Air enters the suction side of the system, between the tank and the injection pump, where the lines are under vacuum. That is why an air leak rarely leaves fuel on the ground. Check filter housing seals, primer pumps, suction hose condition and tank pickup first.
How to confirm it
- Recognize the symptoms of air ingress
Hard starting, an engine that starts and dies, misfire that clears with bleeding, erratic idle and power loss that comes and goes. These are characteristic of air, and they differ from a blocked filter, which produces a steady loss of power rather than an intermittent one.
- Look for fuel weeping even though you expect a vacuum leak
Air enters under vacuum and fuel often weeps out under pressure after shutdown. Oil-damp patches around filter heads, primer pumps or hose ends are a useful clue even though the fault is air ingress.
- Inspect the filter housing and its seals
Most air ingress problems start here, particularly after a filter change. Check the seal is present, correctly seated, undamaged and the right part, and that the housing is tightened correctly. A doubled or missing seal is a common and completely preventable cause.
- Check the primer pump and hand primer
Primer pumps have seals that harden and allow air in. If the engine starts after priming and then dies again, the primer or the suction side is allowing air to enter, and the primer is the first place to look.
- Inspect the suction hose and all its connections
Look for perished or cracked hose, damaged end fittings and loose clamps. A hose that looks sound can still draw air through a pinhole under vacuum, which is why replacing a suspect suction hose is often the fastest route to a resolution.
- Check the tank pickup and the fuel level
A pickup that is cracked, or a standpipe that has corroded, will draw air. It is also worth confirming the fault does not appear only below a certain fuel level, which points at the pickup rather than at the lines.
- Verify the repair by bleeding and running under load
Bleed the system properly, then run the engine under load rather than at idle. Air ingress often only shows up when fuel demand is high. Idling in the yard is a poor test, and a fault that passes it will appear again on site.
What to do about it
Filter housing seals: this is the most common single cause, and it is usually a five-minute fix. Check the seal is the right part and correctly seated — do not assume a new filter came with the correct seal.
Primer pump: replace it. A priming pump with hardened seals is inexpensive and is not worth trying to rescue.
Suction hose: replace it. Attempting to seal a perished suction hose is temporary at best because the vacuum draws air through any remaining weakness.
Tank pickup: repair or replace it. If the fault only appears below a certain fuel level, this is almost certainly the answer, and it is a cheap part.
After any repair, bleed thoroughly and load-test the engine. Also consider whether the same maintenance practice that introduced the fault is being repeated across a fleet — filter changes are a frequent cause of air ingress, and correct technique prevents it entirely.
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Request a quoteFrequently asked
Why is there no fuel leak if air is getting in?
Because the suction side of the system runs under vacuum. Air is drawn in through openings that fuel does not escape from while the engine is running. Fuel weeping often appears after shutdown when the line is still pressurized, so damp patches are still worth looking for.
My engine starts after bleeding and then dies. Why?
Air is entering somewhere while the engine runs. Bleeding removes the air that has accumulated; it does not stop more entering. Find the entry point rather than repeating the bleed.
Can a restricted filter cause the same symptoms?
It can cause similar symptoms but the pattern differs. A restricted filter produces steady power loss that worsens with demand. Air ingress tends to produce intermittent symptoms and an engine that responds to bleeding.