Fuel Return System Diagnosis

Fuel Return System Diagnosis

Diagnostic Instructions

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

Circuit/System Description

Fuel is used to cool and lubricate the fuel injectors and fuel injection pump. The fuel return system is designed to return this fuel to the fuel tank. If the high side fuel pressure becomes excessive, the fuel rail pressure relief valve releases the fuel into the fuel return system. The return fuel travels through the diesel fuel conditioning module and the fuel cooler, then to the fuel tank.

Diagnostic Aids

Bent or pinched fuel return lines may cause an external fuel leak or engine performance concerns.

Circuit/System Verification

Inspect the fuel return hoses and lines on the engine and chassis for any external fuel leaks.

Circuit/System Testing

Important: DO NOT apply more than the specified air pressure to the return line.

1. Disconnect the fuel return line at the engine.
2. Remove the fuel filler cap.
3. Apply 1 psi of air into the fuel return at the engine and listen for air flowing from the tank at the filler tube.
4. If air flow was not detected, disconnect the fuel return line at the fuel sender and retest using 1 psi of air.

• 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. Repair or replace the fuel return line.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

* Fuel Return Pipe Replacement
* Fuel Sender Assembly Replacement Verification Tests