When engine is cold, the injection system will operate on open loop mode:
- The airflow sensor plate will actuate the fuel distributor to provide a mixture proportional to the airflow
When oxygen sensor is warm, the injection system should operate in closed loop mode:
- The oxygen sensor will measure whether the mixture is correct and provide this information to the ECU
- The ECU will control the EHA (Electrohydraulic Actuator) to provide richer or leaner mixture according to the feedbadk from oxygen sensor and the input for engine temperature
Apparrently, the closed loop mode does not function correctly.
You already replaced the oxygen sensor, so remaining issues are coolant temperature sensor, ECU and EHA.
One thing you should check first though is the overvoltage protection (OVP) relay. If this fails (which it is known to do), the ECU will not get power and therefor never enter closed loop mode.
The OVP relay is behind a shield behind the battery, and it has a fuse on top of it. Check the fuse and try to replace the relay (not expensive).