2004 mercury grand marquis starts then dies immediately

I have a vehicle that starts then dies immediately. I was
driving it around for work then it started having a pretty bad
clacking noise, that escalated it to running like crap then
the next day it was having trouble getting power then
eventually it resulted in it not starting or staying started for
very long  I could also smell gas.   I tested the alternator
and it was bad.   I replaced it.  I checked the fuel filter,
relays and replaced the bad relay.  It popped a lean air fuel
mixture code for bank 1 sensor 1 and did the same thing
for bank 2 sensor 1 a while back.  I replaced both sensors.
Now it will turn over once and then shut back off
immediately. If might stay or turn back on a second time if I
give it some gas but sometimes it doesn't feel like me
putting my foot on the pedal is giving it any gas.  Kind of in
a tight spot and have seen great results from these forums.  
Thank you for reading this if you have made it this far.