Remove the sensing bulb from the engine and immerse it in a pan of hot (or boiling) water. If the gauge doesn't respond, you've probably got a pinhole leak in the capillary tube somewhere. That type of gauge isn't repairable without some pretty sophisticated equipment to refill and seal the sensing bulb.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!