I own a 1999 Chevy Silverado, 5.3 L V8. I have no heat, and my truck over heats. I recently replaced the thermostat, and flushed the system, running water through the heater core and checking the flow of water through the whole system. I have let the air pockets out, and have kept an eye on the coolant level after draining it for the fourth time in two weeks. I know its a new thermostat (i boiled it) and i know the heater core is not clogged. My only thing left is to replace the water pump but i can't see spending the time or money when it seems to be operating, I have ran it with the supply to the core off and seen that it pushes water through the system. I cant find any crinks or crushed hoses and there is no leak that has appeared to me yet.
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After replacing the thermostat, you must bleed the air from the cooling system.
To bleed air from the 2.2L and 2.5L engines, remove the plug or sensor
on the top of the thermostat housing. Fill the radiator with coolant
until the coolant comes out the hole. Since the plug is made out of
steel and the thermostat housing is aluminum, it is a good idea to
apply an anti-seizing compound or Teflon® tape on the plug threads
prior to installation. Install the plug and continue to fill the
radiator. This will vent all trapped air from the engine.
Any trapped air in the heating system will have to be displaced by coolant. Once the cooling system is filled, with the radiator cap off, turn of the heater at it's highest setting. Start the engine and allow it to reach operating temp. You should see a drop in the coolant level as the air in the heating system is displaced by coolant. Add coolant to the proper level and replace the radiator cap.
Keep a close eye on the coolant level for at least the next couple of weeks. The cooling system is a "closed" system. Any significant decrease in coolant level indicates a problem.
Had the same problem with my Mazda6 awhile back. Took it to the dealer and they found the radiator wasn't holding pressure. They replaced the whole radiator under warranty (WHEW!). Not sure how to check for that kind of thing, but it's a start.