[37 Coupe]

The metal (scale) part of the gauge snaps around the thermometer tube are you able to move it up or down? Mayby bumped from proper position,otherwise I aggree with an air bubble somewhere in the line. This sounds crazy but I have done it,centrifugal force is the cure,carefully attach strong string like clothesline to upper gauge part,mayby hold on with duct tape and swing it around in a circle above your head or whatever,it will work.