You should be able to call a local dealer's parts department and they can look up your engine number if they have your VIN. You said you experienced the problem after the car sat a few weeks. Did you notice any preignition (valve ping) under acceleration?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles