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Fuel System Diagnosis

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    • Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Circuit/System Description

The control module enables the fuel pump relay when the ignition switch is turned ON. The control module will disable the fuel pump relay within 2 s unless the control module detects crankshaft reference pulses. The control module continues to enable the fuel pump relay as long as crankshaft reference pulses are detected. The control module disables the fuel pump relay within 2 s if crankshaft reference pulses cease to be detected and the ignition remains ON.

The fuel tank stores the fuel supply. An electric fuel pump attaches to the fuel pump module inside the fuel tank. Fuel flows to the fuel filter/heater element housing, which combines a water separator, fuel heater element, and a filter element. The fuel is then delivered to the high-pressure pump. The mechanical fuel injection pump is located at the front of the engine. The mechanical fuel injection pump is engine-driven by the timing belt. From the high-pressure pump, the pressurised fuel flows to the fuel rail. The fuel rail is equipped with a fuel rail pressure sensor and a fuel pressure regulator.

The Fuel Return System returns un-used fuel to the fuel tank. Fuel is used to cool and lubricate the fuel injectors and fuel injection pump. If the high side fuel pressure becomes excessive, the fuel pressure regulator releases the fuel into the fuel return system. The return fuel travels through the fuel lines and then to the fuel tank.

Diagnostic Aids

A Cranks But Will Not Run or Hard Start symptom may exist if air is being drawn into the fuel injection system due to the following conditions:

    • Deformed or cut O-rings at the fuel supply line connections
    • Improperly seated fuel supply line fittings
    • Porous or weathered rubber fuel supply lines
    • Fuel filter vent screw not tighten or cross threaded
    • Fuel filter not tighten properly

Reference Information

Schematic Reference

Engine Controls Schematics : LLW

Description and Operation

Fuel System Description

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Special Tools

    • EN-49799 Fuel Pressure Gauge Adapter
    • EN 49495 Fuel Pressure Gauge

For equivalent regional tools, refer to Special Tools .

Circuit/System Verification

Note: 

   • Inspect the fuel system for damage or external leaks before proceeding.
   • Verify that adequate fuel is in the fuel tank before proceeding.

  1. Ignition ON, command the fuel pump relay ON and OFF with a scan tool. You should hear the fuel pump turn ON and OFF.
    2. ⇒If the fuel pump does not turn ON, refer to Fuel Pump Electrical Circuit Diagnosis .
  2. Observe the Actual Fuel Rail Pressure parameter with a scan tool. During engine cranking, the pressure should be at least 415 kPa (60 PSI). With the engine running at idle the pressure should be close to the Desired Fuel Rail Pressure. As the engine speed is increased, the Desired Fuel Rail Pressure and the Actual Fuel Rail Pressure should closely match each other.

Circuit/System Testing

    Note: It may be necessary to remove engine components for the visual inspections.

  1. Verify the following conditions do not exist:
    2. • Hoses and lines that are flattening when the engine is running, or have kinks that may restrict the flow of fuel.
    • Fuel leaks between the fuel tank and the fuel injection pump.

       Note: A fuel return line that is leaking , is a good indication there is a restriction in the return system.

    • Fuel leaks in the fuel return lines.
    ⇒Repair any leaks, restrictions, or replace any components found to be damaged or leaking.
  2. Ignition OFF, install the EN-49799 Fuel Pressure Gauge Adapter and the EN 49495 Fuel Pressure Gauge to the fuel supply pipe at the fuel filter assembly. Refer to Fuel Pressure Gauge Installation and Removal .
  3. Prime the fuel system. Refer to Fuel System Priming .

    4. Note: The fuel pump relay may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.

  4. Ignition ON, command the fuel pump relay ON with a scan tool. Verify the fuel pressure is between 415-535 kPa (60-78 PSI) and remains steady for 5 minutes.
    5. ⇒If less than the specified range, remove the fuel tank and test, inspect, and repair the items listed below. If all items test normal, replace the G12 Fuel Pump.
    • Leaking fuel lines
    • Restricted fuel feed pipe
    • Restricted or plugged strainer
    • Stuck or binding fuel level float
    • Inspect the harness connectors and the ground circuits of the fuel pump for poor connections.
  5. If the fuel system components test normal, test or replace the fuel filter.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Fuel Filter Replacement
    •  Fuel Tank Replacement
   


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