Occasionally the truck seems to be on steroid and doesn't want to stop when I step on brake pedal. I wonder if anyone has experienced the throttle plate getting stuck at a partially open position? Any other possibility?

 

A defective IAC valve or a vacuum leak can also cause that type of symptom.

 

Thank you for the suggestions. I know there is a vacuum leak. Long term fuel trim has been consistently high. I will pay attention to IAC.

 

In addition to the IAC if there is a lot of carbon build up inside the throttle body the butterflies could be getting held open slightly by it.

 

Thank you. I cleaned the throttle body not too far back, but will clean it again. In the morning I start the engine, drive off, stop at a tee (100 feet from driveway). I feel "resisting stopping". Sometimes further braking cause brake locking and truck jumps.

 

Since you've cleaned it already then I doubt it's the issue. Focus your attention back on the IAC valve and any vacuum leaks.

 

Since you KNOW there is a vacuum leak, then address it first. Always fix the obvious stuff first as that often fixes stuff that is less obvious.

 

I wish I could address it. Long term fuel trim stays about 15% at idling and drops to 5% at cruising. So I know there is a vacuum leak. I tried propane gas and also home-made smoke machine, but could not locate the leak. I am guessing intake manifold gasket is leaking somewhere. I am not sure if I should go ahead and replace the gasket based on guess work. Any advice?

 

First inspect every single inch of the PCV hose, from the PCV valve to the intake. Then inspect the brake booster and it's vacuum line. Aside from intake gaskets, these would be the biggest sources for a vacuum leak and they're much easier to repair.

 

It's also possible for a dirty or defective MAF sensor to cause skewed fuel trims due to misreporting of incoming mass of the air. Clean the MAF sensor elements if you haven't already done so.

Intake gaskets and isolator bolts have been common reasons for intake leaks on the 4.2L over the years.

 

Just an FWIW - but I had similar issue with high idle and it was the TPS.

 

Certainly a possibility. One would have to look at its output voltage at closed throttle to see if its output is wrong or right.

 

Mine was intermittent at closed throttle which made it hard to diagnose. Once I had eliminated vacuum leaks (note a vacuum leak should cause the condition - in his case what I'm guessing is a surging intermittent idle - to be constant unless its also intermittent), and I had changed the IAC to a known good one from another vehicle - I had pretty much eliminated components that could cause idle issues besides the TPS.

For the OP - when your idle is high (for instance when you sense its harder braking when coming to a stop, shift it into park and see if the idle goes high. If so and its not always like this, its likely not a vacuum leak and therefore the IAC or TPS is the culprit.

If idle returns to below <<700 rpm when you disconnect the IAC connector - this tells you the IAC is returning to its closed position with no voltage applied which indicates its likely not sticking and therefore the idle is being commanded high by the TPS. You can confirm bad TPS by reconnecting the IAC and disconnecting the TPS. You should have a normal idle at that point. Or at least that's what I recall during my issues earlier this year.

I'll see if I can find my old thread on my issue and copy the link.

 

https://www.ford-trucks.com/forums/1...high-idle.html

 

Thank you much for wonderful tips. The problem got solved pretty much by itself. I retraced all vacuum hoses per ProjectSHO's advice. Disconnected, examined, and reconnected. Now Long term fuel trim has been 5% and lower for both banks @idling and cruising for 3-4 days. No jerking at stopping either. I wonder if the connector to the brake booster (or any other connector) was not sitting correctly, causing a constant small leak. The PCV hose was replaced earlier. Thank you again for encouraging me to re-test the hoses. Sorry for not reporting earlier but just wanted to make sure over several days.