Either the sensor is broken or Your gaug on the dash is not working.
The gauge has failed.
The 1990 Ford Aerostar Maintenance Information manual says 21 gallons.
It might fit but it will take you a while.
You have a 21 gallon tank.
The proper gap for 1990 aerostar spark plug is .044. This was taken from Dept of Motor Vehicles in Delaware. - Bill R. Jr.
It's inside the gas tank
In a 1990 Ford Aerostar : You either have the 3.0 liter V6 engine or the 4.0 liter V6 engine from the factory ( each size of engine has 6 engine cylinders )
The distributor for a 3.0 liter in a 1990 Ford Aerostar rotates CLOCKWISE ( looking from the top ) The spark plug firing order is ( 1 - 4 - 2 - 5 - 3 - 6 ) The engine cylinder locations are numbered : 3 - 6 2 - 5 1 - 4 front of Ford Aerostar
21 gal. probably, full it up and see.
1 - 4 - 2 - 5 - 3 - 6
Under tyhe dash driver side