Unless your wiring is really butchered, when the key is turned to "OFF" the temperature circuit receives no power. The gauge simply parks where ever, it is not an actual reading until the ignition is switched to "ON".

 

I am having the same problem as you describe. I beliee it is due to a short. I have a fsm and the wire from sender unit goes through the firewall junction box where the wire splits one to the gauge and the other to the ignition switch at bottom of the steering wheel.

The fix idea(for me).

Seperate the wire from sender to junction box (purple with yellow stripe). I already have a hole in the firewall drilled, so I am going to use the hole instead of box. Cut same color wire from gauge harness and replace it with new wire. I'm either doing 1 of 2 thing for gauge power. Cut and replace purple/ yellow from ignitin switch and tie into the wire for sender/gauge. Or wire.a power wire from cigarette lighter and tie into the sender wire. Hopefully that will remove the short and get the gauge going. I plan on doing this in the am today and ill let everyone know how it turned out. -matt