For some time the heater on the passenger side of my 99 740iL fails to heat when set normally. By setting that side up around 80+ degrees it blows some heat. Dealer tech looked at it and claimed it worked fine when the wheel between the dash vents was set to red. That was not the fix. Just replaced valley pan, water pump, and all hoses so put in a new auxiliary waterpump, (there was a fault code on that), but I don't expect that to correct problem. I think it is probably a bad heater thermostst on that side. Has anybody encountered this?
Nope, but I have seen a bad heater control valve cause this. It may also be the flap on that side not opening properly.
The aux water pump did come with new water valves right? It could be a couple things like mentioned above it could be a flap the way to tell is if the air is blowing less intense then the other side. In the e38 the heater/ac system does not use what is called a blend door to control the cabin temperature it uses two water valves to control coolant flow into the heater core. One for each side of the passenger compartment The other could be the IHKA control unit failing not commanding the water valve to open all the way for full heat. To test this function is to unplug the electrical connector to the water valves if it gets hotter inside its most likely the control unit but I would do some wire testing before just replacing it. Finally it could be the water valves themselves that have stuck closed, I take a small hammer and lightly tap on them and sometimes it's enough to free them up to move. But you might have already replaced them so no need to worry about them then. I had the same issue on my old e38 and it ended up being the valves failing.
