Problem: The vehicle came in for a leak from the fuel tank area.
The technician found the fuel tank had collapsed and the seams were beginning to split. Strangely, there was no MIL on or DTCs from any modules. Early testing revealed that the EVAP system had no restrictions. Acting on the advice of the Tech-Assist consultant, he used a scan tool to command the canister purge and vent valve solenoids to operate. If either one was sticking open or closed, the gas tank could be subject to engine vacuum.
On test drive with the scan tool still connected, the tech noticed the fuel tank pressure sensor PID indicated very high vacuum. The canister purge and vent valve solenoids were operating properly so the technician checked the EVAP canister. He found the canister was restricted causing engine vacuum to be applied to the fuel tank.
Confirmed Repair: The EVAP canister and the gas tank was replaced, then the truck was taken on a long test drive. The fuel tank pressure sensor PID was within factory specifications and the tank showed no signs of collapsing. Problem solved!