Good plan. Performing normal maintenance is always a good place to start. Then, when all filters, plugs, plug wires, distributor cap and rotor, voltage regulator brushes, OVP and fuel pump relay have been replaced, and maybe a couple of bottles of Redline injector cleaner run through the motor, then diagnosing the other mechanical/electrical systems on the car will be much more fruitful.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't