Mercedes-Benz W201 low oil pressure at idle

According to the owner's manual, the minimum idle pressure of a warm petrol engine is 0.3 bar. For example, 1–1.5 bar is plenty. When cold the pressure is higher and it falls as the engine warms up, because hot oil thins.

First check the oil level, the oil and the oil filter: a clogged filter is a common cause on M102 engines. Other causes are a stuck pressure relief valve, a clogged pump pickup screen and a faulty gauge or pressure sensor.

Measure the pressure with a separate gauge to make sure the fault is not in the car's own gauge or sensor. If the actual pressure when warm stays below the minimum value, the cause is a worn oil pump or worn bearings, and they should be replaced.