Isuzu N-Series. Manual - part 723

6E-314    Engine Control System (4HK1) 

Rough, Unstable, or Incorrect Idle And Stalling

Exhaust System Checks

Inspect the exhaust system for a possible restriction. Refer to the Exhaust System
section.

• Inspect the exhaust brake valve for a stuck closed position.
• Inspect for a restriction in the exhaust pipes.

Engine Mechanical Checks

Inspect the engine mechanical for the following conditions. Refer to the Engine
Mechanical section.

• Inspect for poor cylinder compression. Proper compression is more than 1960 kPa

(284 psi) and variation of each cylinder is less than 294 kPa (43 psi).

• Improper mechanical timing (timing gear)
• Improper valve gap
• Broken or weak valve springs
• Worn camshaft lobes

Electrical System Checks

Inspect the engine electrical for the following conditions. Refer to the Engine Electrical
section.

• Inspect the glow plug control (preheating) system operation. Refer to Glow Control

System Check in this section.

• Inspect the fuel heater operation.
• Inspect for slow cranking speed.
• Inspect for weakened batteries.

Checks

Action

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

• 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 Engine Control Module (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 Tech 2 Data List in this section.

Sensor Checks

Inspect the engine control sensors for the following conditions. Refer to the Tech 2
Data List in this section.

• Use the Tech 2 to compare the Coolant Temperature with the Intake Air

Temperature (IAT) and Fuel Temperature (FT) 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 on the low reference circuit and signal circuit or for a
skewed sensor.

• Use the Tech 2 to observe the Fuel Rail Pressure (FRP) Sensor parameter with the

engine OFF. The FRP Sensor on the Tech 2 should read 0.9 – 1.0 volt with the key
ON and engine OFF after the engine has stopped running for a minimum of 2
minutes. If not, check for high resistance in the 5 volts reference circuit, low
reference circuit, signal circuits or for a skewed sensor.

• Use the Tech 2 to observe the Differential Fuel Rail Pressure parameter at idle in

Neutral. Differential Fuel Rail Pressure parameter should always be within 

± 5 MPa

(

± 725 psi).

• Inspect the crankshaft position (CKP) sensor is tight and the flywheel teeth are not

damaged.

• Inspect the camshaft position (CMP) sensor is tight and the camshaft gear flange

surface is not damaged.

Checks

Action

 Engine Control System (4HK1)    6E-315

Fuel System Checks

Inspect the fuel system for the following conditions. Refer to Fuel System Check Chart
and appropriate procedure in the Fuel System section.

• Inspect for air in the fuel system.
• Inspect for water contamination in the fuel.
• Inspect for external fuel leaks or high engine oil level.
• Measure the fuel return rate from the fuel injectors.
• Inspect the fuel lines between the fuel tank and fuel supply pump for being crushed

or kinked.

• Inspect for a plugged fuel tank vent hose.
• Inspect inside the fuel tank for any foreign material that may be getting drawn into

the fuel line pickup causing a blocked condition.

• Inspect the fuel lines between the fuel tank and fuel supply pump for tightness and

all fuel hoses for cuts, cracks and for the use of proper clamps.

• Inspect the 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 Injector Balancing test with the Tech 2. Repair the wire or replace the

appropriate fuel injector that does not change engine speed when commanded
OFF.

• Perform the Injector Forced Drive test with the Tech 2. Repair the wire or replace

the appropriate fuel injector that does not create a clicking noise (solenoid
operating noise), contents an interrupted noise or has abnormal nose when
commanded ON.

• Use the Tech 2 to observe the Cylinder Compensation for each cylinder at idle. If

there is a cylinder that is excessively high or low value, it may indicate faulty fuel
injector, weak or slightly seized cylinder or an incorrectly programmed fuel injector
ID code.

• Inspect the fuel rail pressure (FRP) regulator feedback current on the Tech 2.

Air Intake System Checks

Inspect the air intake system for the following conditions.

• Inspect the air cleaner and air intake ducts for a restriction, holes, or leaks.
• Inspect for a restriction in the turbocharger inlet duct.
• Inspect the intake throttle valve for a stuck closed position.
• Inspect for a restriction or leak in the intake manifold.

Exhaust System Checks

Inspect the exhaust system for a possible restriction. Refer to the Exhaust System
section.

• Inspect the exhaust brake valve for a stuck closed position.
• Inspect for a restriction in the exhaust pipes.

Engine Mechanical Checks

Inspect the engine mechanical for the following conditions. Refer to the Engine
Mechanical section.

• Inspect for poor cylinder compression. Proper compression is more than 1960 kPa

(284 psi) and variation of each cylinder is less than 294 kPa (43 psi).

• Improper mechanical timing (timing gear)
• Improper valve gap
• Broken or weak valve springs
• Worn camshaft lobes
• Inspect for incorrect basic engine parts such as camshaft, cylinder head, pistons,

etc.

Checks

Action

6E-316    Engine Control System (4HK1) 

High Idle

Additional Checks

• Electromagnetic interference (EMI) on the reference circuit can cause an engine

miss condition. The Tech 2 can usually detect EMI by monitoring the engine speed.
A sudden increase in speed with little change in actual engine speed change
indicates that EMI is present. If a problem exists, check routing of high voltage
components, such as fuel injector solenoid valve wiring, near the sensor circuits.

• Inspect for faulty engine mounts.
• Inspect faulty crank pulley.
• Inspect faulty generator & A/C compressor.
• Inspect the generator output voltage.
• Inspect the EGR system operating correctly. Refer to EGR Control System Check

in this section.

