their is a piece that has to be put on in the filler neck and if it is backwards it will cause fuel to backup. I have the small fillerneck on mine and i took the middle small vent tube out of it. It helps fill up a little bit more but it still spits out.
If you really want to fix it for good, take off the piece and make sure he installed it with the angle of the return fitting pointing towards the tank and not the filler neck and also this would help out too.....GM fillneck kits
still have no idea why they would ever put a small filler neck on a diesel truck.....makes no sence.
2003 CCSB SOLD
EFI LIVE with Max Effort from ATP TRUCKS
, 5" MBRP with DPF delete, SDP EGR delete tube, S&B, PCV reroute, Airdog II 165, Fuel shim, Fuel fittings ported, Edge Insite CTS, Glowshift boost & fuel psi, Transgo Jr., BDS 4.5, Cognito UCA, Ready Lift keys, 35 x 12.5 nitto mt, 20 x 8.5 rockstars