Rough, Unstable, Or Incorrect Idle And Stalling - Isuzu N 2008 Series Workshop Manual

1A-328 Engine Control System (4JJ1)

Rough, Unstable, or Incorrect Idle and Stalling

Checks
Definition:
Engine runs unevenly at idle. If severe, the engine or vehicle may shake. Engine idle speed may vary in RPM. Either condition
may be severe enough to stall the engine.
Preliminary Checks
Sensor Checks
Fuel System Checks
• Diagnostic System Check - Engine Controls.
• Remove the air cleaner and check for dirt, or for air ducts being plugged or
restricted. Replace as necessary.
• Inspect the ECM grounds for being clean, tight, and in their proper locations.
• Inspect that the harness connectors are correctly connected.
• Inspect the fuel type and quality.
• Inspect the programmed fuel injector ID code for each cylinder.
• Inspect the Scan Tool Data List in this section.
• Inspect the Service Bulletin.
Inspect the engine control sensors for the following conditions. Refer to Scan Tool Data
List in this section.
• Compare the Coolant Temperature with the Intake Air Temperature (IAT) and Fuel
Temperature (FT) parameters on a cold engine condition. If the difference among
temperature reading is more than 5°C (9°F) on a cold engine, check for high
resistance in each circuit or for a skewed sensor.
Notice: The mass air flow (MAF) sensor is heated and as a result the IAT may indicate
a higher than normal intake air temperature if the ignition switch is being ON.
• Observe the Fuel Rail Pressure (FRP) Sensor parameter with the engine OFF. The
FRP Sensor should read 0.9 to 1.0 volt with the ignition switch ON and engine OFF
after the engine has stopped running for a minimum of 1 minute. If not, check for
high resistance in each circuit or for a skewed sensor.
• Observe the Fuel Rail Pressure parameter at idle in Neutral. The Fuel Rail Pressure
should always be within 27 to 33 MPa (3,900 to 4,800 psi) after warm up.
• Crankshaft position (CKP) sensor is tight and the sensor rotor is not damaged.
• Camshaft position (CMP) sensor is tight and the timing chain sprocket is not
damaged.
Inspect the fuel system for the following conditions. Refer to Fuel System Check chart
and appropriate procedure in Section 1D Engine Fuel.
• Air in the fuel system.
• Water contamination in the fuel.
• Fuel waxing or icing.
• Fuel filter clogging.
• External fuel leaks or high engine oil level.
• Fuel leak off from the fuel pressure limiter valve and fuel injectors.
• Fuel lines between the fuel tank and fuel supply pump for being crushed, kinked,
tightness, cracks and plugged.
• A plugged fuel tank vent hose.
• Inside the fuel tank for any foreign material that may be getting drawn into the fuel
line pickup causing a blocked condition.
• Fuel supply pump operation.
Notice: The fuel supply pump must be timed to the engine and adjustment value must
be learned to the ECM.
• Perform the Cylinder Balance Test with a scan tool.
• Perform the Injector Forced Drive with a scan tool.
• Observe the Fuel Compensation for each cylinder at idle on the scan tool.
• Observe the FRP Regulator Feedback Current on the scan tool.
Action