Ford Grand Marquis Owner's Manual page 32

Fuel Gauge
The fuel gauge
The fuel gauge tells you approximately how
many gallons/liters of fuel you have in the tank.
When your tank reaches 1/8 or approximately 2
gallons (8 liters), a flashing fuel pump will
appear.
Turn the ignition switch off while fueling to
obtain an accurate fuel gauge indication.
NOTE: At least 2.5 gallons (9.5 liters) of fuel
must be added for the fuel gauge to
immediately show the new level of
fuel. If less than 2.5 gallons (9.5 liters)
of fuel is added, the gauge will reach
the new level slowly.
If the fuel gauge displays only the top two bars
and the bottom two bars, or, if fuel remaining or
distance to empty functions display CO or CS,
then there is a problem. Take your vehicle in for
service.
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