Background...2001 F150 Lariat w/ 5.4L / AOD transmission 126K miles.

A few days ago I went on a very short drive (3 miles or so). When I put the truck in Park to shut it off the idle raced and stayed near 4000 rpm. This happened again on the way into work this morning. The rpm is fine when driving, but when I put it in park or neutral it jumps (not all the time, but more and more often) up near 4000rpm. I tapped the gas pedal hoping to get it to drop down but no luck. I'm forced to put it in gear at that rpm to get moving again. The check engine light is not lit.

Not sure if this is related, but several months back it had the problem where when I started it up it would start and then immediately shut off (idle would drop and stall). This hasn't happened in many months but I figured I'd mention it in case it helps. Thanks for any advice!

 

Good news bad news. Bad news is that it's doing it all the time now. Good news is that it's doing it all the time now. At least now I can have the dealership see the problem when I get there. Bad news is that it's $80 for them to plug it into their computer. I've heard of problems where the vehicle can shut off if the idle air control valve is bad (assuming I have one). But not sure if that can cause the current problem I'm having. Hopefully I can have it diagnosed and then just fix it myself.

If anyone has any ideas before I take it to the dealership next week, feel free to let me know!

 

Sounds like a sticking iac or a huge vacumme leak in the pcv hose

 

Thanks. I'll definitely look into those ideas. I'm going to disconnect the battery tonight (I seriously doubt it's a computer learning problem but figured I'll disconnect it overnight). Tomorrow I'll check out the two items you mentioned.

 

The battery was disconnected all weekend. I hooked it up today and the problem was still there. I decided to chance it and replace the idle air control valve without spending the $80 on the dealership checkup.

I replaced it and started it up and the idle was still racing (~2000rpm). After a few minutes it went down to 1800. I shut it off, disconnected the battery and waited 15 minutes and hooked it up and tried again (hoping that the computer had to relearn things). Idle was still 1800. I then blipped the throttle a few times and it dropped to 1200...blipped a few more times and it dropped down to normal. It was like trying to kick down a high idle setting from a carbureted choke system. Oh, and yes I did try hitting the gas a few times before the fix hoping for the same results but it had no effect on the idle.

I then disconnected the battery again (paranoia by now) and hooked it up and started it and it idled properly. I took it for a test drive and it was perfect except that around 50mph with no gas (coasting in gear) it seemed to jump between gears. I suspect that's because the tranny was forced to not shift right for the past week (the high idle caused shifting problems). Other than this slight shifting issue it seems resolved. I have the battery disconnected one last time so that when I go to work tomorrow it will hopefully relearn the shifting/idling/etc. In other words, I believe everything is fixed and the truck is running well again! $50 fix vs $80 just for a diagnosis.

ETA: I assume that the IAC is set to normal settings. When I looked at the screw adjustment inside the old one, it was VERY different from the new one. Another reason that I think it was indeed the IAC. It seems that it was bound up and some spring came loose inside the adjustment piece. I also looked around for any vacuum leak and all hoses looked fine. I have to admit that there was a slight concern that somehow I caused this problem when I yanked the dash to replace the heater core back in November, but that doesn't seem to be the case.

 

You have disconnected the battery... This will yes as you stated reset fuel learn strategy but it also reset shift strategy...... Drive it normally for a little while and see how it goes.. Also check your fluid level an make sure your fluid is good and not burnt.

 

Still not fixed...a few miles into my commute and it went back to the high idle. After ~10 miles of revving it a few times I was able to get it to drop back to normal for the remaining miles but it's obviously not fixed. And until it acts up 100% of the time again I won't be taking it to the dealer to pay for nothing.

 

check to make sure the throttle body butterfly is closing all the way.did you check the pcv hose for a large hole in it on the back of the intake

 

I did check the pcv hose but haven't checked the the throttle body. That could be the case because on the way into work it twice acted up and by revving it a few times I was able to get it to come back down. Thanks for the suggestion, I'll check it tonight.

When it's fast idling I did see that the throttle linkage was resting up against the idle screw. Can the butterfly be open even if the throttle is in the 'closed' position?

Actually, I've never replaced the pcv so I'll do that too.

 

Replaced the pcv valve and verified (best I could) that the pcv hose isn't leaking. There's foam around the hose that I can't get off unless I disconnect the entire line (didn't have the time last night). But I didn't feel any problem with it. I'll see if I can remove it this weekend and double check it.

Verified that the butterfly is directly connected to the mechanism that connects to the accelerator cable. Because it's connected and it rests on the idle screw adjustment (even during fast idle) it's not the case that the butterfly is partially opened.

Truck is definitely not acting up like it did before I replaced the IAC and when it does act up I can almost always bump the idle back down. So it's better, but still not fixed.

 

problem seems to be coming back a bit more often again. Not consistently, but more so than the past week. I'm going to try replacing the Throttle Position Sensor and see if that helps. Interestingly the past few days it's rained and that's when it just acted up. I don't know if there's a pattern yet but will keep an eye on weather conditions and see if there's a pattern.

 

I replaced the Throttle Position Sensor on Tuesday. 15 minutes to change it, 20 minutes to retrieve the small socket that fell onto the intake... in a place where my hands couldn't reach.

Anyhow, that was about 70 miles ago and so far the truck hasn't acted up ::knockingonwood::. Hopefully this solved it, but time will tell.

 

good to hear man.have to remember that.

 

Hopefully this is my last reply to this thread! :-) The truck has been driving normal. I've now logged 160 miles without a problem.

 

Wow, you diagnosed your own problem and found the fix! That sure must have made you feel good! Thanks for sharing the knoweldge and steps, I'll now be prepared if my 5.4 starts actin up in this way!