Air appears after an oil-filter service
An oil burner that previously operated reliably begins losing prime after a filter element is replaced. Once the supply is restored by the specified procedure, it runs for an extended period, but the fault returns after a long shutdown. The tank indicates adequate fuel, and there is no obvious outward oil drip at the filter cover. The fuel unit draws through the filter from a lower tank. The previous service record does not document the gasket type or a post-service suction-integrity check. The technician has access to the filter and pump instructions and can compare the actual condition before making further changes.
1. Why does the absence of a visible drip not rule out the filter assembly?
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The operating suction section can be below atmospheric pressure, allowing air to enter a defective seal without oil flowing outward. The disturbed gasket and cover are relevant evidence, but the actual leak or other prime-loss cause must be established using approved tests.
2. What should the diagnosis compare?
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Check correct gasket and assembly, the supply arrangement, suction readings at known points and the shutdown behaviour. Account for lift and restriction rather than assuming the tank gauge proves delivery. Verify the repair and affected burner operation afterward.