Just right...

As I posted earlier, I've been struggling with getting the heating to work properly at high engine RPMs.

I looked through forums and my service manual, and found these excellent repair instructions from a different model Mercedes that has the exact same problem.

The problem is that Mercs have an automated climate control system controlled by a valve that sometimes wears out. You can purchase repair kits (consisting of the part that wears out) online for not much.

So, I ordered a new monovalve repair kit and installed it. After the repair, instead of running too hot or too cold, I only got mildly lukewarm air.

I reinspected my installation yesterday, only to find that (d'oh!) I left the cap of the coolant reservoir loose, thus releasing any pressure that might force hot antifreeze through the climate control monovalve.

The climate control is now working great, the valve responds perfectly to the control inputs and the automated climate control works like a champ, even when I bury the pedal in the floorboards, and go faster than the speedometer allows.

Thanks to the commentors who pointed me in the right direction!