'83 300D.

I have been having a problem lately with my heater core flooding with hot coolant during short stops. As a result, I get to enjoy a couple minutes of very hot air when I get back underway.

Obviously, this isn't supposed to happen. And it didn't happen until recently. A visual inspection of the monovalve reveals no obvious problems, except that the check valve in the outlet does not seal completely. It is free, however.

Considering that the monovalve opens when not powered, it is not clear to me what mechanism or feature is supposed to prevent coolant from getting to the heater core in the first place, when the car is shut down. Obviously, the check valve should prevent reverse flow. What prevents flow in the normal direction?

The check valve is indeed the problem. The monovalve used to be rebuildable but I don't think those parts are available any more. Short of finding a good unit in a junkyard you'll have to buy a new one if you can't repair yours.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970