That sounds like the idle mixture adjustment to me. the fuel pressure regulator is a check valve that regulates fuel pressure, downstream of the fuel distributor. Its not adjustable.

My only other point would be that these components, at least on my 87 300E, are from a K-Jetronic CIS-E fuel injection system. Someone else may be able to confirm that this was in fact used on an 83 model 380SL.

Presuming that you may have a non-electric CIS system, adjusting the idle fuel mixture is, iirc, anti-clockwise is lean, clockwise is rich. And a little goes a long way. A search on idle mixture adjustment will probably help.

good luck

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1987 300e manual 250,000 km (sold)