Monday, August 19, 2013

One cause of erratic idling of the Bosch KE Jetronic/CISE driven W124

Below is the diagram of the engine vacuum and related components that drives the idling mechanism.

Vacuum and Idle Mechanism of the KE Jetronic/CISE driven W124s

The following is that I understand how the idle mechanism works

1) When feet is fully off throttle, idle microswitch is activated [closed circuit].

2) This trigger the IACV(Idle Air Control Valve) to alternate the open and close position to allow air to flow into engine as there is not enough air from the closed throttle to maintain minimum rpm for engine to be running.

3) When that happens, the vacuum pressure detected by the Fuel Pressure Regulator(FPR) will react to the change and maintain the fuel pressure to be constant.

4) If the primary crankcase breather hose that supplies the vacuum is not secured proper, the vacuum pressure will be unsteady.

5) Unsteady vacuum will cause the fuel pressure regulator to behave erratically and hence cause an unsteady fuel pressure, resulting in an idle condition that is not stable.

It is advisable to replace the crankcase breather hose that is directly connected to the valve cover/cylinder head and to secure them properly with a small hose clamp as shown.

Secured Engine Vacuum Crankcase Breather Hose with Hose Clamp on the 8v M102 engine
This is be easily DIY-ed by first removing the air filter box to enable access to the vacuum port and crankcase breather hoses.

If your crankcase breather hoses have aged and hardened, it may crack and break.

Do exercise case and caution when fiddling with these hoses. If unsure, always bring it to your preferred workshop's mechanic for this job.

The above suggestion takes into consideration that the fuel pressure regulator, idle microswitch, IACV, fuel distributor, fuel injectors and engine is in good working condition. 

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