Fuel Return System Diagnosis

Fuel Return System Diagnosis

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) 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 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 fuel cooler to the fuel tank.

Diagnostic Aids

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

Reference Information
Special Tools

J 44581 Fuel Line Disconnect Tool

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. 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 at the engine. 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. Repair or replace the fuel return line.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Fuel Feed Pipe Replacement (Service and Repair)
* Fuel Sender Assembly Replacement (Service and Repair)