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Q: Could you please define what can be root cause?

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Hi, I have Mercedes E240 2002 and has idle pulsing problem when it is cold and sometimes when I drive and brake to stop fully and waiting my foot on brake pedal idle starts pulsing. It also vibrates the car and feels like engine loosing power a bit. I understand it from lights and air conditioning air flow noise which also pulsates when idle pulses.

My car has 180000 miles.
My car has an automatic transmission.

Surging can be caused by a number or mechanical or electronic issues. A vacuum leak for example. If there is a crack in the air intake hose, or a vacuum line or loose or broken, this can effect idle speed and air/fuel mixture. The engine control unit will try to compensate for this and can continuously "hunt" for the proper idle speed. Electronic components such as the mass airflow sensor can cause this as can a bad coolant sensor, or any one of the dozen or so sensors used to control the engine and transmission. Since any of dozens of components can be the cause, the best route to take is to have a certified technician look into the surging and poor idle to find the exact cause. Replacing parts at ransom can get quite expensive.

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