I am receiving a code of po300. I have had it at the mechanic for several days and he finally called me and told me that he gave up. He can't fix it. He has no idea what's wrong with it. The car starts, but idles really loud and acts like its not getting gas. This started after I got gas from speedway. The mechanic said it could be bad gas, but the mechanic tested the gas, and said there was no water in it. The mechanic said he checked the gas, confirmed good condition of the fuel, fuel pump, fuel filter, fuel pressure regulator, ignition module, coils, wires, plugs, map sensor, crank shaft sensor, can shaft sensor, ext vacuum leaks, exhaust restriction, battery, alternator, ac current, dc current, it says needs alt but that's not the cause of my main problem. Do u all have any other ideas for me?