2001 Volkswagen Jetta Hard start

The car cranks a long time before starting, back fires through exhaust and intake sometimes and gas can be smelled. I hold throttle down about half way to get it to catch and sometimes it will try start but not, then try again and it will start. Codes were for oxy sensor, cat, maf and iat. Replaced maf/iat and cleared codes. All codes never came back but car still starts same way and will only stay running if revs are kept at 1500 or better. I had also pulled battery cable to clean posts. Any help will be appreciated.