A solid maintenance routine will keep a diesel engine serving faithfully, and that maintenance doesn’t have to be intimidating. For optimal performance, a diesel engine’s primary need is a continuous supply of clean fuel. A symptom known as “airlock” occurs when this flow is interrupted by an air bubble, causing the engine to shut down and refuse to start.
By bleeding trapped air from the fuel system, you can restore the flow of fuel and get your marine diesel engine running again. This skill is crucial for anyone who intends to be on the water for any considerable length of time. The steps involved are straightforward and applicable to the vast majority of engines, though it is wise to check your owner’s manual for any procedures specific to your model.
Step One: Identify the Leak
Before you can get the air out of the system, you have to know how it got in. Diesel engines should never run dry, as this is guaranteed to cause airlock. Outside of this scenario, air intrusions typically occur when the fuel filter is changed. This can be prevented by filling the new filter with fuel and topping it off as it is absorbed, then applying fuel to the inside of the gasket before installation.
If you’ve taken the proper steps or haven’t changed filters recently, inspect your connection points and the fuel line itself for leaks. Once you’ve identified the issue and corrected it, you’re ready to bleed your diesel engine’s fuel system.
Step Two: Back off the Bleed Screw
Where is the bleed screw on a diesel engine? There may be more than one, depending on the size of the motor. You can find them on top of the fuel filters fitted to the engine. Start by loosening the screw on the first filter by a half turn. You can loosen it further if necessary, but don’t pull it out. Giving it three or four turns at most is generally sufficient.
Once you’ve loosened the bleed screw, go ahead and grab plenty of rags, as the next step is going to be messy.
Step Three: Operate the Manual Fuel Pump Primer
Knowing how to prime a diesel engine involves finding the fuel pump lever or plunger. Your fuel lift pump will be mounted on the engine and connected to the primary fuel filter. Begin pumping the lever or plunger while keeping an eye on the bleed screw. You will see bubbles form as air leaves the system.
Use your rags or other absorbent material to sop up the fuel as it bleeds out. When clean, bubble-free fuel is all you see, tighten the screw back down while continuing to pump — this will maintain pressure and eliminate the chance of air reentering the system.
Step Four: Bleed Any Additional Points
Continue this process with the secondary fuel filter, if one is present. That may be all you need to get your engine running again. If the airlock persists, you will also need to bleed the injector pump and the individual injectors using the same procedure. Your owner’s manual will have additional information on your motor’s specific components.
Contact Diesel Pro Power for Your Marine Engine Needs
Knowing how to bleed and prime a diesel engine is critical for keeping your boat on the water — and for getting it back to the dock. When that doesn’t do the trick, the professionals at Diesel Pro Power have an afterhours fast help resource and a vast collection of marine diesel engine parts with 24-hour worldwide shipping. No matter what you need, we’re here to help. Give us a call at 888-433-4735 in the U.S. or 305-545-5588 internationally.