If my car has been sitting for a bit, say 15-20 minutes, I start the car and I'll get some heat out of the vents for a bit till the outside temperature is able to overcome the leftover heat (sorry for the run-on sentance). Not a ton of heat, but just enough for me to wonder whether the valve leaks after shutoff.

This still happens if the car is sitting out in the sun for hours, the vent heat still needs to be purged while driving.

Is this normal mono valve behavior?

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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)

The monovalve contains a check valve to prevent hot coolant from backflowing into the heater core; it could leak, but what you may have is simply a heat-soaked climate control system from the hot engine and sitting in the sun.

I notice that when I take the car out of the cool garage, the a/c system does a great job, regardless of the outside temp. If the car has been sitting out in the sun, however, it has to work much harder. And if the car has been run and then parked in the sun, it's even slower to cool things off. Subsequent sitting in traffic is the final straw, it really struggles.

So your car may just be acting normally. However, you might consider pulling the monovalve apart sometime and see if it is crudded up inside to where the check valve isn't working correctly.

Jeremy

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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

It is anywhere from 120 to 140 degrees in cars down here when we get into them during the summer. It is a blast furnace out of the vents for several minutes......

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

I have the same problem (intermittently) that Eric describes. It's definitely the result of uncommanded heater operation following a short period of engine shut down.

What prevents hot coolant from "forward flowing" into the heater core when power is removed from the monovalve?

I don't know! Actually, with the engine off and no pumps running, you would think that the coolant would not flow at all. I suppose there must be some flow due to convection, and the check valve is there to stop that. The answer then has to be that the hot coolant would not naturally flow in the other direction, "with" the check valve. I guess we need someone who worked at Mercedes designing engines to explain it to us.

__________________

"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