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O2 sensors wouldn't cause that, they are not used at idle or for starting.
But if engine has over 150k miles then they needed to be changed.
On the fuel rail, passenger side front, on lower intake will be the FPR(Fuel Pressure Regulator), by the alternator.
It will have a Fuel line connected, the return fuel line, and it will have a vacuum line connected.
You need to check this vacuum line for fuel or fuel smell, it runs from FPR to rear of engine then around the back of upper intake to the Vacuum manifold on drivers side upper intake, where brake booster is connected.
If FPR diaphragm is leaking then raw fuel will be sucked into the intake via this vacuum line when vacuum is high, like at low RPMs/idle.
If there is fuel in the vacuum line then replace FPR.
Another possibility is the IAC(idle air control) Valve.
This wouldn't cause a gas smell though, but can cause engine to stall and be hard to restart.
It is on the upper front of the intake looks like a tube mounted sideways and has a two wire connector, this can be cleaned.
How-to with pictures here: http://www.explorerforum.com/forums/index.php?threads/how-to-iac-valve-cleaning-thread-w-pics.84220/
Has your MPG dropped recently?