Bleeding The Fuel System in a Marine Diesel Engine

To bleed the fuel system, or part thereof, means to remove all the air from the fuel lines and chambers in the system. This is done by pumping fuel through the lines and venting the air/fuel from various bleed points in the system.

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A fuel system will need to be bled after changing the fuel filters, running out of fuel, if there is water in the fuel, if there are loose connections causing air leaks, if you have taken on poor quality fuel, or if there is bacterial growth in the marine diesel fuel tanks.

Always follow the correct progression of bleeding as shown in the diagram. To pump the fuel you must first operate the lift pump to bleed the low pressure side of the system. The lift pump must be pumped many times to ensure complete bleeding. Ensure your lift pump works well. Aftermarket bleeding pumps can be fitted.

fuel system schematics

If pump does not operate (that is, the lever is slack), check if rocker arm is depressed. Turn engine over and try again.

First bleed primary filter by loosening bleed screw on top of housing. Holding a container under filter, pump fuel until no bubbles are visible and fuel is running freely (that means it’s going everywhere!). Tighten bleed screw whilst pumping. Repeat process on engine filter and fuel injection pump. Put throttle wide open in neutral and crank over engine for 25 seconds. If engine won’t start, crack one or more injectors until fuel leaks out, then tighten whilst cranking over engine. If engine still won’t start, repeat entire bleeding process.

Have plenty of rags and fuel clean-up equipment on hand. This is a messy job!


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