What causes miss firing and black smoke in Mitsubishi Galant 98 model when on idling. And when airflow sensor is removed its OK.

Black smoke is un burnt fuel. Which motor do you have?

Its a Mitsubishi Galant 1998 model

Which engine is in it? The 2.0L DOHC or the 2.4L SOHC?

The engine is a 1.8L GDI petrol 16v

I would try cleaning the air flow sensor. When you say removed -- do you mean taking it off or just unplugging it from wiring harness? Also check and clean air filter.

I mean unplugging it from the wiring hatness

Oh ok. That leads me to think that the sensor either needs cleaning or to be replaced. I would try cleaning first. Only use cleaner for that type sensor and make sure that you don't touch any parts of the sensor with your hands.

I have replaced more than 3 sensors,replaced the GDI pump,replaced the fuel pump as well as the plugs. Now I am to check my ECU

Thank you very much. I will try that and get back to you on Monday. Nice day