Q: 1998 GMC keeps overheating on the temperature gauge. It's full of water and has a new thermostat.

Hi and thanks for writing to YourMechanic. There are several things to look at. First off, have the coolant tested and make sure it is within spec. If coolant is not mixed at the correct ratio, it can cause the engine to overheat.

Second, being that your truck is a 1998 there is a possibility of a water jacket getting obstructed, not allowing proper coolant to flow and cool the engine.

In this case a coolant system flush can cure this problem. Check and make sure that the radiator fins are clean and clear of any debris. Not enough air flowing through the radiator fin will also cause overheating.

Finally, there could be an electrical issue. The water temperature sensor might have corrosion which acts as resistance and will read high temperature. In this case, the temperature sensor switch will need to be replaced. You will need to have the coolant temperature electronic system checked for resistance and/or any broken/bare wire. If you would like to have this taken care of, a qualified pro from YourMechanic can come to your car’s location to inspect the overheating problem and proceed with the proper repairs.