Mercedes-Benz W202 petrol engine uneven idle
Why does the idle of my W202 petrol engine hunt or fluctuate?
The most common causes are a dirty or faulty throttle body, the mass air flow sensor (MAF) and an intake air leak. First try cleaning the throttle body and the mass air flow sensor. Other suspects are the throttle idle microswitch, the accelerator pedal sensor and the wiring harness. It is also worth checking the condition of the overvoltage relay.
How do I find an intake air leak on a W202?
With the engine idling, spray spray fluid on the joints of the hoses and rubbers. If the revs rise, the leak is at that spot. Check the vacuum hoses and the rubber and O-ring between the throttle body and the intake manifold, as they crack and become brittle.
Why is the idle bad only when the engine is cold?
When cold, the throttle can freeze where cold outside air meets moist breather air. Water collected in the vacuum hoses can also freeze in frost. In addition, a fault in the engine temperature sensor has caused cold-running symptoms, so it is worth checking too.