Possible Causes & Corrections

WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Savana, 2003 Chevrolet Silverado, and 2003 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.

Check the following items:

A contaminated or clogged fill valve filter. A service interval is not specified for the fill valve filter. It's condition depends on fuel quality. Use of CNG with excessive amounts of contaminants or compressor oil will increase the frequency of service. Service if necessary.
A fill line union check valve that is restricted or improperly installed. The check valve flow direction arrow should point towards the fuel tank.