i have a gmc 2015 lml that will crank but won't start just recently. it has has not started three times now, i thought it was the DEF or something to do with that. i was informed it will run but you can't drive so fast, then i thought it might be the fuel pump. when i pumped it up i didn't see any fuel come out of it so i figured it was good. i just don't know what else to look at, help please!!!!