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How To Bleed A Diesel Engine

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

Once upon a time, 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. This procedure can vary from engine to engine, so consult your operator’s manual for the correct steps to follow. But generally speaking, this is the idea.


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

Second, begin working the lever (plunger, 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.

NOTE: This procedure may vary from engine to engine, so consult your operator’s manual for the proper method.

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