Very common (25% of cases), especially after running out of fuel, changing fuel filters, or sitting unused. Air in the fuel lines prevents diesel from reaching the injectors. The engine cranks fine but won't fire.
What you'll see
If cranking is slow or labored, the problem is more likely a weak battery or oil-flooded screw element -- not air in fuel. See: Weak or Dead Battery. Also make sure there's actually fuel in the tank -- sounds obvious, but fuel gauges on portable compressors are notoriously inaccurate.
How to diagnose
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Check the obvious: is there fuel in the tank?
Open the fuel cap and look inside. Don't trust the fuel gauge -- many portable compressor fuel gauges are unreliable, especially after the machine has been sitting on uneven ground. Also check that the fuel shutoff valve (if fitted) is open.Result: Fuel visible in tank, shutoff valve open. -
Check the low-pressure fuel system
Disconnect or loosen a low-pressure fuel line after the fuel filter and crank the engine. Does fuel come out? If yes, the low-pressure side is OK. If no fuel or just air bubbles, there's air in the system. Check the fuel filter bowl (if fitted with a clear bowl) for air bubbles too. If there's an electric fuel pump, listen for it -- you should hear or feel it running when the key is turned on.Result: Steady fuel flow = low-pressure side OK. Air bubbles or no flow = air in the low-pressure system. -
Look for the air entry point
Air gets into the fuel system through loose connections, cracked fuel lines, bad o-rings on the fuel filter housing, or a faulty primer pump diaphragm. Inspect all fuel line connections from the tank to the injection pump. Look for wet spots (diesel seeping out is also where air gets in). Flexible fuel lines harden and crack over time, especially in sun-exposed portables.Result: Found loose connection, cracked line, or degraded seal.
How to fix it
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Bleed the low-pressure fuel system
Most diesel compressors have a hand primer pump on or near the fuel filter. Pump it repeatedly until you feel resistance and no more air bubbles come out of the bleed screw. On machines without a hand primer, loosen the outlet connection on the fuel filter, crank the engine until fuel flows steadily, then tighten.
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Bleed the high-pressure fuel system
If bleeding the low-pressure side isn't enough, you may have air in the high-pressure lines to the injectors. Loosen the fuel line connections at the injectors one by one. Crank the engine until fuel seeps out around each connection, then tighten. Work from cylinder 1 to the last cylinder. Be careful -- high-pressure diesel can penetrate skin.
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Fix the source of the air ingress
Bleeding only fixes the symptom. Find and fix where the air is getting in. Replace cracked fuel lines, tighten loose connections, replace fuel filter o-rings. If the machine ran out of fuel, fill the tank and bleed -- that's the fix. But if it keeps happening, there's a leak somewhere that's sucking air.
Don't crank the engine for more than 15-20 seconds at a time -- you'll overheat the starter motor. Wait 30 seconds between attempts. Also: after changing fuel filters, always fill the new filter with clean diesel before installing it. This pre-filling reduces the amount of air that needs bleeding and gets you started faster. Old diesel that has been sitting for months can grow algae and clog filters -- if the fuel looks dark or cloudy, it's suspect.
Open-end wrenches for fuel line connections. Clean rags. Container to catch fuel. Replacement fuel filter o-rings. Fresh diesel fuel if the old fuel is contaminated.
Diesel fuel is flammable. Have a rag ready to catch fuel when bleeding lines. Don't smoke near the fuel system. Clean up any spilled diesel immediately.
This issue can also cause
- Engine Starts Then Shuts Down Diesel engine starts but shuts down shortly after—usually a fuel, sensor, or loading issue.