My heater works sort of. As long as I leave the fan on low I get heat out of my heater where it is supposed to be, but when I turn fan on high by the swich or by turning on the deforster it gets cold in about 15 seconds. After doing a search I keep seeing the mono valve being a problem with heat. Can that cause the problem I am having? The problem is on my '91 300SE.

Thanks,
Davis