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There could be lots of reasons that the ECU is over fueling.
1. a cable is trapped and earthing out that feeds one of the injectors, they are normally switched earther by the ECU.
2. try removing the plugs from items like the air flow meter and see if you note a change in running.
3. has the lambda sensor gone, has it got a power feed?
The lambda sensor used on the your car has 3 wires – two white heater wires and one black signal wire. Generally it is best to tap into the wires using wire piercing test probes which makes a tiny hole in the insulation. I make the connection at the car side of the plug where the leads from the sensor join the main loom because the wires attached to the probe are sheathed in a very hard heat proof insulating material which is very difficult to puncture. If when you switch on the ignition you get a reading of 12+ volts you have tapped into the heater power lead by mistake.
For this test you must leave the sensor connected to the ECU. Connect the positive multimeter probe to the sensor output wire and the negative probe to earth. With the engine running if all is well the voltage shown on the meter should flicker up and down the scale changing at least twice a second between about 0.1 and 0.8 volts -- the average reading should be about 0.4-0.5 volts but it should never be steady at this voltage. Flick the throttle open and the voltage should flicker, rise for a second or so then fall again. This indicates the ECU is receiving input from the sensor and is responding properly.
A steady voltage of between 0.4 and 0.5 V is a sign that the ECU is running in open loop mode . Either the engine is not fully warm or it may indicate a bad connection or a faulty water temperature sensor or an ECU fault. It could also indicate a problem with the sensor heater so check the heater is getting power and is properly earthed.