High Idle Speed - Isuzu 2008MY N Series Workshop Manual

1A-352 Engine Control System (4HK1)

High Idle Speed

Checks
Definition:
Engine idle speed is higher than normal in regardless of engine coolant temperature or DPD system regeneration.
Preliminary Checks
Sensor Checks
Fuel System Checks
• Diagnostic System Check - Engine Controls.
• Inspect that the harness connectors are correctly connected.
• Use the scan tool to compare the engine speed and tachometer on the
instrument panel (IP) cluster.
• Inspect the A/C operation.
• Inspect the fuel type and quality.
• Inspect the engine oil level.
• Inspect the Scan Tool Data List in this section.
• Inspect the Service Bulletin.
Inspect the engine control sensors for the following conditions. Refer to the
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.
• Observe the Accelerator Pedal Position (APP). APP parameter should
change linearly from 0 to 100% according to the accelerator pedal
operation.
• Observe the Idle Up Sensor. Idle Up Sensor parameter should read less
than 0.6 volts at full counterclockwise. If not, check for high resistance in
the low reference circuit or skewed sensor.
• Observe the PTO Remote Throttle Sensor. PTO Remote Throttle Sensor
parameter should read less than 0.4 volts at lowest position. If not, check
for high resistance in the low reference circuit or skewed sensor.
Inspect the fuel system for the following conditions. Refer to Fuel System in
Section 1D Engine Fuel.
• Fuel injectors. Remove the injectors and visually inspect. (Injector tip(s)
may be damaged)
Action