Join Date: Jun 2004
Location: West Michigan
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Rep Power: 14
Also, check to see where the return line is routed into the tank, it could be disturbing the float. Quick test would be to disconnect the return, and route it into a fuel can.
Generally engine caused vibrations are too 'close' to make the gauge fluctuate, typically if visible it will be a slight blurring of the needle from the vibration acting directly on it.
Check voltage, connect a voltmeter somewhere in line and see if the voltage fluctuates, fuel senders are resistors and fluctuating voltage will make the gauge read exactly as if the float was moving. An alternator/voltage regulator not functioning exactly right, or some electric fuel pumps can cause it.