Is it time to do my duovalve?
 

Hi everyone. '97 S420 here...

So it's been very warm in Richmond lately. I went to go pick my kids up from a class they were attending and while I was waiting I left my engine running to keep the A/C going. After idling 10-15 minutes, I noticed it was starting to get warm inside the car. Being the geek that I am, I put the A/C controller into diagnostic mode and checked the evap temperature. It showed a balmy 38 def F, but the air coming out of the vents was very warm. So I shut off the side vents and pressed the blue button on the center vents and once again the air was nice and cold.

Once my kids showed up and we started driving for a while everything returned to normal. Today, I did a round trip from Richmond to Philadelphia (8+ hours of driving) and the A/C worked flawlessly.

The only thing I can think of at this point is that the coils on the duovalve solenoids became heat soaked and thus weak, and weren't able to shut off the flow of hot water to the heater cores.

So, is my duovalve on the way out or am I missing something? Thanks in advance for any advice you can offer.