I have a 2003 F-150, 2 wd 6 cylinder, runs great, starts great. But when idling in gear with foot on brake, it will drop rpms from 700 down to 500 for just a couple seconds, causing the whole truck to tremble. Anyone got any hints i can give my mech, its been in the shop 5 times. thanks for any help forthcoming.

 

I'm surprised you don't have a check engine light on. The idle is supposed to be @ 500 to 700 rpm in that situation. It sound like you have a misfire or a major vacuum leak which is most common on the V6. The upper intake manifold has gasket leaks or vacuum lines under it that rot out. Try carb cleaner at the heads to eliminate that as the gasket leak. Also the PCV valve can be the source of the leak.

 

Thanks for the tips, it did have check engine light on it is currently not on but like i said its been back to the same shop 5 times since June. It has 257,500 miles on it, still runs 80 mph with no strain thanks again for the tip

 

Welcome to the forum.

Tony

 

Typical symptoms of a vacuum leak. With all those miles you got, it would be a good idea to thoroughly check all vacuum lines for dry rot & cracking. The plenum bolts are also known to cause air leaks too. The rubber seals around them deteriorate & leak. I'll bet the codes you're getting are 0171/0174. (lean cond.)

And, welcome to the forum.