Painless Bleeding

You typically can’t replace fuel filters without bleeding air from the fuel lines, so it’s a technique all do-it-yourselfers must master.

The first step is to thoroughly check all lines, filter gaskets, and unions for air leaks. The most common place for such leaks is between the lift pump and the tank, since this is where suction is taking place. Air trapped in high spots can be especially hard to dislodge.

Bleeding begins downstream from where air leaked in. If you’ve just replaced the primary, lift-pump, and
secondary filters, bleeding begins at the secondary filter, because it’s downstream of the other two. Most secondary filters have a bleed screw somewhere on the top of the filter body. Your service manual should indicate where it’s located.


Clean all dirt or loose paint around the screw and open it a full turn. Place an oil-absorbent pad or rag beneath the filter, and begin pumping the manual lift pump or activate the electric lift pump. If, on a manual lift pump, you have trouble making a full stroke, rotate the crankshaft half a turn to free it up. Keep pumping until a steady flow of fuel¿without bubbles or air¿exits from under this screw. While fuel is still pumping out, close the screw with light pressure from a wrench.

If air didn’t enter the system downstream of the secondary filter¿it usually doesn’t with filter changes¿your work is done. Start the engine, check for leaks, and run it at a high idle (1,200 to 1,500 rpm) for a few minutes. This will allow small amounts of air to pass through the injector pump.

If, during your filter change, air has entered the lines downstream of the secondary filter, you’ll need to purge the injector pump and the injectors. Again, your service manual should indicate where the injection pump’s bleed screws are located and in which order you should bleed them. After bleeding the injection pump by using the same technique as you used for the primary filter, move on to the injectors. On some engines, you can bleed the injectors by simply advancing the throttle to full open and cranking the engine for 15-second intervals. (It’s a good idea to close the raw-water-intake seacock to prevent engine damage. Be sure to open it again before starting the engine.)


In some cases, you’ll have to take one more step. Loosen the high-pressure fuel-line fittings (not the return-line fittings) attached to each injector. Once again, crank the engine with a fully open throttle. Fuel, free of air bubbles, should dribble out of each loosened pipe union as each piston reaches its firing position¿every two revolutions of the crankshaft. (You may have to completely loosen the unions to confirm fuel flow.)

When this has been confirmed, tighten each union snugly; don’t overtighten. Preferably, you should tighten the union with a flare-nut wrench, which is designed specifically for this type of fitting. If fuel isn’t uniformly dribbling (note: It’s a dribble, not a squirt) out of each pipe, the injection pump may require further bleeding. It’s also possible that you
didn’t remove all the air from the low-pressure side of the system.

Your first try at bleeding fuel lines can be frustrating, so practice first at a calm dock. Learn to do it well. Someday, you may need to do it in a hurry.


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