i got a 99 with a 3.9,and lately the truck will be running fine and all of a sudden the rpm guage goes erratic and seems to be jumping around and the engine wont rev up or respond to any change on the gas pedal.any ideas?

 

So when the tach starts to act up the engine acts up also? On my '95 the tach will act up the same way but the engine still runs normal. I think mine is a ground wire issue on the gauge. The computer controls the truck's ignition and uses that same computation to drive the tach, so you could have a bad computer. Thats my first guess. Or it could be something as simple as a bad crankshaft position sensor, camshaft position sensor or distributor pickup.

 

is it just the tach?
i know my entire gauge cluster will do that, bad ground.