Throttle actuator, wiring harness, engine breather hoses, motor mounts, belt tensioner are a few things come into my mind.

Timing chain and its accessories, distributor caps and rotors(earlier model) need some cares, a lot of owners complain mysterious idle problems, mine was solved by fixing a fuel leak near fuel pumps.

Airpump and power steering pump cause some troubles but these are not M119 related issues.

Most of the things above are not really related to the engine mechanicals, these engines are built to last, at 230K kms my camshafts showed absolutely no wear at all.

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99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.