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Home >> Saab >> 2001 >> 9-5 Base, 4D Sedan, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Fault Code Diagnosis >> P0300: Random Misfire Detected >> Fault Isolation Procedure

Fault Isolation Procedure

  1. Check additional DTCs in the ECM system:
    • Turn the ignition switch to the ON position.
    • Contact ECM.
    • See whether any of the following DTCs have been recorded: DTCs P1300, P1301, P1302, P1303, P1304, P1305, P1306, P1390, P1391, P1392, P1393, P1394, P1395, P1396.
    If any of the above trouble codes are present, follow the appropriate fault isolation procedure. If none of the above codes are present in the system, go to next step.
  2. Checking further DTCs in ECM: See whether any of the following DTCs have been recorded: DTCs P1676, P1641, P0171, P0172, P1171, P1172, P1181, P1182. If any of these trouble codes are present, follow the appropriate fault isolation procedure, otherwise go to step 3.
  3. Checking cylinder compression:
    • Read Freeze Frame and make note of the following data: Engine RPM, Airmass / Combustion and Coolant Temperature.
    • Check visually that the hose connections are coupled between the compressor and the throttle body and that the vacuum hose between the fuel pressure regulator and intake manifold is undamaged.
    • Check with the diagnostic tool that the coolant temperature sensor indicates a reasonable value. Rectify as necessary.
    • Check the compression of the cylinders.
    If the function is correct, go to next step, otherwise rectify the fault and go to step  13.
  4. Checking the ignition discharge module:
    • Change all spark plugs.
    • Check visually that the rubber seals are intact, the contact springs are OK and the connector is free from corrosion and moisture.
    If the function is correct, go to next step, otherwise rectify the fault and go to step  13.
  5. Checking the ignition discharge module (continued):
    • Fit the test spark plug in the ignition discharge module.
    • Ground the spark plugs.
    • Position the ignition discharge module so that the spark plugs are inclined downwards and the oil in the ignition coils covers the HT outputs.
    • Activate the ignition trigger by selecting automatic activation of the ignition coil for the misfiring cylinder. (Ignition Coil Cyl #)
    • Select automatic activation.
    • Check whether the relevant test spark plug produces a spark. If necessary, compare it by activating another ignition coil.
    If the function is correct, go to step  7, otherwise go to next step.
  6. Changing components: Change the Ignition Discharge Module. When finished, go to step  13.
  7. Checking mass air flow when idling:
    • Turn the ignition switch to the ON position.
    • Contact ECM.
    • Obtain readings for the absolute pressure sensors as described in the instructions below. If any reading differs by more than 10 kPa from the others, the sensor is defective and should be changed.
    • Read value: Manifold Absolute Pressure (ECM).
      NOTE: The manifold absolute pressure sensor value will be compared with the mass air flow sensor value later on in the course of fault diagnosis. Take note of it for later use.
    • Read value: Charge Air Absolute Pressure (ECM).
    • Read value: Atmosphere Absolute Pressure (ECM).
    • Unplug the connector: EVAP canister purge valve (321).
    • Connect a test lamp to the female connector.
    • Start the engine and run it at idling speed.
    • Turn off the A/C.
    • Read value: Airmass Deviation from Calculated (ECM).

    If the diagnostic tool shows -15% to -25%, go to step  12, otherwise go to next step.

  8. Checking mass air flow when under partial load:
    • Turn the ignition switch to the OFF position.
    • Plug in the connector: EVAP canister purge valve.
    • Drive the car in top gear with a steady high engine load. Driving uphill is recommended.
    • Read value: Airmass Deviation from Calculated (ECM).
    If the diagnostic tool shows 15% to 25%, go to step  10, otherwise go to next step.
  9. Checking the fuel supply: Check the fuel pressure, fuel pump flow rate and injector flow rate capacity. Rectify any faults and go to step  14.
  10. Checking air leakage before flap: Check for an air leak in the system before the flap. If there is a fault in the system, rectify and go to step  13, otherwise go to next step.
  11. Changing components: Change the Mass Air Flow Sensor. When finished, go to step  13.
  12. Checking air leakage after flap:
    • Check that the purge valve is airtight, the brake servo is not faulty and crankcase ventilation is airtight.
    • Check that no air leakage occurs in the vacuum hoses.
    • Pinch off one hose at a time and observe the diagnostic tool's display to determine whether a leak occurs. It is OK if crankcase ventilation gives an increment of about 5% via the thin hose. The other hoses must not give any noticeable increment.
      NOTE: The EVAP canister purge valve's electrical connection must be disconnected for this check.
    If there is a leak in the system, first plug in the connector, EVAP canister purge valve, then rectify the fault and go to next step. If there is no leak in the system, plug in the connector, EVAP canister purge valve and go to step  11.
  13. Checking misfiring:
    • Activate: Reset Misfire and Knock Counters.
    • Take a current reading with the diagnostic tool. (Misfire Cyl #)
    • Drive the car with the same engine speed, air mass/combustion and coolant temperature as shown by Freeze Frame.
      NOTE: A few misfires in this load and engine speed range are acceptable.
    If any of the counters are incrementing continuously, the fault is still present. If the counters are all remaining relatively stable, the fault has been rectified.
  14. Implementation of driving cycle:
    • Clear DTCs.
    • Unplug the connector: EVAP canister purge valve (321).
    • Connect Test Lamp (86 1 857) to the female connector.
    • Start the engine and run it at idling speed until the coolant temperature exceeds 60°C.
    • Read value: Purge Adaptation (ECM).
    If the diagnostic tool shows -15% to +15%, go to step 15, otherwise the fault has not been rectified.
  15. Implementation of driving cycle (continued):
    • Turn the ignition switch to the OFF position.
    • Plug in the connector: EVAP canister purge valve.
    • Drive the car between 35-60 MPH (60-100 KM/H) with constant accelerator depression.
    • Read value: Multiplicative Adaptation (ECM).
    • Wait until the reading changes. Another speed can then be chosen immediately or the car stopped completely if desired. The engine must not be turned off however.
    • Wait for the reading to stabilize.
    If the diagnostic tool shows -15% to +15%, the fault has been rectified, otherwise the fault is still present in the system.
Fig 1: System Overview, Charge Air (4-Cyl)
G00207353Courtesy of SAAB-SCANIA OF AMERICA, INC.
Fig 2: Charge Air Cooling System (4-Cyl)
G00207354Courtesy of SAAB-SCANIA OF AMERICA, INC.
Fig 3: System Overview, Charge Air (V6)
G00207407Courtesy of SAAB-SCANIA OF AMERICA, INC.
Fig 4: Charge Air Cooling System (V6)
G00207408Courtesy of SAAB-SCANIA OF AMERICA, INC.
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