1998 Buick Riviera Owner's Manual - Page 247

The fuel doorcan also be opened by using the release in the trunk compartment, on the driver's side of the vehicle. To remove the cap, turn it slowly to the left (counterclockwise). The cap has a spring in if you let it; go of the cap too soon, will spring back to the right. it While refueling, hang tethered cap in the notch on the the fuel door.