I have the same car, and the '97 is a problematic year as far as EGR deletion is concerned. The only technique I have discovered that will not trigger a check engine light is to block the exhaust and provide filtered air to the EGR valve instead of exhaust. The sensor on top of the valve needs air flowing by. Not sure if this would address your 400 code, but it might. I have not yet done this to my car as in VA the emissions stuff must appear stock, even though our inspections do not check emissions.

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'97 E 300 D