According to my Chilton's Auto Repair manual :
A 1988 Mercury Grand Marquis has a ( 20 U.S. gallon ) gas tank
12 Gallons (US)
As much as the gas tank will hold... ---------------------------------------------------------------------------------------------- On a 1997 Mercury Grand Marquis , the gas tank capacity is ( 20.0 U.S. gallons )
My manual says fuel tank capacity is 19.0 gallons (71.9L).
According to the 1999 Mercury Grand Marquis Owner Guide : ( 71.9 liters / 19.0 U.S. gallons )
According to the 2003 Mercury Grand Marquis Owner Guide : ( 71.9 liters / 19.0 U.S. gallons )
According to the 2000 Mercury Grand Marquis Owner Guide : ( The gas tank capacity is 19.0 U.S. gallons )
According to the 2003 Mercury Grand Marquis Owner Guide : The fuel tank capacity is ( 19.0 U.S. gallons )
According to the 1997 Mercury Grand Marquis owners manual : With engine oil filter change : ( 5.0 U.S. quarts / 4.7 litres of engine oil )
On a 2001 Mercury Grand Marquis : The fuel tank is mounted vertically behind the rear seat
( 20 U.S. gallons ) according to my Chilton's repair manual
15.5 quarts plus 1.5 quarts in overflow tank
A replacement tank is 20 gallon. I had a Lincoln town car and it was 20 gallons so I would think that 20 gallons is probably what you have.