for the last 3 yrs when the out side temp gets below 50degrees my 2000 5.4f150 idles around 400 rpm in a loop cycle and some times kills .i replaced the air flow senser twice with not much results.mechanic can 't find any vacuum leaks, codes seem fine. Any body have a clue?

 

I'm no mechanic, but did he check for the leaks in the cold or a nice and warm garage?

 

Welcome to F150forum.com .. Lots of good guys and info here... Be sure to post some pics

 

Moved to the appropriate section. You should get some better responses here.