Question: My 2003 Dodge Durango will sometimes overheat. I just had a new radiator put in. I'm not sure if it is the water pump, a bad thermostat, or air in the system.
Answer: Typically a water pump will leak coolant and/or make all kinds of noise when it fails. A pump reducing or eliminating the flow of coolant -- causing an overheat -- is a rarity, and not a first thought when diagnosing an overheat condition. When working on any vehicle, you want to be sure that air is bled from the coolant system after a service. But that really shouldn't be an issue on a Durango because it's rear drive with a forward-mounted engine, and has a fillable radiator with a radiator cap. Inspect for any bleeder ports at a high point of the engine. If present, open the port, fill the radiator until coolant comes out of the port, close the port, and continue filling the radiator. Keep adding coolant as it heats up and the thermostat opens. It's also good practice to raise rpm and have the heat on. This opens the hot water valve (if equipped) and allows coolant to circulate through the heater core. Getting hot air from the ducts confirms the flow of coolant from the engine and through the HVAC system.
Thermostats can be tricky. Carefully feel the radiator and upper and lower hoses as the engine warms up. When the thermostat opens, allowing coolant flow, the radiator and both hoses will become hot. You can also visibly see the flow by looking into the filler neck where the cap came off. If there's no flow after a long period of run time, remove the thermostat and visually inspect it. It should be fully closed when cold, and you should be able to manually open it against spring tension with your fingers. Replace it regardless.
A very common issue is the possibility of a leaking cylinder-head gasket. This is where coolant is being burned in the combustion chamber, and combustion pressure is forced into the coolant system. This is often the case when a system is significantly low on coolant, but there is no sign of an external leak. A quick way to check is to take a whiff of the coolant in the radiator and the expansion tank. If you get a strong smell of exhaust (hydrocarbons), that's a dead giveaway. Also, when a head gasket leak is really bad, coolant might erupt out of the radiator on startup. Smaller leaks will require pressure testing to confirm the fault. Other items to check are airflow obstructions at the radiator, coolant fan operation, slippage of a drive belt or pulley, and the accuracy of the temperature gauge.
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