From the service manual:
Cooling System Capacity 15.1 Liters (16.0 Quarts)
Cap pressure 89-124 kPa (13-18 psi)
Thermostat Opening Temperature
Starts to Open 90°C (194°F)
Fully Open 99°C (210°F)
I believe that the coolant system capacity is 8.6 quarts ( 8.1 liters ) for your Explorer ( if it has air conditioning )
The two door , 1999 Ford Explorer has a ( 17.3 U.S. gallon gas tank )
The 1999 Ford Explorer owners manual shows ( 20.8 U.S. gallons ) for the 4 door
The repair manual reads...7.8 to 13.2 qts.
( 16.8 gallons ) in the 2003 Ford Explorer Sport
According to the 2013 Ford Explorer Owner Guide : ( 18.6 U.S. gallons )
( 21 ) U.S. gallons on the 2000 Ford Explorer four door
A 1999 Ford F-250 typically has a coolant capacity of about 30 to 40 quarts, depending on the engine type. For example, the 5.4L V8 engine holds approximately 30 quarts, while the 7.3L diesel engine can hold around 38 quarts. It's important to check the owner's manual for the specific engine configuration to get the exact capacity.
The 2000 Ford Explorer 4 door is - 21 U.S. gallons ( 79.5 liters )
18.6 gallons ( U.S. ) according to the 2012 Ford Explorer Owner Guide
The gas tank on the 2010 Ford Explorer holds 22.5 gal..
The gas tank on the 2013 Ford Explorer holds 18.6 gal..