Feel of the hoses at the heater core. If one is hot and the other is cold the heater cont balbe is not working. If they are both hot, the mixing door inside is not operating. Make sure you have vacuum going into the cab from the intake.
The 1990 Ford Aerostar Maintenance Information manual says 21 gallons.
A Haynes manual from Autozone and the like could have the answers you are looking for.
The 1990 Ford Aerostar and the 1989 model share a similar design and platform, as they are part of the same generation of minivans. However, the 1990 model introduced some minor updates, including improvements in interior features and safety enhancements. Overall, while they maintain a similar overall appearance and functionality, the 1990 Aerostar has subtle refinements that differentiate it from the 1989 version.
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
You can find a kickdown cable for a 1990 Ford Aerostar with an A4LD transmission at auto parts stores like AutoZone or O'Reilly Auto Parts. Additionally, online retailers such as RockAuto, eBay, or Amazon may have the part available. If you're looking for OEM parts, checking with a Ford dealership or specialized Ford parts suppliers could also be beneficial.
the radiotor or the cams
Ford Aerostar was created in 1986.
Check the fuses first!
21 gal. probably, full it up and see.
1 - 4 - 2 - 5 - 3 - 6
Under tyhe dash driver side