Mixture control

OXYGEN SENSOR VOLTAGE (#05)

Reading #05 on the XR25 diagnostic tool: the value read is the voltage sent to the ECU by the oxygen sensor, expressed in volts (in fact the value varies between 0 and 1,000 millivolts).

When the engine is in closed loop, the voltage must oscillate rapidly and be between 50 ± 50 mV (lean mixture) and 850 ± 50 mV (rich mixture), and vice versa.

The smaller the max-min difference, the poorer the oxygen sensor signal (this difference is generally at least 500 mV).

MIXTURE CORRECTION (#35)

The value read at #35 on the XR25 diagnostic tool is the average mixture correction applied by the ECU according to the mixture richness seen by the oxygen sensor (the oxygen sensor actually analyzes the oxygen content of the exhaust gases, which results directly from the mixture richness).

The correction value has a midpoint of 128 and limits of 0 and 255 (in practice, under normal operating conditions #35 sits and varies slightly around a value close to 128).

• Value below 128: lean request.
• Value above 128: rich request.

ENTRY INTO MIXTURE CONTROL

Closed-loop phase

Entry into mixture control takes effect after the start delay:

• with throttle released, if the coolant temperature has reached:

• 46°C for the E7J engine,
• 30°C for the D7F engine.

• with throttle not released, if the coolant temperature is above:

• + 20°C for the E7J engine,
• + 20°C for the D7F engine.

The start delay depends on the coolant temperature:

• at 20°C it is at most:

• 3 minutes for the E7J engine,
• 1 minute 20 seconds for the D7F engine,

• at 80°C it is at most:

• 1 minute 30 seconds for the E7J engine,
• 35 seconds for the D7F engine,

until mixture control is entered, #35 = 128

Open-loop phase

When in mixture control, the operating phases during which the ECU ignores the voltage signal from the sensor are:

• at full throttle: #35 = variable and above 128,
• during heavy acceleration: #35 = variable and above 128,
• during deceleration with throttle released (fuel cut-off): #35 = 128,
• in case of oxygen sensor failure: #35 = 128.

LIMP-HOME MODE IN CASE OF OXYGEN SENSOR FAULT

When the oxygen sensor voltage is incorrect (#05 varying very little or not at all) in mixture control, the ECU only switches to limp-home mode (#35 = 128) if the fault is confirmed present for 3 to 5 minutes. Only in this case is the fault stored.

When a present oxygen sensor fault is detected and the fault has already been stored, the system goes directly to open loop (#35 = 128).