When it's running (and knocking), remove the rear spark plug wire. If it goes away, you have a bad piston rod.

Are you sure it's knocking, and not clicking or pinging, etc...? Different sounds mean different things.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.