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Your mileage suffers when the temp valve doesn't regulate and provide a more constant warm air temp into the AMM and intake. This is especially true for the LH 2.4 and 3.1 cars during cold running. Once the ouitside air temp gets above ~60 deg F it doesn't matter and the ambient cold air provides better combustion.
Also note that the valve isn't an OPEN/SHUT type of valve. It is a dynamically varable valve to mix warm and cold air when the car is driving at highway speeds. In most cases when the temp is below freezing the valve can be 1/2 way oopen and mix warm and cold air.
The temp sensor in the airbox is supercooled by a pitot-type tube to bring high velocity cold air to the sensor, and when the outside air is very cold the valve shuts intentionally at a colder "Wind Chillled" temp than the outside air. This will open the valve to warm air only during cruise and warm the intake air more. At idle even in cold outside air temps the valve will close and block the warm air with the engine bay heat and let in partly cold air.
I found this can attribute to 2-3 mpg loss in <32 degree F cold running. The car also runs smoother and better starts. I have two boxes, one for winter with a working thermostat, which I put in once a year when I winterize the car, and one for Spring/Summer & Fall with the damper and temp sesnor removed.
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