Maybe on the S type, but on the X350 one heater core, one hose in and one hose out, no water valve. Everything controlled by diverter/blend flaps. Aux pump is up front of the engine, if it affects the heater, which I doubt, it would not affect just one side.

Stu

 

OK,thanks psg---made an untested assumption!

 

After playing with various combination of the modes, I found that if I turn the A/C off, the passenger side starts to blow warmer air. Not as warm as driver, but warmer than with the A/C switched on. Is it possible a sensor is providing an incorrect signal, making the system turn the A/C on to try and get the temperature right? I drive about 25 minutes at highway speeds, and the heater never blows hot air, only warm. Engine temp gauge is right in the middle. This may not be related, but ambient air around the steering column where it meets the dash seems quite cool.

 

I have an 08 XJ8L and tend to get warmer air on the driver's side prompting me to ignore my wife's complaining about no warm air on her side. It never occurred to me that her whining was valid until I felt the cooler air coming out of the passenger vent (when she was not in the car). This is terrible, I will now have to pay more attention to her!