Fuel System Diagnosis
Fuel System Diagnosis
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions)provides an overview of each diagnostic category.
Circuit/System Description
Fuel is supplied to the fuel injection pump from the fuel tank sending unit, an electric fuel pump draws fuel from the tank and pushes fuel through the lines to the fuel conditioning module. The fuel conditioning module consists of a fuel filter, water in fuel sensor, fuel filter vacuum switch and a hand primer. The fuel injection pump has a low pressure pump that draws fuel in from the fuel conditioning module and supplies the high pressure side of the fuel injection pump, exhaust after treatment fuel injector and the fuel injector return system, with 4 bar (58 psi) of pressure. Excess fuel is returned back to the fuel tank or the conditioning module. Fuel that is returned to the conditioning module helps to heat the fuel going to the fuel injection pump.
This diagnostic is used to test the low pressure fuel supply to the fuel injection pump.
Diagnostic Aids
* An Engine 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 tightened properly
* Contaminated or improper fuel may cause driveablity or starting issues. Refer to Fuel System Specifications (Fuel System Specifications) and Contaminants-in-Fuel Diagnosis (Contaminants-In-Fuel Diagnosis).
* A leaking fuel injector return line constant pressure regulator may cause No-Start, Hard Start, or Stall conditions.
Reference Information
Description and Operation
Fuel System Description (Fuel System Description)
Scan Tool Reference
Control Module References (Programming and Relearning) for scan tool information
Special Tools
* CH-48027 Digital Pressure Gauge
* EN-47969 Fuel Supply Diagnostic Hose
* J 44581 Fuel Line Disconnect Tool
* J-23738-A MityVac
Circuit/System Verification
1. Inspect the fuel system for damage or external leaks before proceeding.
2. Verify that adequate fuel is in the fuel tank before proceeding.
3. Observe the Actual Fuel Rail Pressure parameter with a scan tool. During engine cranking, the pressure should be at least 10 MPa (1450 psi). With the engine running at idle the pressure should be close to the Desired Fuel Rail Pressure. As the engine speed increase, the Desired Fuel Rail Pressure and the Actual Fuel Rail Pressure should be within 2 MPa (290 psi) of each other.
4. Verify the fuel filter vacuum switch parameter does not display restricted.
Circuit/System Testing
Note: You must perform the Circuit/System Verification before proceeding with Circuit/System Testing
1. Install the CH-48027 to the fuel system service port on the right front side of the engine.
2. Command the fuel pump ON with a scan tool and observe the fuel pressure gauge to ensure that fuel pressure is present. Visually inspect the engine and chassis for leaks or damage to the fuel hoses, fuel lines, and fuel system components.
• Repair any leaks or replace any components found to be damaged or leaking.
• If the fuel pump did not run and fuel pressure was not observed, refer to Fuel Pump Electrical Circuit Diagnosis (Fuel Pump Electrical Circuit Diagnosis).
• If the fuel pump did run and fuel pressure was not observed , inspect the fuel feed line for a vacuum leak between the primary fuel filter assembly and the fuel tank sender.
3. If leaks or damage were not found on the fuel system, turn OFF the fuel pump and disconnect the fuel supply line at the engine. Refer to Metal Collar Quick Connect Fitting Service (Metal Collar Quick Connect Fitting Service).
4. Cap off the fuel supply pipe on the engine with a rubber plug and a clamp.
5. Remove the fuel pump relay.
6. Engine cranking, verify the CH-48027 displays greater the 10 in. Hg of vacuum.
• If vacuum is present during cranking but bleeds off rapidly, repair the leak in the fuel supply line.
• If less than the specified value, replace the G18 high pressure fuel pump.
Note: Changes in engine speed can cause an increase of air bubbles that enter the EN-47969 . An air pocket at the top of the EN-47969 loop is considered a normal condition.
7. Remove the rubber plug from the fuel supply line and install the EN-47969 . Create a vertical loop in the hose to observe the incoming fuel for air bubbles. Command the Fuel Pump ON with a scan tool and prime the fuel system until all air has been purged. Start and run the engine for at least 10 minutes to allow the fuel system to stabilize. At a stabilized idle, observe the fuel entering the EN-47969 for air bubbles.
• If air bubbles were observed entering the transparent hose, test the fuel sender for leaks. Refer to Fuel Sender Assembly Replacement (Service and Repair).
• If no air bubbles are present , remove the EN-47969 and run the engine to ensure no fuel leaks exist.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Fuel Injection Pump Replacement (Fuel Injection Pump Replacement)
* Fuel Sender Assembly Replacement (Service and Repair)