How to Bleed Air From a Marine Diesel Fuel System

If the tank ran dry, refuel. If the engine died on its own, check the fuel filters and fuel-water separator for water and dirt, and make sure the fuel valve is open (see the fuel-water separator and diesel won't start guides). Look along the fuel line from the tank to the engine for wet fittings or loose clamps. Bleeding won't help if air keeps getting in.

Engine off, key out. Put absorbent pads under the filters, the lift pump and the injection pump. Have a sealable bag for oily rags and keep the engine-room hatch open for air. Find the bleed screws in your manual: usually one on top of the engine-mounted filter and one or more on the injection pump.

Soundings suggests filling new filters (or the filter bowl) with clean diesel before you install them. It's far quicker than pulling all that fuel through by hand with the primer. Fill carefully so no dirt gets into the clean side of the filter, and refit the seals and O-rings exactly as the kit shows.

Most mechanical diesels have a lift pump on the engine with a small hand lever, or a separate plunger-style hand primer. Some engines use an electric lift pump. Cruising World's diesel writer Steve D'Antonio explains that turning the key to "on" often runs an electric pump, but on some engines it only runs with the glow plugs on or with the engine running; check your manual rather than improvising wiring.

If the lift pump lever feels stiff or gives only a short stroke, the pump's drive cam may be holding it up. Cruising World's fix is to turn the engine over a little (about half a turn of the crankshaft) and try again; BoatUS describes the same trick, done with a very brief "bump" of the starter. Do this the way your manual describes.

Clean the area around the bleed screw so dirt can't get in. Open the screw about a turn. Pump the primer steadily. At first you'll get froth and bubbles; keep going until a steady stream of fuel with no bubbles comes out. Then, while fuel is still flowing, close the screw snugly. Don't overtighten small bleed screws; they strip easily.

If you changed a primary filter or separator on its own, bleed any vent on it first, then move to the engine filter.

Cruising World says that after a routine filter change, air usually doesn't get past the engine filter, so you're often done at this point. Try starting the engine. If it starts, run it at a fast idle for a few minutes to carry tiny bubbles through, then check every filter and fitting for drips.

Each time you crank a raw-water-cooled or wet-exhaust engine that doesn't start, its raw-water pump pushes water into the exhaust, and with no exhaust gas to blow it out it collects in the muffler. Steve D'Antonio warns that if you crank for more than about 20 seconds in total, you should close the raw-water seacock so water can't back up into the engine, and open it again as soon as the engine starts. He suggests cranking in roughly 10-second bursts with about a minute's rest between them so the starter can cool. Northern Lights gives its generator owners the same kind of warning (also relayed by Southern Boating): don't crank more than about 60 seconds in total after a filter change; if it still won't start, drain the muffler per the manual and repeat the priming.

The trade-off with a closed seacock. The raw-water pump impeller is lubricated by the water it pumps, so cranking with the seacock closed runs it dry. BoatUS's bleeding article handles this two ways: take the impeller out until bleeding is finished, or leave the seacock open and disconnect the water injection hose at the exhaust so the water runs into the bilge instead of the muffler (then pump the bilge). Either way, put the impeller or hose back before you try to start the engine. Keep cranking bursts short whichever method you use, and if you're not sure which suits your engine, ask a mechanic. Watch the battery too; a weak battery cranks slowly and makes bleeding harder (see the marine battery guide).

If air got past the engine filter (common when the tank ran dry), the injection pump needs bleeding. Your manual shows which bleed screws and in which order. Use the same method: open one screw, pump the primer until bubble-free fuel flows, close it while it's still flowing, then move to the next. Then try a start, observing the cranking limits above.

Some older mechanical diesels need the last air pushed out at the injectors. Do this only if your engine manual lists it, and never on a common-rail engine. Soundings warns that common-rail fuel pressure is enough to cut skin, and that a common-rail line that's been opened should be replaced, not reused.

The usual method on engines that call for it, described by both Cruising World articles: with the engine off, loosen the nuts where the steel lines meet the injectors about a half to one turn (not the small return-line fittings). Keep your face and hands back and pads around the injectors. Crank in short bursts with the throttle set as your manual says. Fuel should start to seep out at each loosened nut; Cruising World describes it as a dribble, not a squirt. When it comes out free of bubbles, stop cranking and tighten that nut snugly, then try to start.

Use a flare-nut wrench, which Cruising World recommends for these fittings because it doesn't round off the nuts. Don't overtighten. And never loosen or touch these lines once the engine is running.

D'Antonio suggests a small amount of throttle for the first start after bleeding. The engine may run roughly for a few seconds as the last air clears; adjust the throttle until it smooths out. Open the raw-water seacock if you closed it and check that cooling water is coming out of the exhaust.

With the engine running, look (don't touch) at every bleed screw, filter seal and line nut for weeping fuel. Shut down and tighten anything that seeps, then restart and check again. Bag the fuel-soaked rags and dispose of them as hazardous waste; never leave them in the engine room.

An engine that needs bleeding again and again has an air leak or a supply problem. Cruising World points to the suction side between the tank and the lift pump: loose hose clamps, a cracked fitting, a bad filter seal, a worn lift pump diaphragm or a partly blocked tank pickup. Mechanics can test with a clear section of hose to watch for bubbles. This is a good point to hand the job to a diesel mechanic.

Cruising World's advice is to learn this at a calm dock, because one day you may have to do it in a hurry offshore. Change a filter, bleed the system, and write the bleed screw locations and order inside the engine-room hatch. Carry spare filters, the wrenches that fit and plenty of pads (see the onboard toolkit guide).