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Check lambda probe heating > < Poor performance after cold start

A4 Mk1

Motronic fuel injection and ignition system (4-cyl.turbo) > 07.96
Test lambda control
Testing operation

Check lambda probe voltage

  • ‒ Read measured value block, Display Group 21, engine at idling speed =>Page 01-49.
 
→ Indicated on display:
Read measured value block 21    ⇒
    860 rpm    1.15 ms    34.2 5C        l-control ON
  • ‒ Check display in display zone 4.
  • Specified result: λ-Control ON (lambda control active)

Do not continue with check until lambda control is active.

  • ‒ Press the C key.
  • ‒ Press keys 0 and 9 for "Display Group 09" and confirm entry by pressing Q key.
 
→ Indicated on display:
Read measured value block 9    ⇒
    860 rpm    - 0.7 %    0.755 V    0.4 %
  • ‒ Check display in display zone 3.
 
→ Specified result: Display fluctuates between - 0.1 V and 1.0 V.
Read measured value block 9    ⇒
    860 rpm    - 0.7 %    0.185 V    0.4 %

Note:

The voltage must fluctuate about 15 ... 30 times per minute.

If the voltage fluctuates too slowly, the following faults are possible:

  • ◆ Lambda probe heating defective
  • ◆ Lambda probe defective
 
→ If a constant voltage of 0.45 ... 0.50 V is displayed, the following faults are possible:
Read measured value block 9    ⇒
    860 rpm    - 0.7 %    0.475 V    0.4 %
  • ◆ Line break to lambda probe.
  • ◆ Lambda probe heating defective.
  • ◆ Lambda probe defective.
 
→ If the display shows a constant voltage reading of - 0.0 ... +0.3 V (mixture too lean), this means that the lambda control has reached the control stop in the "enrich" direction but the lambda probe is still registering "Mixture too lean". The following faults are possible:
Read measured value block 9    ⇒
    860 rpm    - 0.7 %    0.185 V    0.4 %
  • ◆ Unmetered air in intake system.
  • ◆ Fuel pressure too low.
 
→ If the display shows a constant voltage reading of + 0.7 ... 1.0 V (mixture too rich), this means that the lambda control has reached the control stop in the "lean" direction but the lambda probe is still registering "Mixture too rich". The following faults are possible:
Read measured value block 9    ⇒
860 rpm        - 0.7 %        0.830 V    0.4 %
  • ◆ Intake system blocked.
  • ◆ Fuel pressure too high.
  • ◆ Solenoid valve for active carbon canister constantly open.

Check learned value for mixture formation.

The mixture adaptation system has learning ability, i.e. the lambda control can recognise differences between various engines (injector nozzle delivery, compression, fuel pressure etc.) and compensates by adjusting the pre-programmed basic injection periods in the map in the control unit. The injection periods are extended or reduced until a mixture of λ = 1 is achieved. The difference between the actual injection period and the basic injection time programmed in the map is given in %.

  • ‒ Read measured value block, Display Group 08, engine at idling speed =>Page 01-49.
 
→ Indicated on display:
Read measured value block 8    ⇒
    4.37 ms    0.4 %    0.1 %    TB active
  • ‒ Check display in display zone 2.
  • Specified result: - 12...+ 12 %
  • ‒ Check display in display zone 3.
  • Specified result: - 12...+ 12 %

Positive learned value (+...%): Pre programmed basic injection period is too short, actual injection period longer by ...% in order to achieve a "λ = 1" mixture composition.

If the learned value is too high, the following faults are possible:

  • ◆ Unmetered air in intake system.
  • ◆ Unmetered air at exhaust manifold.
  • ◆ Airflow meter defective.
  • ◆ Fuel pressure too low.
  • ◆ Injector blocked.

Negative learned value (-...%): Pre programmed basic injection period is too long, actual injection period shorter by ...% in order to achieve "λ = 1".

If the learned value is too low, the following faults are possible:

  • ◆ Oil may be diluted (high petrol content in oil).
  • ◆ Airflow meter defective.
  • ◆ Fuel pressure too high.
  • ◆ Injector leaking.
  • ◆ Solenoid valve for active carbon canister constantly open.

Check lambda probe heating > < Poor performance after cold start