My tachometer hangs out around the 600 RPM spot when I first start my truck, but lately it will end up dropping to zero when idling, and then bounce up to about the 200 RPM point when accelerating. Its not the Cam Positioning Sensor (I had that happen with my 99 7.3) and this truck runs great. I am thinking it could be at the tach. Question is:
Where does the Tachometer signal generate and does the PCM do anything with it before it sends the signal to the Tachometer?
The reason for focusing on the Tach is since the engine runs and idles fine, then that tells me the PCM is receiving the correct signal from the engine and making changes accordingly.
I read about a 5.4 gas engine with rain issues, but the only issues going on here is extreme cold (tonight, -15). I keep the truck garaged and when I go out, it is for purpose and then return home to a heated garage.
Thanks for the help
Spencer

 

Having same issue. Any fix?

 

I have a 2006 and had problems with the speedometer being all over the place when driving. There is a Ford Technical Service Alert about the face of the gauge set warping and causing the needles to malfunction. This may be your problem.

Take a close look at the black plastic circle at the base of the needle and see if it is touching the warped plastic gauge face. If it is you have to replace the whole gauge panel (expensive), or I took some sand paper and sanded the black plastic circle down some so it would not hit the face. Mine has been working for years now.

 

Thanks, i inspected my gauges and they seem fine, It seems my issue has gone away since last winter and i havent experienced another issue. I think it might have something to do with the gauges themselves becoming cold. Guess i'll see once this winter starts back up again!