On my 500 SE, 126 chassis, I notice that when the push button is set on EC and temp on MIN for outside vent air, the temperature coming thru the vent is slightly warmer than the actual outside air temp. This occurs also at nite or cloudy days and so it is not due to radiant heat buildup. I have measured the temp. differences at about 8 to 10 deg. F. (70 deg outside & 78 inside) Is this normal? I do not detect any heat coming through the heat exchanger but I cannot be absolutely sure. When it gets real hot, I don't want my AC fighting the heater.

Are you sure?
 

How did you measure this? Best way is to use a 'good' thermometer stuck in the center vent.

Also due to the engine heat, it's natural that it heats the air venting so that it may be a little bit warmer by the time it gets through into the car.

So it sounds normal, although 8-10 F may be a little too much. What happens are highway speed, after everything cools down. Is there less differential under those conditions?

I am measuring temperature at the center vent. Tonite during freeway driving I was getting about 82 deg. vent air while outside temp. was 70 deg. The fresh air vent running through the center console to serve the back seat area is much cooler by feel than the center and side front vents. This rear vent is independent of the heating system and only provides fresh air or A/C air. I have searched this site for discussions about the monovalve which I replaced several months ago. I rechecked monovalve and it appears OK. No torn diaphragm and I can certainly get HOT air if I turn the temp. selector toward heat. I appear to be getting some hot water seepage when temp. control is set on MIN. Are there other areas of the monovalve that can cause this? I tried clamping the input heater hose to see if this would eliminate the warmer air but, with limited work space for a clamp, I was unsuccessful in completely blocking off the hose. Certainly the system is designed to allow outside fresh air into the cabin. Any suggestions?