You have a 99 Chevy Malibu that is overheating and you've replaced the thermostat and checked the water pump and its OK and the radiator cap is good to Do you know what else it could be?

Answer 1

What size engine? Are you losing coolant(this may be internal or external)? Is your oil level rising? This is critical to anwer these questions. The 3.1 (if you have it)motor can be tricky. 2 common problems are intake manifold gasket and the water pump. When the water pump leaks on these, it can be difficult to verify. Look for dried coolant spray on the hood or on the passenger side of the radiator in relation to the water pump. Also look for a rusted flat surfaced pulley of the water pump, or try to check for moisture on the inside the pulley-easier said than done.

You may havt bleed off the air that is in the cooling system. Their is a bledder just above the #2 spark plug ( it is a very small bolt that turns ccwise) I t is connected to a solid blck tube that runs horizontily above the front valve cover and connects to the block. Good Luck