Originally Posted by Richard Wooldridge

I suspect that your auxiliary water pump is not working. The mono/duo valves need to have the pressure that the auxiliary water pump produces to make the valves work properly. when the car is idling at a stop light, there's not much pressure from the engine water pump, so the valves cannot stay closed. When the car runs at a higher engine rpm, the valves start closing again. The solution is to replace the auxiliary water pump, or change to a different type of valve. I drew up a drawing of the change I made on my '82 380SL, which had the same problem. It's on one of the diesel threads, if you have trouble finding it I can probably look it up.