rough. That's why, after changing the IAC valve and replacing the PCV valve/unit for safe measure, i'm thinking it's the TPS on the intake.

I'm definitely open to suggestion, but in my little own process of elimination, that seems to be the next stopping point. I will say that tomorrow morning I will go out with a screw driver and tap the IAC when she starts in running rough to see if it changes as ya'll have me curious on that now

 

You can test the TPS with a voltometer:

-Closed Throttle can be anywhere between 0.6-1.0 volts
-Part Throttle is triggered @ 0.04 volts above Closed Throttle
-Full Throttle is triggered @ 2.71 volts above Closed Throttle

A smoothe increase of voltage as the throttle is opened indicates the TPS is working properly

If there is no CEL indicating TPS voltage, and the voltage increases with throttle movement, you can rule out the TPS as a problem.

 

Don't forget the MAF sensor- it tlls the computer how much ait is coming through the intake. A faulty MAF sensor may tell the computer that you are at WOT when you are idling. This could cause stumbling because of the too rich fuel mixture. If you have a scan tool you can check its performance.