Check Valve

[calcite]

i am in mexico working and will be coming out in a month,but i know i have gotten bad gas(suprise) i never had a problem on start up,but after smelling sulfur,i had idea bad gas,i have yukon xl 2007,,when i shut down engine and go to restart it takes 3 cranks to crank,seems as if a check valve is stuck and the gas in the line is running back to the tank,any one can help i will be thanful,i went to chevrolet dealer here and he said before he could help he would have to change oil and filters,i told him i just had that done,but he said thats the law,(another rip off)i said well give me a paper saying i was here because my milage was 35,000,he said yes we can do that but you need to give me 50 dollars for the paper(another rip off) wow you cant believe the service here,,any one help please