The truck has had a slightly rough idle since I bought it and I have poured money trying to find the root cause. Today I was leaving work and after 4 minutes of driving I came to a stop light. The engine started idling very rough, much more than normal. As I took off the engine remained running rough up to about 25 mph until I slowed for the following light (only a few hundred feet from the prior red light.) When I took off from that light the truck ran smooth and normally the entire rest of the journey home. I've changed the plugs, fresh oil change, new fuel filter, clean MAF sensor. Anyone know what could have caused this?

 

clean throttle body ..

 

Mine was doing this exact same thing....i fixed it yesterday. Check your 90 rubber elbow fitting on the back of the throttle body. Mine was cracked and collapsed

 

In our 5.4s how do we get to that 90deg. Rubber fitting. Behind the TB I can barely see it. Mine may have a similar issue and is also hooked to the brake booster and having issue with that as well

 

Check your FPDM.

 

^^ this, ~$100 on ebay, replaced it on my 04 lariat and brought the idle back up and smoothed it out.

 

Check all the vacuum lines and clean the throttle body and clean the injectors