Mercedes-Benz W140 idle hunts or surges
S 320600 SEL400 SE420 SEL300 S
Why does the W140 idle hunt or surge?
Common causes are a faulty idle control valve, small vacuum hoses that have come off near the throttle body, and the lambda sensor. The symptom often shows only after the engine has warmed up. The cause is found one thing at a time.
What should be checked first?
Listen to the idle with the air conditioning off: if the surging eases, the fault relates to the idle air control. Then check the throttle body vacuum hoses and the idle control valve. Securing a hose or repairing the valve may have removed the hunting.
Can the lambda sensor cause hunting?
Yes, a faulty lambda sensor can cause idle surging, higher consumption and uneven running when warm. The sensor's condition can be found by measuring. An intake air temperature sensor giving a wrong value can also make the car run unevenly.
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