Vortex

how would I start diagnosing it?

it was idling ~1,500rpm about 6-7 months ago, then went away. recently, roughly every 3rd/4th drive, when I come to stop the truck is idling high and wants to creep forward. Once, while parked, I let it idle high for a few minutes to see if it would come back down, but instead stalled. I restarted the truck, and it stalled again. I restarted the truck again, revved the motor, and now it would stay running but still idling high. I let it sit a week, and now a cold start, it runs fine.

I'm 100% sure the problem will come back the moment I take a girl on a date

so how do I fix it in advance?

background:
-2001 F150 bi-fuel (cng currently not working)
-5.4L V8
-recent oil change
-177k miles

youtube videos/threads point to different issues - vacuum leak (how do I diagonose this?), PCV valve, IAC, gap in throttle body, etc.

akdoggie

First 2 things I'd check are the PCV valve and tube behind the throttle body. These are well known to crack and give some of your symptoms. The second is the IAC motor. This controls idle speed, so it also a very likely culprit. That's where I'd start, and eliminate the possibilities one at a time.

NOTE: If it is the PCV valve, don't use any brand other than Motorcraft to replace the old one. The rubber tube can be anyone's brand, but the PCV needs to be Motorcraft. This keeps these modular motors from using excessive oil. The aftermarket brands are not part specific and can be used in numerous applications. They are not built to a specific engine family. Motorcraft number for the PCV is EV238. OEM is F75Z-6A666-BA.

Vortex

much thanks!

just ordered Motorcraft EV238 from Amazon for $14 with a new air filter...going to swap that and the hose first, then go to the IAC

10thGenScab

I had this same issue in cold weather and it was the IAC valve assembly. When it happened, I was able to take a wrench and tap on the IAC body... that would immediately free it up. You can likely take it off and clean it (check youtube for a video on that). Just be careful not to damage the gasket if you don';t have another one handy.

10thGenScab

Also, the PCV itself may or may not be ok, but the line from the PCV to the back of the throttle body does not come with the valve you ordered... i know because i just replace mine yesterday. It's that line that gets brittle and leaks.

Vortex

good info!

I found the IAC on Amazon - it's called "Motorcraft CX1867 Idle Air Control Motor"

ordered that + gasket for $86.28 shipped

going to replace one at a time...first the PCV valve...then drive the truck. If not fixed, IAC motor, then drive the truck. If not, replace the PCV hoses.

unfortunately the high idle is getting more frequent with driving, indicating that some part is failing. On the other hand, if the problem is repeatable, I can replace parts and be sure it's sorted if it all goes away after 1 part is swapped.

Vortex

Replaced the PCV with the motor craft, and still no difference.

Truck also did not start cold today. Cranked, but just stalled. Needed a foot full of gas to Rev then stay running.

Going to try IAC next.

Then replace the PCV hose
​​​​​​
replaced the air filter while I was at it

BoF97

One thing I would try since it's simple and cheap and just to be sure, normaly they cause rough idle but have you cleaned the mass airflow sensor or throttle plate? If you do make sure you get the special cleaner for them

Jbrew

Since it's a 5four, I'd give the PCV hose a couple shots of WD40. Or just go through the throttle body and hit the IAC with lub. Just to free it up..the plunger is probably sticking. Well , I'd say it is for sure from the description.

10thGenScab

While it's idling high, reach in with a tool (ratchet or something similar) and tap the IAC body a few times... not too hard, but with enough authority to jar it a little bit. If the plunger is sticking, that might free it up and will confirm the IAC replacement will cure the problem. This is the same exact condition I had back in January.