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One thing you might try is blocking the radiator on the downhill - if it's all or mostly downhill. Use a piece of coroplast (the stuff they make political signs out of) or similar. Use the ScanGauge to monitor coolant temps. The point is that going downhill the engine is under little load, and so doesn't produce much heat. The airflow may cool the engine way below optimum operating temp, and the engine computer may even be burning extra gas trying to maintain a minimum temp.
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