Purging Air from a Marine Diesel Fuel System.
A diesel injection system relies on a two-tiered pump mechanism: a low-pressure lift pump ferrying fuel from the tank to the high-pressure injection pump, which then propels the fuel to the injectors. These injectors deliver atomised fuel into the cylinders at precisely calculated intervals, governed by the timing of the injection pump.
The Fuel Filtering Process.
Typically, a primary fuel filter is positioned near the tank for initial filtration. In many diesel inboard engine setups, a time-honoured CAV 296 filter is very commonly used as the primary filter, supported by a secondary filter on the engine for finer fuel purification.
The CAV 296, a stalwart in the industry for decades, incorporates angled slots that not only filter impurities but also expel water from the fuel. This two-tiered system ensures that only purified fuel reaches the injection pump and injectors.
Once in the injectors, the fuel is subjected to intense pressure, opening a spring-loaded valve and enabling precise atomisation within the cylinders. Excess fuel, essential for cooling the injector bodies, is cycled back to the tank via return pipes.
Why Air in the System Is Problematic.
Air infiltrating the system—whether from vibration, filter replacement, or fuel line disconnections—can obstruct the pump’s ability to function. This is because the injection pump compresses air instead of fuel, leading to injector malfunction. Pressures within these systems can soar to 3,000 psi (approximately 200 bar), rendering air-induced disruptions especially problematic.
Essential Safety Precautions
Working with high-pressure diesel systems demands utmost caution. Diesel can irritate the skin, while fuel under pressure poses serious risks. Always wear gloves, and safety goggles, and have a fire extinguisher at hand. Double-check filter seals and connections during maintenance to avoid fuel leakage.
Tools and Preparations.
To bleed the fuel system, gather the following:
Your engine’s owner's manual.
Absorbent cloths or rags.
A correctly sized wrench for bleed screws.
A container for collecting spilt fuel.
Protective gloves and eyewear.
Diagnosing Air Intrusion.
Air can enter the system through several avenues:
Changing filters.
Running out of fuel.
Loose or deteriorated fuel lines.
Before beginning, inspect for leaks using talcum powder around suspect areas; this can reveal even the most elusive of breaches.
The Bleeding Process.
Preparation: Locate all fuel filters and bleed screws on your engine. On some systems, the installation of a hand-operated primer bulb as used with some outboards can simplify the process by assisting fuel flow through the lines. The bulb should be fitted between the diesel tank and the primary fuel filter.
Priming the Filters: Fill the primary filter with clean diesel, and bleed air by slightly opening the bleed screw and pumping fuel until it flows without air bubbles. Secure the screw tightly.
Sequential Bleeding: Begin with the lowest bleed screw—often on the secondary filter or lift pump. Place a rag beneath it to catch spillage, then pump the lift lever until fuel flows bubble-free. Tighten the screw while maintaining pump pressure.
Bleeding Injector Pipes: Loosen the injector pipe nuts at the cylinder head and crank the engine until fuel emerges. Tighten the nuts securely to complete this step.
Final Checks: After ensuring all bleed screws and fittings are secure, attempt to start the engine. If the engine fails to start after multiple times, it can be prudent to close the through-hull raw water stopcock to prevent hydro-locking. This is caused when there is a build-up of water and no exhaust gasses to push the raw water out through the exhaust. There is a danger of burning out a raw water impeller if the engine is turned over too many times when dry so it may also be simpler to remove the impeller temporarily.
Proactive Maintenance and Troubleshooting.
Persistent issues post-bleeding may indicate deeper problems. Regularly replacing filters and inspecting for wear can prevent recurrence. If difficulties persist, consult your manual or seek assistance from a marine mechanic.
By diligently following these steps, you’ll ensure the reliable operation of your marine diesel engine while safeguarding against air intrusion. Always prioritise safety and refer to professional expertise when uncertain.