Isuzu N-Series. Service manual - part 525

6C-8   Fuel System

Trouble Shooting

Problems with starting

Unstable idling

Insufficient power

Maximum engine speed is too low

Condition

Possible Cause

Correction

Problems with starting

Fuel tank is empty

Fill the tank.

Air has entered the fuel system.

Bleed the air.

Fuel line is clogged or damaged.
Connection to the fuel line is loose.

Repair or replace the fuel line. 
Re-tighten the connection.

Fuel filter element is clogged.

Replace the cartridge.

Fault in the feed pump

Replace the fuel supply pump.

Regulating valve is open.

Replace.

Sticking of the fuel injector nozzle

Replace the fuel injector.

Defective engine control system

Diagnose 

the 

engine 

control

system.

Condition

Possible Cause

Correction

Hunting during idling

Air has entered the fuel system.

Bleed air from the fuel system.

Leakage or clogging of the fuel
system

Repair or replace the fuel system.

Water particles or foreign matter in
the fuel system.

Replace the fuel.

Fuel 

filter 

element 

is 

clogged.

Replace the fuel filter element
(cartridge).

Sticking of the fuel injector nozzle

Replace the fuel injector.

Defective engine control system

Diagnose 

the 

engine 

control

system.

Condition

Possible Cause

Correction

Insufficient horsepower

Air has entered the fuel system.

Bleed air from the fuel system.

Leakage or clogging of the fuel
system

Repair or replace the fuel system.

Water mixes in the fuel system

Replace the fuel.

Fuel filter element is clogged.

Replace  the  element  or  the
cartridge.

Sticking of the fuel injector nozzle

Replace the fuel injector.

Defective engine control system

Diagnose 

the 

engine 

control

system.

Condition

Possible Cause

Correction

Maximum engine speed is too low

Fuel line is clogged or damaged.

Repair or replace the fuel line.

Defective engine control system

Diagnose 

the 

engine 

control

system.

Fuel System   6C-9

Engine does not stop

Exhaust gas is blue or black

Fuel System Check

Description
The common rail fuel system is comprised of two fuel
Pressure sections: a suction side between the fuel tank
and the fuel supply pump and a high-pressure side
between the fuel supply pump and the fuel injectors.
Fuel is drawn from the fuel tank via a feed pump and
then pumped into the fuel rail by two plungers, all of
which are internal to the fuel supply pump. This high
pressure is regulated by the engine control module
(ECM) using the fuel rail pressure (FRP) regulator
dependant upon values from the FRP sensor attached
to the fuel rail. In case of fuel rail overpressure, a
pressure limiter valve threaded into the fuel rail will
open to release overpressure and return fuel back to
the fuel tank. 
The Fuel System Check diagnostic table directs the
service technician to the appropriate fuel system
diagnosis. The diagnostic table assumes the following
conditions are met:

• The batteries are completely charged.
• The engine cranking speed is normal.
• There is adequate fuel in the fuel tank.
• There is no air in the fuel line.

Diagnostic Aids

• The fuel system from the fuel tank(s) to the fuel

supply 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 or if there
is a crack in one of the fuel hoses. Air in the fuel
system will cause fuel rail pressure fluctuations
especially at high engine speed and load.

• If the fuel tank is empty or near empty, air might be

allowed to go into the fuel system. With air in the
fuel system, smooth flow of fuel into the supply
pump is interrupted. Perform bleeding of fuel
system after refilling.

Condition

Possible Cause

Correction

Engine does not stop

Defective engine control system

Diagnose 

the 

engine 

control

system.

Condition

Possible Cause

Correction

Exhaust gas is blue or black.

Reduction 

in 

injection-valve

opening pressure or defective
atomizing status

Replace the fuel injector.

Engine control system malfunction

System diagnosis.

Step

Action

Value(s)

Yes

No

1

1.

Install the Tech 2.

2.

Turn OFF the ignition for 30 seconds.

3.

Turn ON the ignition, with the engine
OFF.

4.

Observe the Diagnostic Trouble Code
(DTC) Information of the engine control
module with a Tech 2.

Are any DTC(s) set?

—

Refer to 

Applicable DTC in 

Engine Control 

Section

Go to Step 2

6C-10   Fuel System

2

1.

Inspect the high pressure side between
the fuel supply pump and the fuel
injectors for fuel leakage. The following
components may contain an external
leak:

• Fuel supply pump
• Fuel rail
• Pressure limiter valve
• Flow  damper  valve
• Fuel rail pressure (FRP) sensor
• Fuel pipe between the fuel supply

pump and fuel rail

• Fuel pipe between the fuel rail and

fuel injectors

• Each fuel pipe sleeve nuts

Notice:
Fuel may leak under the cylinder head cover
from the inlet high pressure line. In such case,
the engine oil level will rise. Inspect for fuel in
the engine oil.

2.

Repair any fuel system leaks as
necessary.

Did you find and correct the condition?

—

Go to Step 18

Go to Step 3

3

1.

Turn OFF the ignition.

2.

Check the fuel system line connections
between the fuel tank and the fuel supply
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 supply 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
rail pressure fluctuations especially at high
engine speed and load.

3.

Pump the priming pump on the fuel filter
until it becomes firm .If there is a leak on
the suction side of the fuel system
between the priming pump and the fuel
supply pump, the priming pump will not
build up sufficient firmness and fuel
leakage may occur.

4.

Repair any fuel system leaks as
necessary.

Did you find and correct the condition?

—

Go to Step 18

Go to Step 4

Step

Action

Value(s)

Yes

No

Fuel System   6C-11

4

1.

Make sure the fuel tank(s) have adequate
fuel and the fuel quality is good (take a
sample).

2.

Bleed the air from the fuel system as
necessary. Refer to Fuel System Air
Bleeding in this section.

3.

Start the engine and let idle. If the engine
does not start, continue to crank. 

4.

Observe the Differential Fuel Rail
Pressure parameter with the Tech 2 while
cranking over the engine or running the
engine.

Is the Differential Fuel Rail Pressure
parameter within the specified value?

±5MPa

±(725 psi)

System OK

Go to Step 5

5

1.

Turn OFF the ignition.

2.

Disconnect the fuel injector in-line
harness connector (H-125) from the
cylinder head cover case.

3.

Remove the rubber fuel hose from the
fuel filter housing (fuel supply pump side).
Use a pan to catch the fuel leakage from
the removed fuel line.

Important:
The fuel vacuum pump/pressure gauge
connector and the adapter hose must be
cleaned before connecting to the fuel line. 
Otherwise, foreign material internal to the
tools line may damage the fuel supply pump.

4.

Connect the suction side fuel pressure
adapter 5-8840-2844-0 (EN-47667) with
fuel vacuum/pressure gauge assembly
5-8840-2844-0 (J-44638) in series with
the filter housing and the disconnected
fuel hose. Ensure the service tool and
fuel line connections are tight.

5.

Bleed the fuel system by priming the
priming pump until it becomes firm, then
crank over the engine for a maximum of 5
seconds. Perform procedure three times.

6.

Fully clamp off a fuel hose as close to the
fuel tank as possible (this will draw
vacuum on the fuel system). You can also
disconnect a fuel line and plug it.

7.

Crank over the engine (for no longer than
15 seconds) while monitoring the fuel
pressure/vacuum gauge.

Notice:
Release the clamp or open the plug when the
gauge is likely to be more than 8 inHg during
the test.

Can a vacuum of at least the specified amount
be pulled on the fuel system?

8 inHg

Go to Step 8

Go to Step 6

Step

Action

Value(s)

Yes

No