My truck (2006 FX4, 5.4 Screw) has been running flawless for over a year, then on Monday morning it started acting weird. I can't figure out if it's the engine, or transmission, or both. I am not getting any engine codes.
Here is what it is doing: As I move from a dead stop, the transmission feels like it is in a gear too high, by 35 to 40 mph it is already in top gear. Between 30 and 40 mph the transmission hunts for the proper gear to be in, and it shifts hard. Gas mileage is way down since this started.
Also, when I come to a stop, I am having to use more brake pressure than in the past. And, just as the truck is almost stopped, a heavy vibration can be felt though-out the truck, almost feels like I am driving over a miniature cattle guard crossing. I threw the truck in neutral once while stopping, and no vibration. It's almost like the transmission is not downshifting at stops.

 

To me sounds like a trans issue, I would check the fluid just to make sure you have some in there, and then take it in to the dealer..

 

If it feels like youre running over a cattle guard it could be your torque converter. Do u have a friend with a scanner that could scan transmission codes?

 

I hear transmission issues are common in those years

 

There is a really good transmission shop close to where I work, looks like I will drop it off there for a full diagnoses.

 

It was the tranny. Shift module warped, torque converter bad, and 3rd gear solenoid wasted. According to the shop, the 3rd gear solenoid was causing all the problems, which in turn took out the other parts. Per the shops recommendation, doing a complete rebuild since the tranny has 135,000 miles on it. $2,800 later and my truck will shift again, at least they are giving me a 24,000 mile/ 2 year warranty.