Is my temperature gauge broke?

[71400lb]

I have a 71 c/20 with a rebuilt 400 small block Chevy 4 bolt main. I was told when I bought the truck, that the motor came out of a 72 Nova, but apparently 72 Nova's never came with 400 SBC's.

Anyway, the guy I bought it from said he removed the thermostat and put in what he called a "restrictor plate" instead! So it has no thermostat.

The truck has been fine for the past year, but today I noticed my temperature gauge was all the way over to the "C" on the left. I drove all the way to work(18 miles) at 79 degrees outside, and the gauge never moved!

When I shut the truck off at work, I noticed when I insert and turn the key clockwise just before the "start" position, the gauge needle moves quickly all the way to the left, like it should,,,so power IS getting to it...and when the truck is started the needle just stays at "C".

Should I be concerned? What's your theory?

Thanks
71400LB

1971 3/4 Ton 2wd
400 SBC/4 bbl Quadrajet
3.73 Open