To bleed a diesel fuel system, you push fuel through the system from the tank to the injection pump until no air is left, usually by filling the new filter with clean fuel, running the lift pump or working the hand primer, and opening the bleed screw on the filter housing until fuel flows without bubbles. Most modern common-rail trucks with an electric lift pump bleed themselves when you cycle the key several times. Older engines with mechanical injection pumps may also need the injector lines cracked open while cranking.
When you need to bleed it
- After replacing a fuel filter
- After running the tank dry
- After replacing a lift pump, injection pump or fuel line
- When air gets in through a leak, causing hard starts or stalling; see diesel won’t start
How to bleed a diesel fuel system
- Fill the tank with enough fuel so the pickup is covered.
- Pre-fill the new filter with clean diesel where the design allows, to reduce air.
- Open the bleed screw or vent on the filter housing, if fitted.
- Prime: work the hand primer, or turn the key to on without cranking to run the electric lift pump, several times.
- Close the bleed screw when fuel flows without bubbles.
- Crank in short bursts, letting the starter cool between attempts.
- Once running, let it idle and check for leaks.
Bleeding the injection pump and injectors
On older engines with mechanical injection pumps, air trapped at the pump or injectors may need extra steps: loosening a fitting at the pump or cracking injector lines while cranking until fuel spurts out, then tightening them. Follow the engine manual. Never loosen the high-pressure lines on a common-rail system while the engine runs; the pressure can inject fuel through skin.
If it still will not start
Look for an air leak at fittings, a failing lift pump, a clogged filter or gelled fuel; see diesel lift pump, fuel filter and diesel fuel gelling. Do not use starting fluid; see diesel hard to start when cold.
Signs of air in the fuel system
Air in the fuel causes long cranking, starting and then dying, rough running, a loss of power under load and, on some engines, knocking from uneven injection. If the symptoms return after bleeding, air is still getting in, usually through a loose filter housing, a cracked line or a worn seal on the suction side between the tank and the lift pump.
Preventing air in the system
- Never let the tank run dry.
- Replace filters with the correct seals and tighten fittings properly.
- Fix leaks on the suction side, which draw air in rather than dripping fuel.
For the high-pressure side, see CP3 pump and bad diesel injector symptoms.
Trouble codes: P0094 Fuel System Small Leak · all OBD2 codes.
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