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You probably have trapped air in the system. When the air gets on the temp sensor it goes hi and then when the coolant hits the temp sensor it goes lower. With a COLD engine remove the radiator cap, and start the engine and let it idle and keep topping off the coolant level in the radiator. MAke sure you see coolant moving or flowing inside the radiator once the thermostat opens. which is usually between 160 and 200 degrees. Once you get the air out of the system, the coolant flowing across the temp sensor will be more consistent and thi should take care of your problem.

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Q: Why would the temp range rise and fall in a 1999 gmc pickup even though the water pump and thermostat are new?
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