The electrical (metal) part is the same; the difference is in the valving (the stuff in the plastic body). The E320 has a simple ON-OFF valve while the E300 has a two-way valve: coolant ALWAYS flows from the back of the head. If it flows through the heater core you get heat in the cabin; if it flows through the bypass line you don't get heat in the cabin. The monovalve decides which way the coolant flows.

IN my opinion, the constant coolant flow keeps the head at a more constant temperature for smog reasons. I don't think it will hurt the engine is that flow were interrupted -- but again, that is just my opinion and I'm afraid to blow up my engine finding out!

You could simply block the bypass line and connect the two lines from the heater core to the E320 monovalve / pump. I don't think the bypass line is absolutely necessary, but that is just an opinion. The gasoline engines have no trouble with such an arrangement, it's only the diesel that has the bypass hose.

Have you taken the valve apart to see what is wrong with it? Here are two pictures that show how the valve works.

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