When I get back in my car after a quick stop I sometimes get cool air from the center vents and hot air from the defrost vents if they're open. Ever so slowly the defrost vents will switch to blowing cool air. Brushing aside the possibility that the defrost vents shouldn't be open, I suspect that:

a) the heater core heat soaks when I turn off the engine and the monovalve opens even if there's no coolant recirculation

b) the monovalve is slow to close when I turn on the engine again.

Is one a more likely scenario than the other? The monovalve cartridge looks pretty new. The screen is clear and the rubber is supple.

I'm tempted to block the heater loop in the summer but I'm not sure how and where to effect the blockage. Any ideas?

Having hot and cold air from the side vents gives bi-level ventilation a bad name.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

I had noticed the same thing in both of the 190's I've driven. I don't have the SDL running yet, so no comparison there.

I had always thought it was heat soak. And I'd never been very happy about it.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)