If the cool heat at highway speeds is due to low coolant temp, then you will need a new thermostat. What temperature does the engine run at on the highway? It should be at at least 80C or higher.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record