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The Fuel System & Bleeding Air
💡 A diesel needs an unbroken, air-free column of clean fuel — so keep the water separator drained and, if air gets in, bleed each point from tank toward injectors until fuel runs bubble-free.
Recall from earlier cards that a diesel relies on compression ignition — no spark plug, just fuel squirted into hot, tightly compressed air. That makes clean, air-free fuel non-negotiable. The fuel path runs: tank → primary filter/water separator → lift pump → secondary (fine) filter → injection pump → injectors. The primary filter catches large dirt and, crucially, water, which is denser than diesel and settles into a clear bowl you can drain from a tap at the bottom. The secondary filter removes the tiny particles that would wreck the high-pressure injection pump.
Why does water even get in? Condensation in a half-empty tank, dodgy fuel, or a leaky deck fill. Water causes corrosion and lets diesel bug (microbial growth) flourish at the fuel-water boundary, clogging filters with black sludge. Checking and draining the separator bowl is routine maintenance.
The classic failure is air in the line. Unlike petrol, a diesel injection pump cannot compress air — an air bubble breaks the solid column of fuel it needs, and the engine sputters and dies. Air gets in when you run the tank dry, change a filter, or have a loose fitting. The fix is bleeding: you open a bleed screw nearest the tank, work the manual lever on the lift pump (or turn the key) to push fuel through, and watch each bleed point until fuel runs out bubble-free, then close it and move to the next point further downstream toward the injectors.
The golden rule: bleed from the tank end toward the injectors, one point at a time, never skipping ahead — otherwise you just push trapped air deeper into the system.