Regulation of the composition of the working mixture (K4M and F4P) (Renault Laguna 2)

Engine with ECU "SAGEM S 2000" equipped with two oxygen sensors: upper and lower.

Heating of sensors

Heating of oxygen sensors is switched on by the ECU command when the following conditions are met:

• the intake pressure is below a certain threshold, depending on the engine speed,
• speed below 135 km/h;
• after a certain time of engine operation according to the program depending on TDC (excluding operating time with the accelerator pedal released) and coolant temperature.

Heating of oxygen sensors stops:

• if the vehicle speed is higher than 140 km/h (reference value),
• under heavy engine load.

Upper sensor signal voltage

Read the value of the parameter: "upstream sensor signal voltage" on the diagnostic tool: this is the voltage value that is transmitted to the ECU by the oxygen sensor installed before the catalytic converter. The voltage value is expressed in millivolts. When the engine is running in a closed control loop, the voltage should quickly fluctuate between two values:

• 100 mV ±100 for lean mixture;
• 800 mV ±100 for rich mixture.

The smaller the difference between the minimum and maximum values, the less accurate the information from the sensor (typically this difference is at least 500 mV).

Note: If this difference is small, check the sensor heating.

Lower sensor signal voltage

It is necessary to read the value of the parameter: "lower sensor signal voltage" on the diagnostic device. This is the voltage value that is transmitted to the ECU by the oxygen sensor installed after the catalytic converter. The voltage value is expressed in millivolts.

This sensor is designed for diagnostics of the catalytic converter and repeated, more accurate, control of the mixture composition (slow control loop). This function is activated only after the engine has been running for some time.

When the motor is running in closed loop, the voltage should vary within the range of 600 mV ±100. When the speed is reduced, the voltage should be below 200 mV.

The voltage value read from the diagnostic tool at idle speed should not be taken into account.

Correction of the composition of the working mixture

The value of the "air mixture correction" parameter, read by the diagnostic tool, is an average value entered by the ECU depending on the information about the air mixture composition transmitted by the oxygen sensor installed in front of the catalytic converter (in reality, the oxygen sensor analyzes the oxygen content in the exhaust gases).

The average correction value is 128, and the extremes are 0 and 225.

Value below 128: lean request.

Value above 128: request for enrichment.

Start of regulation of fuel mixture composition

Phase "closing the control loop"

Regulation of the composition of the working mixture begins after a time delay after starting the engine of no more than 0 s, if the coolant temperature is above 0°C for the F4P engine and 10°C for the K4M engine.

Phase "opening the control loop"

When regulating the composition of the working mixture, the ECU does not take into account the value of the sensor signal voltage in the following cases:

• when you press the accelerator pedal all the way down;
• during sharp acceleration:
• when decelerating, if there is information about the accelerator pedal being released;
• if the oxygen sensor fails.

Backup mode in case of oxygen sensor failure

If the oxygen sensor signal voltage during mixture control is incorrect (changes very little or not at all), then the ECU switches to the backup mode only if the fault has been determined as present for 3 minutes. Only in this case will the fault information be entered into the ECU memory. In this case, the value of the "fuel mixture correction" parameter is 128.

When a fault is detected in the oxygen sensor, if the fault information is already stored in the memory, the ECU immediately switches the engine to open-loop operation.