The heating on my 635CSi has always been, well interesting. The temperature control never seemed to work correctly, either blowing cold when set all the way to the left or roasting-red-hot when even a single click into the red. No big deal.
Last week though, that changed. Suddenly the heating wouldn't stay on when driving above 40MPH! Really odd. Soon as I hit 40.. freezing air! Fine driving through town but chilly as fuck on the motorway :(
Turns out that it's not actually speed-related but rev-related. At higher revs the pressure in the cooling system is higher and in my case it was enough for the hot coolant to blow through a broken seal in the heater control valve and bypass the heat-exchanger in the car.
Two PDFs attached below to show you how to do it! Click to download! :-)
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heaterfix2_925.jpg (132 KB, 0 downloads) 2 months old - heaterfix1_608.jpg (128.3 KB, 1 downloads) 2 months old