Bleed Your Diesel Engine

By Lee Mairs

One routine task that strikes the fear in the hearts of novice mechanics is bleeding a diesel engine fuel system.
Each engine has its personality and the secret to a fearless approach to purging the fuel delivery system of air is to know thy engine. Your engine type might have an electric lift pump, a cam operated lift pump, one fuel-injection pump or a fuel-injection pump for each cylinder.
Sorting all this out requires a little background knowledge of the theory of diesel fuel delivery.

A generic diagram of a typical marine diesel engine fuel system (see photo) shows the lift pump sucking fuel from the tank through the pickup tube, fuel lines and through the primary fuel filter. Fuel is then pushed through the engine-mounted fuel filter and on to the engine's fuel injection pump(s).
The injection pump pressurizes the fuel to about 3,000 psi (this varies among engines), pushing the fuel through the injector pipes to the fuel injectors at the appropriate moment. Highly pressurized fuel causes the pintle to lift from the nozzle in the injector and a fine mist of fuel enters the cylinder combustion chamber, mixes with the compressed air and explodes into energy for the power stroke.
If there is any air in the fuel lines, the fuel injection pump pressurizes air instead of fuel, preventing an atomized mist from entering the cylinder. The engine staggers – its warnings of trouble – and finally stalls. The silence is deafening. The only option is to bleed, or purge, the air from the engine fuel supply lines, because the only thing you want in those lines is clean diesel fuel.
JOE VANVEENEN / LEE MAIRSTypical Marine Diesel Engine Fuel System.
BASIC BLEEDING
The only sticking point about explaining bleeding small engines is the seemingly endless possibilities for engine installations, fuel system design and components and idiosyncrasies, not the least of which is access. There are so many different combinations and configurations to consider that I can only describe the bleeding process in generic terms.
Even if you are comfortable with my explanations, hiring your mechanic for an hour or so to have him walk you through the process in a one-on-one tutoring exercise is money well spent, largely because, when the air hits the lines, he'll likely not be handy for coaching in situ. Make notes and be prepared to mark or label the various bleed screws and other points on the engine so that you can easily find them when your time comes – it's not an "if," it's a "when." Finally, write up the details of the process and post them handy to the engine or in your maintenance log. The stubbiest pencil always beats your memory!
PATRICIA KEARNSCummins OSC series 500-HP six-cylinder shows injection pump with fuel lines to a fuel rail that feeds fuel to the individual injectors that act independently. On smaller engines, the injector lines run directly from the pump to the cylinders.It's impossible to write a detailed, step-by-step, process for bleeding air from every kind of engine, but that doesn't relieve you of the responsibility to know how to bleed your engine when it needs it.
Small diesel engines are built primarily for use in industrial and agricultural applications, such as propelling a heavy street sweeper or plowing ground in a farmer's field. In cases where the fuel tank is mounted over the engine, gravity is usually sufficient to feed fuel from the tank to the engine and no lift pump is required. Marine engines are rarely installed to take advantage of this simple fluid dynamic. Because their position relative to the fuel supply, diesel must be pumped or sucked to the engine via a fuel pump.
Lets start with the assumption that your engine has a mechanically-operated lift pump (see photo). This pump works up and down on a cam, sucking fuel through the inlet valve and pushing it out through the outlet valve. There is a small lever on the pump that allows you to operate that pump manually. Otherwise, the eccentric camshaft works the rocker up and down in your stead.
Now, run your hand along the fuel outlet line until you come to the engine-mounted fuel filter. If the manufacturer used a small cartridge fuel filter, continue tracing the fuel line until you come to the injection pump. Carefully consult your engine owner's manual. Most engine-mounted, canister-style fuel filters and all fuel injection pumps have a bleed screw. Study your manual until you are sure that you can find the correct screw. Once you are certain, crack the bleed screw open a turn or two. At this point, you can probably better appreciate having your mechanic available for a bit of experienced handholding during a first time engine bleeding.
JOE VANVEENEN / LEE MAIRSMechanical fuel lift pump.Now, manually operate the mechanical lift pump lever until fuel begins to seep out the open bleed screw. You will see a frothy, bubbly mess that soon turns into clean fuel flow. As soon as this occurs, tighten the bleed screw the two turns. If you have just changed fuel filters, this pumping process may take awhile. Have faith. Be patient.
PURGING AIR
You have now purged all the air from between the fuel tank and the engine-mounted fuel filter canister. You can bleed any remaining air up to the fuel injection pump by similarly opening the bleed screw, manually pumping the fuel lift pump and then closing the bleed screw after clean fuel appears.
If you have solid fuel flow to the injection pump, most engines will start at this time; however, there are always a few recalcitrant machines out there that will require you to bleed air out of the injector pipes also. Crack open the fuel pipe nut slightly at the injector and crank the engine. Again, as soon as fuel appears tighten the nut and, if necessary, proceed to the next pipe. The engine should begin to cough its way back to life.
PATRICIA KEARNSBleeding a Yanmar 6YA-STE: (1) Loosen the priming pump knob then push it to feed fuel to the engine fuel filter; (2) Loosen bolt on top of fuel filter to release fuel until no air bubbles escape, then tighten bolt; (3) Push down and tighten priming pump knob.If your engine has an electrically powered fuel pump, the process is much the same, except you don't have to operate the lift pump lever with your finger. The click-click-click of the fuel pump pushes the fuel thorough the system until air bleeds out at the bleed point. Continue up the line towards the injection pump, opening and closing the bleed screws once fuel, free of air bubbles, appears.
It shouldn't take you long to master the correct bleeding technique for your specific engine once you understand the generic process and have invested some time with your mechanic. The best thing you can do for your engine to avoid the bleeding process is to be careful when you are changing fuel filters and make absolutely sure that you never run the engine out of fuel.
Fuel line integrity is very important when it comes to keeping air out. The fuel tank, primary filter and the lines connecting them to the lift pump are in a vacuum state relative to the outside world. A pinhole leak often will not reveal itself as a drip. Instead, every time the lift pump tries to suck fuel out of the tank, air can be sucked into the system (air is lighter and easier to suck then diesel fuel). If you suspect an air leak, it's your job to find it and fix it – because it could take hours to get that big engine bled on a Sunday evening, when a mechanic is scarce.

A version of this story previously ran in Mad Mariner's DIY Boat Owner magazaine.