When I have the AC recirculation button pushed on my 16 F150 5.0 sport, why am I smelling everybody’s stank exhaust ? It didn’t do it before but it’s doing it now.

 

From the WSM:

When MAX A/C is selected:

• the air inlet door closes, preventing outside air and allowing only recirculated air.
• the recirculated air indicator is illuminated (recirculated air forced on).
• the air distribution doors operate simultaneously to direct airflow to the instrument panel registers. A small amount of airflow from the floor duct is present.
• the temperature door moves to the full cool position. The air temperature is adjustable.
• the A/C button is illuminated.
• the A/C compressor operates if the outside temperature is above approximately 32.0°F (°C).
• the blower motor is commanded to the highest speed, but can be manually adjusted if desired.

Sounds like your air inlet door is not closing. Do you have access to a high end code scanner or FORScan? A check for codes might confirm this. If there are codes, they would be Ford specific so a low-end code reader likely will not read it as they generally only read generic codes.

 

Even with a charcoal A/C filter, I can still smell undesirable odors with recirc ON, like cattle and diesel/fuel smell, so I wouldn't use that logic to suspect something is wrong. Is the amber light on when you select it? Also remember the stupid thing can turn on and off by itself if you have HVAC in AUTO mode, like stated above. I always use HVAC in manual mode, and recirc never turns itself on or off, except if you use 'Max A/C', which is what it's for (along with max fan speed). The other thing that also turns automatically on when using the front defrost button is the A/C compressor. Finally, what I don't like on this truck is always having to turn on HVAC to get air from the vents. On my other vehicles, don't need to have HVAC on when in fresh mode, saving fan wear and tear, but not a huge deal.

 

Other tricks with the recirc settings, are that if you have it on defrost it will not turn recirc on (light remains off), and if you have defrost and upper or lower vents, it will disengage recirc after a few minutes (light swtiches off).