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How To Bleed A Diesel Engine

Got an airlocked engine on your hands? Here's how to fix it.

Here at Power & Motoryacht we used to “flow fuel,” as they say in boat-testing parlance. That meant disconnecting supply and return fuel lines, splicing in fuel-flow meters, and then running electrical cables from the meters to a computer that would precisely calculate fuel burn. Only one time did I make a bad connection on a hookup and allow air to enter the supply line of an engine, a mistake that promptly airlocked the engine and summarily shut it down. The steps I took to get things going again are fairly universal:

First, back off the bleed screw on top of the secondary fuel filter—three or four turns is usually enough. Some engines, like the one shown, have two secondary filters.

TIP: To prevent airlock after a fuel-water separator element change, top off the separator’s canister with diesel fuel using a bulb-type syringe. Extract the fuel from the jug of spare diesel you keep onboard.


Second, begin working the lever (plunger or whatever) on the fuel lift pump (an engine-mounted component downstream of the primary fuel filter) until bubbles stop forming under the bleed screw and solid fuel comes out. Use rags to keep things neat and then tighten the bleed screw back up. Do the second secondary if necessary.

And third, repeat the bleeding process via the bleed screw on the injector pump (unless it has self-purge capability) and then again (depending on your manufacturer’s recommendations) on each individual injector.

Procedure varies from engine to engine, so consult your operator’s manual for the exact one.

This article originally appeared in the December 2012 issue of Power & Motoryacht magazine.