Dear All,

I have a 1992 W124 300e with CAT and oxygen sensor, EU version and it's produce an annoying issue which I cannot figure out.

So my problem is when I start the car cold the duty cycle sits around 50% as expected because it is in open loop. Once the engine is at operation temperature the duty cycle starts increasing and stops around 92% there no really any fluctuation. It takes a few seconds to reach the 90%. Once I rev the engine to 1500 RPM to duty cycle changes to a 50% without any fluctuation so I am assuming it goes to open loop again. Once I release the pedal it goes back to around 92% within few seconds.

I can smell the mixture is rich and my fuel consumption is around 23 liter / 100 km (~10mpg) and there is a lot of smoke from the exhaust. When the car is cold there is almost no smoke from the exhaust.

Anyway the car starts nicely, little bit rough idle but nothing serious (probably because the rich mixture). Maybe I can feel a little hesitation when I accelerate strongly but again I am suspecting the rich mixture.

What I checked so far:

- Duty cycle when the ignition on - 71%
- Duty cycle when the ignition on and air plate pressed - 11%
- Duty cycle when the ignition on and gas pedal fully pressed - 21%
- Duty cycle when the engine running cold - 50% (no fluctuation)
- Duty cycle when the engine is warm - 92% (no fluctuation)
- Duty cycle when the engine is warm RPM 1500 - 50% (no fluctuation)

- Fuel cut off switch works signal reaches the KE control unit
- Throttle position switch works (both idle and acceleration), signal reaches the KE control unit
- EHA currents seemed ok

- Tested with other KE control unit (used)
- Tested with other EZL ignition unit (used)
- Tested with other EHA (used)
- Tested without the cold start injector

- Tried to adjust the mixture but nothing changed

A lot of things were replaced in the last ~5000km
- Ignition cables
- Ignition cap / rotor brand new
- Ignition coil brand new
- Spark plugs
- O2 sensor replaced multiple times,
- Injectors
- Over protection relay (after market)

Is vacuum leak can cause this kind of symptoms?

I would appreciate any suggestion because at this point I am lost.