I don't have any direct experience on a 300D but I can tell you that on a 190D the auxiliary circulating pump is not needed. In fact, cars delivered in Europe didn't have one. Mine was leaking so instead of replacing it I installed a by bypass. I used a brass hose barb from Lowe's. I believe the size is 5/8".

The replacement pump is about $130 IIRC. Good luck.

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Tom Savage
Vienna, WV
1984 300D Euro
1995 S320