Bleeding A Diesel Engine

You must have this skill in your mental toolbox before you can rely on a diesel inboard

Despite all my years of boating, I’m still surprised at the air of anxiety-inducing mystique that surrounds diesel engines. A comment that the editors at BoatU.S. magazine 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 a little 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.

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 engine in the photos below 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. The BoatU.S. Foundation website has some handy information on proper disposal.

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, 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.
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 cam. Turn the engine over 180 degrees and try again.
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.
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, 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.

Tip: To make the bleed points easier to locate, paint all of them using a color that contrasts with the rest of the engine.

This article was reprinted with permission from BoatU.S. Magazine, flagship publication of the membership organization Boat Owners Association of The United States (BoatU.S.). For more expert articles and videos to make your boating, sailing, or fishing better, visit BoatUS.com.