5.4 F150 rough idle
 

5.4 F150 rough idle
 

I have a 1999 F150 with a 5.4 engine. My question is does anyone else have a rough idle when in drive with foot on brake? This is a very noticable mis at all stop signs and red lights.This problem existed when I picked my truck up new from dealer. I have had it in twise for repairs but Ford cannot smooth out this idle problem. Any suggestions for this problem????

5.4 F150 rough idle
 

Does the problem only occur when your foot is on the brake? If you have cruise control and there is a vacuum dump valve (like on my '91 and '95 Fords), then the vacuum dump valve may be your problem. The vacuum dump valve vents vacuum to the cruise control servo when you press on the brake to cancel the cruise control. If it is stuck open when the brake pedal is pressed, it will be like a vacuum leak and possibly make the truck run lean. It would be easy to check by pulling the vacuum line to the dump valve and plugging it, then see if the problem is there. Just a thought, don't know how likely that this is your problem.

5.4 F150 rough idle
 

I have a '97 F150 with a 4.6 and it does the exact same thing...rough idle (intermittent) when slowing down at lights/stops. It doesn't appear to be a tranny problem, at least with mine. It is somewhat inconsistent, however. For example, if I am slowing down at a light and encounter the vibration, I ease off the brake a little and let it move forward (usually about a foot or two) and the vibration completely disappears. Could this be the same vacuum problem as discussed in the other email? Any info would be appreciated as this is a confusing problem. Thanks.

-Mike