• Inspect the A/C operation.

Checks

Action

DEFINITION: Engine idle speed is higher than normal in regardless of engine coolant temperature.

Preliminary Checks

• Diagnostic System Check – Engine Controls.
• Inspect that the harness connectors are correctly connected.
• Inspect the idle up control knob turned fully counterclockwise.
• Use the Tech 2 to compare the engine speed and tachometer on the instrument

panel cluster (IPC).

• Inspect the fuel type and quality.
• Inspect the engine oil level.
• Inspect the Tech 2 Data List in this section.

Sensor Checks

Inspect the engine control sensors for the following conditions. Refer to the Tech 2
Data List in this section.

• Use the Tech 2 to compare the Coolant Temperature with the Intake Air

Temperature (IAT) and Fuel Temperature (FT) 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 on the low reference circuit and signal circuit or for a
skewed sensor.

• Use the Tech 2 to observe the Fuel Rail Pressure (FRP) Sensor parameter with the

engine OFF. The FRP Sensor on the Tech 2 should read 0.9 – 1.0 volt with the key
ON and engine OFF after the engine has stopped running for a minimum of 2
minutes. If not, check for high resistance in the 5 volts reference circuit, low
reference circuit, signal circuits or for a skewed sensor.

• Use the Tech 2 to observe the Differential Fuel Rail Pressure parameter at idle in

Neutral. Differential Fuel Rail Pressure parameter should always be within 

± 5 MPa

(

± 725 psi).

• Use the Tech 2 to observe the Idle Up Sensor parameter. Idle Up Sensor parameter

should be less than 0.7 volts at full turned counterclockwise. If not, check for high
resistance in low reference circuit or skewed sensor.

• Use the Tech 2 to observe the Accelerator Pedal Position (APP) Angle. APP Angle

parameter should change linearly from 0% to 100% according to the accelerator
pedal operation.

Fuel System Checks

Inspect the fuel system for the following conditions. Refer to the Fuel System section.

• Inspect the fuel injectors. Remove the injectors and visually inspect. (Injector tip(s)

may be damaged)

• Measure the fuel return from the fuel injectors. See Fuel System Check Chart in the

Fuel System section.

Checks

Action

 Engine Control System (4HK1)    6E-317

Cut Out, Misses

Checks

Action

DEFINITION: A constant jerking that follows the engine speed, usually more pronounced as the engine load increase. The
exhaust has a steady spitting sound at idle, low speed, or hard acceleration for the fuel starvation that can cause the engine to
cut-out.

Preliminary Check

• Diagnostic System Check – Engine Controls.
• Inspect that the harness connectors are correctly connected.
• Inspect the engine control module (ECM) grounds for being clean, tight, and in their

proper locations.

• Inspect the Tech 2 Data List in this section.

Sensor Checks

Inspect the engine control sensors for the following conditions. Refer to the Tech 2
Data List in this section.

• Use the Tech 2 to observe the Accelerator Pedal Position (APP) Angle. APP Angle

parameter should change linearly from 0% to 100% according to the accelerator
pedal operation.

• Use the Tech 2 to observe the Fuel Rail Pressure (FRP) Sensor parameter with the

engine OFF. The FRP Sensor on the Tech 2 should read 0.9 – 1.0 volt with the key
ON and engine OFF after the engine has stopped running for a minimum of 2
minutes. If not, check for high resistance in the 5 volts reference circuit, low
reference circuit, signal circuits or for a skewed sensor.

• Use the Tech 2 to observe the Differential Fuel Rail Pressure parameter at idle and

W.O.T. (accelerator pedal full travel) in Neutral. Differential Fuel Rail Pressure
parameter should always be within 

± 5 MPa (± 725 psi).

• Inspect the crankshaft position (CKP) sensor is tight and the flywheel teeth are not

damaged.

• Inspect the camshaft position (CMP) sensor is tight and the camshaft gear flange

surface is not damaged.

Fuel System Checks

Inspect the fuel system for the following conditions. Refer to Fuel System Check Chart
and appropriate procedure in the Fuel System section.

• Inspect for air in the fuel system.
• Inspect for water contamination in the fuel.
• Inspect the fuel lines between the fuel tank and fuel supply pump for being crushed

or kinked.

• Inspect for a plugged fuel tank vent hose.
• Inspect inside the fuel tank for any foreign material that may be getting drawn into

the fuel line pickup causing a blocked condition.

• Inspect the fuel lines between the fuel tank and fuel supply pump for tightness and

all fuel hoses for cuts, cracks and for the use of proper clamps.

• Perform the Injector Balancing test with the Tech 2. Repair the wire or replace the

appropriate fuel injector that does not change engine speed when commanded
OFF.

• Perform the Injector Forced Drive test with the Tech 2. Repair the wire or replace

the appropriate fuel injector that does not create a clicking noise (solenoid
operating noise), contents an interrupted noise or has abnormal nose when
commanded ON.

• Use the Tech 2 to observe the Cylinder Compensation for each cylinder at idle. If

there is a cylinder that is excessively high or low value, it may indicate faulty fuel
injector, weak or slightly seized cylinder or an incorrectly programmed fuel injector
ID code.

Air Intake System Checks

Inspect the air intake system for the following conditions.

• Inspect the air cleaner and air intake ducts for a restriction, holes, or leaks.
• Inspect for a restriction in the turbocharger inlet duct.
• Inspect the intake throttle valve for a stuck closed position.
• Inspect for a restriction or leak in the intake manifold.