i have a 351 windsor, (1989 f150) and i've noticed when i slow down, ex. at a stop light, or put it into park, or any other gear while stopped, or while coasting, the rpms start going up and down from one almost to 0 like it's trying to die but keeps coming back. only thing that's been done to the motor that i know of is EGR delete, and that was only because the egr hose got broken off. besides that; everything seems to run smoothly.

 

Sounds like a bad idle air controller (IAC). Run a self test and see if your truck is throwing any codes.

 

That's a classic symptom of a sticking idle air control valve. Sometimes you can take the valves off, clean and lubricate it, and have it be fine. Sometimes it doesn't work, and you have to replace it.

I would definitely start by checking for codes though.

 

thanks for the answers, that is what was wrong. picked up a new one today and put it on, runs strong again.