1994 F150 5.0 auto. 246k miles EFI. No mods except missing catalytic converter and I don’t know anything about the muffler on it.

Runs great. Crank it and it very quickly comes to life. However about 2 weeks ago when I crank it after it’s been sitting a while the idle will bounce all over the place in park. It will drop very low like it’s going to stall. The zoom up then back down. Like I’m tapping the gas.

However soon as I put it in reverse or drive idle smooths out. Smooth as it’s ever been. Accelerates like it’s supposed to idles like it’s supposed to. Then most of the time it will idle in park just fine after it warms up.

I’m really confused on this one. O and no codes or check engine light. Part of me says just crank it then pop it into drive. However I’m afraid this could be indicative of something about to break and the tuck is down.

I forget what it’s called but the “idle set” screw which I don’t think is actually an idle set screw on the throttle body was messed with a long time ago. When I got the truck I fixed the original issues and got it set to what I thought was correct. My understanding is that’s not supposed to be adjusted. It’s got locktite in place or something. I have had that screw back out and the truck won’t run. That happened recently about a quarter turn and it was running again.

Anyway I don’t have a clue where to start, any ideas?

 

DO NOT TOUCH THAT SCREW. Thats why there is a sticker that says DO NOT TOUCH IT and there is LOCKTITE on it.

STOP playing with it. You need to first clean your throttle body and IAC of carbon and make sure thats good, once it is, check air filter, if good, then you need to reset the screw per fords procedure.

 

The IAC is what broke originally and the previous owner broke the screw loose to keep it running. I will check it but it’s only about 4 years old. I’ll check the throttle body. But when the IAC broke I cleaned it. obviously it’s been 4 years. Air filter was just replaced about 3 months ago.

Thanks for the info on how to reset the “screw.” What I can’t wrap my head around is it runs perfect in gear but surges in park when cold. If it’s cold and in gear it idles perfectly. 🤷‍♂️

 

idle is different strategy than in gear brake held. the IAC is probably reacing its limits except when in gear, also any change needs to see battery power pulled to reset the learning