I bought my 99 f150 4.6 liter 2wd a year ago and Im still having the same problem. Driving around at any speed the rpms will jump up to 4000rpms and then back to normal. Sometimes I will be at a red light or a stop sign and the rpms will shoot up and the truck will lunge forward. (SCARY!!!). At times while Im driving around the engine will just die and I have to pull over take out the key put it back in and restart the engine. The odometer will black out also. Ive had it at ford dealership 4 times now. Ive replaced the plugs, wires, coils, and the alternator by there request, but it hasnt changed a thing. I love the truck but it is not very safe to drive my 3 kids around in. Please somebody help me!!! I cant afford to keep going back to ford to replace things that dont need replaced.

 

sounds more like something deeper than the items you've looked at...
computer related perhaps?

 

what about some basic electrical stuff, like engine block and chassis grounds?

 

im not seeing why that would cause the rpms to surge on their own?

 

Maybe the computer isnt getting suficent power, so it thinks the alternator isnt giving enough power, so it jumps the RPMs. I dunno....

 

ok ok, but 4k seems excessive?

still would be an easy fix and is probably a good idea to do in this situation anyways. try it, but im still leary of the computer its self.

 

agreed, im just tryin to think of something that might be overlooked, or underesitmated by a ford mech

 

try a tps and check wiring going to it. if its shorted or anything that will give a mixed signal to the comp and would send rpms that high

 

Consider this; the engine has to have increasing air and fuel to accelerate in such a manner. I would look hard for physically loose throttle body, top TB manifold, intake manifold and/or gasket leaks. This air has to come from somewhere when the foot is off the throttle.
The PCM, ignition etc only react to a request to accelerate the engine and could hardly be the root cause but will do as it is commanded by sensors, air and the right conditions present..