The sender used with the gauge presents a range of resistance over the range of pressure, i.e. when pressure is zero, the sender has a certain resistance, and it changes as the pressure goes up.
grounding the wire at the engine or applying 12 vols to it is NOT the thing to do.....it may swing the gauge from one extreme to the other and damage it in the process.
The proper thing to do is to attach a resistor equal to the zero psi resistance from the wire to ground to check the gauge reading. Likewise, attach the max PSI equivalent resistance to it to see if the gauge reads properly at the other extreme.
There was a recent post in this forum which stated the actual resistance values to use. I'll see if I can find it.
The OEM senders are not very accurate, and after all these years may very well be defective.
Here's the link:
http://forums.maxperformanceinc.com/...d.php?t=479094
Apparently it may be ok to ground the sender wire as zero ohms corresponds to one end of the scale.
George