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I would check the condition and security of the float for your fuel sending unit.
there is a service bulletin from the manufacturer describing the symptoms you are experiencing since replacing the fuel sending unit on your Cadillac.
The resistance values through the sender are absolutely critical to proper gauge operation. GM has released a kit to correct the erratic gauge operation you are describing since the sender was replaced; if you installed an aftermarket sender or a GM one of the incorrect part number (one that is not the latest design revision) that is likely the cause of what you are experiencing. Installing fuel level sender kit part#25314217 from your local Gm dealer should resolve the problem.
Here is an excerpt from a long TSB detailing various fuel gauge problems and their corrective actions. The TSB is #73-63-14.
Bulletin No.: 73-63-14
Date: June, 1997
Subject: Fuel Tank Will Not Fill to Capacity/Fuel Gauge Stays on Full for Extended Period or Fluctuates (Replace fuel level sensor and/or Fuel Tank)
1997 Cadillac Concours, DeVille, d'Elegance, Eldorado, Seville
Some owners may comment on the following condition(s) relative to the fuel system:
C.Fuel gauge reading is erratic (does not include minimal fluctuation noted during normal operating maneuvers such as braking, turning, or accelerating).
Condition C may be caused by inaccurate resistor values in the fuel level sensor
For Condition C
Replace fuel level sensor and float arm assembly on the fuel sender assembly with fuel level sensor kit, P/N 25314217
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