Fuel Return System Diagnosis
Fuel Return 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 used to cool and lubricate the fuel injectors and the fuel injection pump. The fuel return system is designed to recirculate some of the fuel to the fuel conditioning module and to the fuel tank. The engine control module (ECM) regulates the fuel rail pressure by controlling the two fuel pressure regulators. The fuel pressure regulators bypass the excess fuel to the return side of the fuel system. Some of the excess fuel is used to cool the fuel injectors and provide back pressure to aid in proper injector performance.
The constant pressure regulator maintains 4-11 bar (58-160 psi) of fuel pressure within the fuel injector return lines. This pressure is required to provide proper fuel injector operation. In the event the vehicle runs out of fuel or if injector return line pressure falls below 3 bar (43 psi), a check valve in the low pressure feed line will open allowing fuel from the low pressure pump to fill the injector return line system. The check valve opens at less than 4 bar (58 psi) of injector return line pressure. When the fuel injector return line pressure increases over 11 bar (160 psi) the constant pressure regulator opens to regulate the injector return line pressure.
Diagnostic Aids
* Bent or pinched fuel return lines may cause an external fuel leak or engine performance concerns.
* Low injector return line pressure may cause a No-Start condition.
* Low injector return line pressure may cause performance concerns.
* Contaminated fuel or improper fuel may cause engine performance concerns.
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-50375 Fuel Injector Return Line Adaptor
Circuit/System Verification
Inspect the fuel return hoses and lines on the engine and chassis for any external fuel leaks, bends, pinches or damage.
Circuit/System Testing
Note: DO NOT apply more than the specified air pressure to the return line.
1. Disconnect the fuel return line at the engine. Refer to Metal Collar Quick Connect Fitting Service (Metal Collar Quick Connect Fitting Service).
2. Remove the fuel filler cap.
3. Apply 6.9 kPa (1 psi) of pressure air into the fuel return line to the tank. Air flow should be detected at fuel filler tube.
4. If air flow was not detected at the fuel filler tube, disconnect the fuel return line at the fuel sender and apply 6.9 kPa (1 psi) of air pressure into the fuel return line at the engine.
• If air flow was present, replace the fuel sender assembly.
• If no air flow was present, inspect for a restricted or a damaged fuel return line. Replace the fuel return line.
5. With a scan tool, verify that the fuel rail pressure is above 10 MPa (1450 psi) during crank or 30 MPa (4350 psi) at idle.
• If fuel rail pressure is less than the specified amount, refer to Fuel System Diagnosis (Component Tests and General Diagnostics).
6. Disconnect a fuel injector return line and place the end into a suitable container.
Note: A small amount of fuel will leak out of the fuel injector return fitting. Place a lint free towel around the injector to prevent fuel leakage.
7. Crank the engine for 5 seconds, verify fuel flow is present.
• If no fuel flows into the container, check the fuel feed line for kinks or restrictions. If the fuel feed line tests normal, refer to Fuel System Diagnosis (Component Tests and General Diagnostics).
Note: Idle the engine for 2 minutes to allow pressure to build in the injector return lines. If the vehicle is a No-Start, it may be necessary to crank the engine several times in order to build pressure.
8. Install the CH-50375 and gauge on an injector return line. Crank or start the engine and verify that the fuel pressure is greater than 4 bar (58 psi).
• If less than the specified value, replace the constant pressure regulator.
9. Run the engine at 2000 RPM for 2 minutes and verify the return line fuel pressure is less than 14 bar (200 psi).
• If greater than the specified value, replace the constant pressure regulator.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
Fuel Return Hose Replacement (Fuel Return Hose Replacement)