Pull the radiator and have it checked by a shop for flow. I know it is new but I put a new Behr (made in South Africa) in my 400e last year and it was sh_t. Had to take it back.

Also, make sure you bleed all the air out of the head by removing the bleed screw up front (in the row of sensors)

The last M103 pump I bought came from an MB dealer here on LI. It was new and was under $150. I don't think $400 for an OEM pump here in the US is reasonable.

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88 300e
89 300e
93 400e
93 500e
77 308 GTB