First of all i got a 2004 f150 heritage and got a 4.2 v6 in it. I get in to my truck and start it and the rpms bounce like 500 from 600 to 1100. Then if I let it go then it goes from about 400 to 1600, until it kills on me. Also does this at stop signs, slowing to hit the brakes,any time when I slow down. Also, when I throttle down a bit when accelerating, at like 2100 or 3500 depends on how far I push the pedal down, it shudders for a little while like losing power, then keeps going up, to 3500 rpms then does it again. Also has a rough idle that sounds as if it's gonna kill. Is it going to die?

 

It sounds like the air idler, I had the same thing going on with my 4.6 but all the motors have them. I think it was like $80 at Advance Auto and I installed it myself. No problem since and runs like a champ...

Hope this helps or gives you a direction to go in.

 

Ok start there. Same symptoms? Ha

 

B4 any purchase check vacuum lines for cracks that may let air in.

 

Deal