Isuzu N-Series. Manual — part 793

Engine Control System (4JH1) 6E-223

Temperature Sensitivity

• An intermittent condition may occur when a component/connection reaches normal

operating temperature. The condition may occur only when the component/
connection is cold, or only when the component/connection is hot.

• If the intermittent is related to heat, review the data for a relationship with the

following:

- High ambient temperatures
- Underhood/engine generated heat
- Circuit generated heat due to a poor connection, or high electrical load
- Higher than normal load conditions, towing, etc.

• If the intermittent is related to cold, review the data for the following:

- Low ambient temperatures – In extremely low temperatures, ice may form in a

connection or component. Test for water intrusion.

- The condition only occurs on a cold start.
- The condition goes away when the vehicle warms up.

• Information from the customer may help to determine if the trouble follows a pattern

that is temperature related.

Electromagnetic Interference (EMI)
and Electrical Noise

Some electrical components/circuits are sensitive to EMI or other types of electrical
noise. Inspect for the following conditions:

• A misrouted harness that is too close to high voltage/high current devices such as

injection components, motors, generator etc. These components may induce
electrical noise on a circuit that could interfere with normal circuit operation.

• Electrical system interference caused by a malfunctioning relay, or the engine

control module (ECM) driven solenoid or switch. These conditions can cause a
sharp electrical surge. Normally, the problem will occur when the malfunctioning
component is operating.

• Improper installation of non-factory or aftermarket add on accessories such as

lights, 2-way radios, amplifiers, electric motors, remote starters, alarm systems, cell
phones, etc. These accessories may lead to an emission related failure while in
use, but do not fail when the accessories are not in use.

• Test for an open diode across the A/C compressor clutch and for other open

diodes. Some relays may contain a clamping diode.

• Test the generator for a bad rectifier bridge that may be allowing AC noise into the

electrical system.

Incorrect ECM Programming

• There are only a few situations where reprogramming a ECM is appropriate:

Important:
DO NOT reprogram the ECM with the SAME software/calibration files that are already
present in the ECM. This is not an effective repair for any type of driveability problem.

- A ECM from another vehicle is installed.
- Revised software/calibration files have been released for this vehicle.

• Verify that the ECM contains the correct software/calibration. If incorrect

programming is found, reprogram the ECM with the most current software/
calibration.

Duplicating Failure Conditions

• If none of the previous tests are successful, attempt to duplicate and/or capture the

failure conditions.

• An alternate method is to drive the vehicle with the DMM connected to a suspected

circuit. An abnormal reading on the DMM when the problem occurs, may help you
locate the problem.

Tech 2 Snapshot

The Tech 2 can be set up to take a Snapshot of the parameters available via serial
data. The Snapshot function records live data over a period of time. The recorded
data can be played back and analyzed. The Tech 2 can also graph parameters singly
or in combinations of parameters for comparison. The Snapshot can be triggered
manually at the time the symptom is noticed, or set up in advance to trigger when a
DTC sets.
An abnormal value captured in the recorded data may point to a system or component
that needs to be investigated further. Refer to the Tech 2 user instructions for more
information on the Snapshot function.

Checks

Action

6E-224 Engine Control System (4JH1)

Hard Start

Checks

Action

DEFINITION: The engine cranks OK, but does not start for a long time. The engine does eventually run, or may start but
immediately dies.

Preliminary Checks

• Diagnostic System Check – Engine Controls.
• Ensure the driver is using the correct starting procedure.
• Inspect the engine control module (ECM) and fuel injection pump control unit (PCU)

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 Tech 2 Data List in this section.
• Inspect the Service Bulletins for ECM software updates.

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 Engine Coolant Temperature (ECT) 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.

Notice:
The mass air flow (MAF) sensor is heated and as a result the IAT sensor may indicate
a higher than normal intake air temperature if the ignition switch is being ON. FT
sensor is internal to the PCU and it is part of the fuel injection pump assembly.

• Inspect the crankshaft position (CKP) sensor is tight and the flywheel

circumference is not damaged.

Engine Control System (4JH1) 6E-225

Fuel System Checks

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

• Inspect for water contamination in the fuel.
• Inspect for external fuel leaks or fuel leakage into the engine oil.
• Inspect the fuel lines between the fuel tank and fuel injection pump for tightness

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

Notice:
The fuel system from the fuel tank(s) to the fuel injection pump is under a slight
vacuum with the engine running. As a result, air can enter the fuel system if these
connections are not tight. Air in the fuel system will cause fuel injection pump internal
pressure fluctuations especially at high engine speed and load.

• Inspect for air in the fuel system.

Notice:
If many air bubbles appear in the fuel, check the fuel system line connections between
the fuel tank and the fuel injection pump for tightness and all fuel hoses for cuts, cracks
and for the use of proper clamps.

a. Remove the fuel hose that connects to the fuel injection pump suction side.
b. Substitute a clear hose.

Notice:
A hose must be cleaned.

c. Connect the clear hose to the fuel injection pump.
d. Bleed the fuel system.
e. Let the engine run at idle for at least 2 minutes.
f. Accelerator the engine between idle and W.O.T. (accelerator pedal full travel)

many times while observing the clear hose.

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

the fuel line pickup causing a blocked condition. Draw fuel from the fuel tank at the
fuel line (as close to the fuel tank as possible) going to the fuel pickup tube to verify
a clean stream of fuel comes out (use the hand-held vacuum pump 5-8840-0279-0/
J-23738-A with a clear hose or equivalent). This will ensure the fuel pickup tube is
not cracked drawing air into the fuel line.

• Inspect the fuel injection pump operation.

Notice:
The fuel injection pump must be timed to the engine.

• Inspect the eye bolt for any type of restriction or collapsed gauze filter.

Notice:
If any type of restriction found, check for a condition that causes contaminated fuel,
such as the customer is using an aftermarket fuel filter or extended maintenance
interval. Also inspect fuel waxing or icing that is caused by an incorrect fuel type used
in winter season or water intrusion in the fuel system.

• Inspect the fuel injection nozzle(s) for proper splay condition or operating pressure.

Notice:
Only first stage of operating pressure can be checked.

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 for a restriction or deposit in the intake throttle bore.
• 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 for a restriction in the catalytic converter or exhaust pipes.

Checks

Action

6E-226 Engine Control System (4JH1)

Rough, Unstable, or Incorrect Idle And Stalling

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 2100 kPa

(309 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.
• 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) and fuel injection pump control unit

(PCU) 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 Tech 2 Data List in this section.
• Inspect the Service Bulletins for ECM software updates.

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 Engine Coolant Temperature (ECT) 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.

Notice:
The mass air flow (MAF) sensor is heated and as a result the IAT sensor may indicate
a higher than normal intake air temperature if the ignition switch is being ON. FT
sensor is internal to the PCU and it is part of the fuel injection pump assembly.

• Use the Tech 2 to compare the MAF Sensor parameter with the Desired MAF

parameter. Start the engine and warm up (allow engine coolant temperature to
reach at least 60

°C [140°F]). The MAF Sensor parameter must follow the Desired

MAF parameter within 100mg/strk. If not, inspect the air intake system, EGR
system components and contaminated, skewed or slow MAF sensor.

• Inspect the crankshaft position (CKP) sensor is tight and the flywheel

circumference is not damaged.

Checks

Action