oldskewel

I think this is the explanation. I have a 91 with a relatively new heater control valve, and have the same symptoms. So I'd call it a design feature. If the heater is set to a temp that causes the heater control valve to open, and then the climate control is turned off, the climate fans turn off, but the valve remains open, keeping the heater core warm. Even with the fans off, driving will cause some air to move through the vents, and it will be warm due to the heater core. If you don't want this feature, before turning the climate control off, reduce the temp setpoint so that the heater valve closes. oldskewel 1991 LS400 1985 911

My 91 idles very well at 600. 1200 sounds too high. Probably a very easy fix for them to do - I'd take it back and make them fix it. They will know they messed it up. But probably no permanent harm done.