my heater works ok when engine is idling at a standstill but when i start driving and turn the fan up the heat gradually turns to cold air. is this the shut off not opening fully in the matrix. also the temp gauge drops.
I had the same problem, which I solved by changing the heater valve, which shuts the water flow into the heater matrix when you select cold air, and opens it again when you select warm air.
Unfortunately, it has failed again after only 6 months. I now wonder if something else is wrong.
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