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There are several other possible causes besides a fuel issue. They are: improperly gapped electrodes, a weak transformer, a dirty or faulty cad cell or cad cell socket, even a bad nozzle. I agree that a fuel pump is an unlikely culprit. If your oil line fittings are not all flared fittings then the "vacuum leak" is a possibility. When this boiler was installed some of us were still using compression fittings which can develop a suction leak (not likely but possible).