Sending unit time.

Its a common "GM thing" Trail blazers/Envoys, Safari/Astro vans, Montana and its sister van Full size pickups, S-series, all have the same basic type of Sending unit and pump assy.

There is a replacement part you can get at the parts store for under $30. Installs on your pump assembly in the tank. Removal of the pump assy from the tank is required.

Jay