I have a 1984 300D with no heat when driving over 45 MPH, and any advice on how to fix this would be appreciated. (it's cold in CT). At speeds below 45 MPH, the heat is fine, and the climate control thermostat settings are fine.

When I say "no" heat, that's not entirely true - the temp at the vent drops to about outside temp + ~10°F. So, if it's 25°F, the air coming out of the vent is between 35°F and 45°F.

It's not a problem with the cooling system. The temp gauge is working fine, and even at speed in 15°F weather the engine temp never drops below 80°C.

I have replaced the climate control unit, the mono valve, the engine thermostat (just in case)... No joy. (except the buttons on CC head are nice and new)

My working theory is that at speeds in excess of 45 MPH, cold air is entering the system and mixing with the warm. Unfortunately, I can't figure out where that much air would be coming in from, or if it isn't a completely different problem.

Any clues?

Thanks,
Chris