I have a autometer water pressure gauge that I removed from boat and it fell in my seat when pushed out. Now it reads on five. So I put on dash of truck today and it goes to zero after heated up then cooled and goes back to five . Any suggestions

Yeh, there should be a pressure release on the guage. Contact the manufacturer for where it is(usually under a black rubber cap) and how to use it.

I ended up putting a pin hole in the rubber plug on my non-autometer to allow it to adjust for the heat in the engine bay