Sorry for asking 2 years later, but are you saying that if the gauge doesn't go all the way hot when shorting the sender wire to ground, the gauge is bad? Mine goes to about 3/4 of the way to hot then stops.
either the IVR is week, the gauge needs calibration, or there is some resistance in the sending wire path. (a bad ground from the engine to the chassis could do that, assuming you grounded the sender wire to the engine)
