I haven't run across this particular problem throughout the reading of the questions in the archives.

If my 300D has been sitting for 15 minutes to an hour, as soon as I start the car backup, heat enters the interior (the fan needs to be on of course). It seems as if the mono-valve leaks heat into the heater core even though I have not run the heater. My mono-valve performs normally while the car is running, but the amount of heat that enters the a/c system while the car is shut down seems to be abnormal.

Anyone? Bueller?

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

I had that issue at one point and ended up replacing the entire monovalve, I suspect that the check valve that's built into the monovalve was leaking and letting hot coolant circulate through the heater core when the engine was off.

X2 -- check valve is supposed to stop convection flow of hot coolant while engine is shut off. It may be possible/necessary to repair the check valve, I believe the 123's monovalve is NLA.

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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