I have a 1986 BMW 535i - the heater only blows hot air when the car is idling or slightly above. At higher rpm the air turns cooler. When you return to idle or lower rpm the air returns to hot. The firewall heater valve seems to be working fine, when you turn the temp control on dash from hot to cold it responds correctly. This is a recently purchased used car and I found that the interior temp. Sensor had been totally disconnected and appears to be broken. It has a vacuum hose connection to it, which makes me think there maybe a connection to this sensor and my problem (since the temperature does seem to change with engine vacuum). I just don't know what the interior temp. Sensor controls. Could this be related to my problem?