I am having trouble figuring out what triggers the locking of the fuel door on an 04 S-80, I have located the release button and the actuator. the release button works fine and if I physically extract the actuator it holds the fuel door locked, then I push the release and it unlocks and it remains unlocked all the time swinging open and shut while I drive, any suggestions?

The spring should hold the door closed. Most likely your spring is missing or the hinge is broken. As for the locking function, there are 2 possible ways. Can't remember when they changed it, , think mid 04. either locks when you drive off,(newer method) or 10 minutes after you lock the car. At any rate, it should not flop open and closed when driving whether it is locked or not.

thank you, I will investigate the presence of a spring.

Anyone with an s-80 feel like trying to determine at what point the fuel door lock engages? and perhaps what may trigger this function?
Thank you, any information on this would be helpful.