2001 Grand Marquis cranks but won't start???

Got in the vehicle last week to take my little sister to school, started up fine
and warmed up but right before leaving the car stalled and shut off. Cranks
fine but will not completely turn over without flooring accelerator *which I've
still only started twice doing so* but of course it dies as soon as I let off the
gas. I'm getting good spark and good fuel injector pulse. Fuel pressure on
the rail is sufficient. Idle Air Control valve is good, I've changed out the Oil
pressure sending unit as well as the Camshaft position sensor and still no
luck. My mind is rattled. Any suggestions??

Also, It seems as though sometimes on the first crank in a while it is almost
as if it is getting back pressure through the air intake as I hear a popping
noise from the air filter cover. after the first crank though, it will just crank
with no success or changes in noise.