I started my truck today and let it run for about 5-10 minutes because it is 10 below zero today. On the way home my temperature gauge read around 190-200 and I noticed that everytime I let off the throttle from 45mph the temperature gauge would drop to about 140. Is this normal? If not what is wrong with it. It's a 2003 chevy silverado 1500.
My first thought is a defective thermostat. If by low, you mean the over flow tank is near empty you need to bring it at least close to the normal level. Recheck it over several days. The other possibility is an air pocket. When the air pocket settles in the area of the temp sensor the gauge drops. This is due to air being a poor conductor of heat. This is more likely if the overflow tank emptied due to a small leak that went unnoticed.