Bleeding A Marine Diesel Engine

Story and Photos by Mark Corke

To rely on a diesel inboard, and to help you get to know your engine better, put this skill in your mental toolbox.

Despite all my years of boating, I'm still surprised at the air of anxiety-inducing mystique that surrounds diesel engines. A comment I often hear is that diesels are complicated to work on and not as simple as gas engines. Nothing could be further from the truth. A diesel is, in fact, less complicated than a gas engine, uses a less dangerous fuel, gets better mileage, and doesn't rely on a spark to ignite the gas. But there is one skill that will help demystify the diesel, and every diesel-engine owner should learn it: how to bleed trapped air from the fuel system. If the fuel system has air in it, the engine won't start. While there are engines now on the market that self-bleed, most of them don't. Learning how to bleed your diesel engine will help you understand the engine better and can get you out of trouble without resorting to a mechanic.

Before you begin, it helps if you understand a few basic principles of diesel operation. Almost without exception, fuel is drawn from the tank with a lift pump attached to the engine. The fuel first passes through a primary fuel filter and water separator, which is often quite large, then through a secondary filter, which is sometimes fitted to the engine. From there, fuel goes through the injector pump to one or more injectors, which squirt precisely measured amounts of it into the cylinders at the correct moment. The pump delivers more fuel than can actually be used by the engine, so there's a return line for the excess fuel from the injector pump to the fuel tank.

Photo of a diesel boat engine

Here's how to go about bleeding the engine. The idea is to force diesel all the way through the fuel system to push trapped air out, a process that will then allow the engine to start. Most engines need to be bled in a specific order of components, usually following the sequence of secondary filter, injector pump, injectors. Your manual can be a big help here. The specific steps for your particular model should be outlined within, but the general procedure looks like the one laid out below. The engine in the photos is a small, single-cylinder Yanmar (1GM10), but all engines will be similar, even if some of the components look slightly different. Spilled fuel creates a mess, is smelly, and poses environmental hazards, so use plenty of oil-absorbent pads to soak up spills. When the job's done, place used pads in a plastic bag and dispose of them properly.

Raw water pump impellors rely on seawater to keep them lubricated so it's essential they are not run dry. To prevent damage either remove the impeller until the job is finished, or leave the raw water seacock open, but remove the water injection hose to the exhaust, and allow any water to run into the bilge as the engine is cranked over. When you have finished bleeding the engine don't forget to replace the impeller or injection hose straightaway.

  • Photo of draining off water and sediment from diesel engine
    Photo of lift pump and secondary fuel filter

1. The primary fuel filter should have a clear sight bowl. Check the bowl, and drain off any water or sediment at the bottom of the element holder. Catch the runoff in a cup, bowl, or absorbent cloth. If you're replacing the filter element, be sure it's full of fuel before you start bleeding.

2. Locate the lift pump and secondary fuel filter attached to the engine. If they're not readily apparent, try tracing the fuel line, or look in your owner's manual. It's worth noting that not all engines require bleeding at the injector pump. Some larger engines only require bleeding at the secondary filter and the injectors.

  • Photo of opening the bleed screw on the top of the secondary filter
    Photo of retighting the pump screw

3. A small lever that can be manually operated, shown here marked in yellow paint, pumps fuel through the lift pump. While operating the lever on the lift pump, open the bleed screw on the top of the secondary filter. You only need to loosen the screw about half a turn. Don't remove it completely. Note that if the lever doesn't seem to work, the lift pump may be in the wrong position on the internal camshaft. Turn the engine over 180 degrees and try again. Do this by very briefly pushing the starter button, to "bump" the engine over.

4. When you start pumping, you'll likely see small bubbles coming through the bleed screw. Continue pumping until the bubbles stop and clean fuel begins to weep from the hole. Retighten the screw as you pump. The pressure from the fuel escaping will prevent any air from migrating back into the fuel line. Wipe up any spilled fuel.

  • Photo of releasing the screw in the fuel delivery line
    Photo of locating fuel lines connected to injector

5. Repeat the pumping procedure, only this time release the screw in the fuel-delivery line on the injector pump. Loosen the screw by half a turn, then tighten it as soon as bubbles stop and fuel is ejected. The run from the fuel-lift pump to the injector pump is generally short, so a couple of pumps are often all it takes. The fuel will squirt out with greater force because it's on the pressure side of the fuel pump.

6. Locate the other end of one of the fuel lines where it connects to the injector; pick the shortest one first. It will look something like the picture shown here. Loosen the fitting, no more than one half to one complete turn. Do not unscrew it all the way. Then pump until the diesel runs clear, then tighten. Repeat the sequence for each injector. On larger engines, you may have to loosen the injector lines, open the throttle, and crank over the engine with the starter to force fuel through the lines. (Don't worry; it won't start because of the loose injector lines.) If so, be sure to close the engine-cooling-intake seacock so water can't back up into the muffler as the engine spins. (Don't forget to open it again before you start the engine.)

The job is done, and now the engine should start. If it doesn't start after 10 seconds of cranking, double-check that everything is tight. If it still won't start, try bleeding the system once more, because even a tiny bit of air can prevent the fuel from getting through.  

— Published: October/November 2015


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