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 driveability or starting issues. Refer to Fuel System Specifications [1][2]Fuel Delivery and Air Induction 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 Control Module References for scan tool information
Special Tools
* CH-48027 - Digital Pressure Gauge
* J 44581 - Fuel Line Disconnect Tool
* J-23738-A - MityVac
Circuit/System Verification
1. Inspect the fuel system for damage or external leaks.
• If damage or external leaks were found
Repair as necessary.
• If no damage or external leaks were found
2. Verify that adequate fuel is in the fuel tank.
• If there is not adequate fuel in the fuel tank
Add fuel and revaluate.
• If there is adequate fuel in the fuel tank
3. Verify the fuel filter pressure switch parameter does not display restricted.
• If displaying restricted
1. Test the fuel filter and fuel feed line for a restriction
• If restriction is found
Repair as necessary.
• If no restriction is found
Refer to DTC P1082 P1082 for further diagnosis.
• If not displaying restricted
4. Observe the Fuel Rail Pressure Sensor parameter with a scan tool. During engine cranking, the pressure should be at least 10 MPa (1450 psi).
• If less than 10 MPa (1450 psi)
Refer to Fuel Pressure Regulator Diagnosis Testing and Inspection.
• If 10 MPa (1450 psi) or greater
5. Observe the Fuel Rail Pressure Sensor parameter with a scan tool, at idle the pressure should be close to the Desired Fuel Rail Pressure. As the engine speed increases, the Desired Fuel Rail Pressure and the Actual Fuel Rail Pressure should closely match each other
• If pressure does not closely match
Refer to Fuel Pressure Regulator Diagnosis Testing and Inspection.
• If pressure closely matches
6. Refer to Circuit/System Testing.
Circuit/System Testing
Note: You must perform the Circuit/System Verification before proceeding with Circuit/System Testing
1. Install the CH-48027 - Digital Pressure Gauge to the fuel system service port on the right front side of the engine.
2. Command the fuel pump ON with a scan tool until 68 kPa (10 psi) is indicated on the CH-48027 - digital pressure gauge. Fuel pressure should not drop below 13.7 kPa (2 psi) in less than 1 minute.
• If the fuel pump did not run and no fuel pressure was observed
Refer to Fuel Pump Electrical Circuit Diagnosis Fuel Pump Electrical Circuit Diagnosis.
• If fuel pressure drops below 13.7 kPa (2 psi)
Repair leaks or replace any components found to be damaged or leaking.
• If unable to achieve 68 kPa (10 psi)
Test the fuel supply lines and fuel filter for restrictions, damage or leakage. If the lines and filter test normal, replace the fuel sender assembly.
• If 68 kPa (10 psi) fuel pressure can be achieved and does not drop below 13.7 kPa (2 psi)
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 Service and Repair.
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 - Digital Pressure Gauge displays greater the 10 in. Hg of vacuum.
• If less than 10 in. Hg
Replace the G18 High Pressure Fuel Pump.
• If vacuum is present during cranking but bleeds off rapidly
Repair the leak in the fuel supply line.
• If greater than 10 in. Hg
Note: Changes in engine speed can cause an increase of air bubbles that enter the clear hose. An air pocket at the top of the clear hose loop is considered a normal condition.
7. Remove the rubber plug from the fuel supply line and install a piece of clear hose. 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 clear hose for air bubbles.
• If air bubbles were observed entering the clear hose
Test the fuel feed lines and sender for leaks.
• If no air bubbles were present
8. All OK.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* Engine Fuel Vacuum Switch Replacement Service and Repair
* Fuel Injection Pump Replacement Service and Repair
* Fuel Sender Assembly Replacement Service and Repair