pnfireman

I have a 07 NBS Silverado. I got a tune-up and 100k service done at 97k miles a local shop (not dealership). Ever since this work was done my gas mileage sux and there has been a smell of gas coming from the vents when the air is not on recirculate. Before tune up mpg was up around 18.5-19 hwy and 15-16.5 cty. Now I'm lucky to get 15.5- 16 hwy and 13.5 -14 city. Nothing else was changed or has been done to my truck. Same tires & pressure, same gas stations, same driving habits, clean air filter, etc. The gas smell will go away after driving a short distance but comes back on next engine restart. I checked the engine compartment after start up and there is a strong smell of gas. Truck runs great except for this.

Took it back to the shop that did the work twice and they witnessed the same symptoms I described but were unable to find anything after keeping it for two days. No leaks, no visible cause.

Does anyone have any ideas on what this could be? I guess trying to save $250-350 by not getting the service at the dealership is going to cost me the same to have it corrected if I can ever find out what it is. THANKS for any help!!!!