Mercedes-Benz W124 petrol engine rough idle

The most common causes are vacuum leaks, for example a cracked rubber boot, the neck of the breather pipe or the O-ring of the intake air temperature sensor. Others are a worn potentiometer in the air flow meter, the idle control valve, the lambda sensor and poor solder joints in the overvoltage relay (YJ relay). On the M104, a dead ignition coil can also cause jerky running.

First check all the vacuum hoses, because a rubber hose can look intact but crack when it flexes. A leak also shows up by blocking the vacuum outlets one at a time or by spraying starter fluid or brake cleaner onto suspected joints with the engine running. The running changes when the fluid hits the leak. The fluids ignite easily, so open flames must be removed.

The adjustment is made with a warm engine at idle: measure the voltage between the middle terminal and the outermost terminal of the potentiometer, the target is about 0.7 V. Tighten the mounting screws before adjusting, because tightening changes the reading. The potentiometer is adjusted first and the mixture only afterwards, otherwise the mixture adjustment is wasted.

The valve should be slightly open at rest. When 12 V is applied to it directly, it should click and close the air passage. The valve can be cleaned with brake cleaner and compressed air, after which it should be lubricated.