Weird in cabin smell, and its not my feet.
 

I have a 09 nismo with 6500 miles. The car recently started to pump in a weird smell into the cabin. It smells like your home central air heater heating up dust the first time you use it in the season, mixed with drying interior house paint, and carbon monoxide. Its the most distinct smell. Its unlike anything else. It happens when A/C is off and just running the fans. Also, it never happens with the interior air set to recycling mode. The problem only exists when the fans are pulling in outside air and the A/C off. The smell comes in at totally random times so its not the physical locations of the car. The smell is particularly strong when the car is warmed up sitting at a red light. Its not the clutch or brake smell.

Might wanna check to see if you don't have a blown exhaust gasket...??

Have you tried cranking and popping the hood to see if you can smell the same thing in the engine bay?

The first thing I would do is replace the air filters, they could have picked something up which has an unpleasant smell and is just sitting on the filters. That would also be why you don't smell it with the air con recirculating, it bypasses these external air filtration filters.

When you park your Nismo for the most part is it on an incline? You're getting the standard smelly sock odor because the compartment that houses the core for the AC is not draining properly. Check to see that the drain hose is not blocked where it exits from underneath the Z.

Here is what happens: When the a/c is used, it creates condensation. So as this condensation lingers and doesn't evaporate, it begins to collect bacteria and mold, which is the source of the A/C stink. This is very very very common across all makes and models of vehicles. You can start by inserting a carbon particulate cabin filter into the A/C case, you can use one from either an altima, maxima, toyota avalon or camry. If this doesn't work or last, try buying an evaporator case cleaner, which can be bought at any local auto parts store, or you can have the service done by the dealer. Good luck!

What they said - Oh, and you can't smell carbon monoxide - it's odorless....

mold would be my guess

Other things that lead to weird smells:
1) using the wiper fluid jets. The drainage off the windshield drips on the cats. I've noticed a burnt smell after using the wiper jets quite often actually
2) road grime on aftermarket cats. This is especially noticeable in the winter here in Chicago. Salt and grime gets on the cats during a wet road day and you smell it burning off.
3) plastic stuck to aftermarket exhausts. Drive over a plastic bag and it will instantly stick to the hot piping. You'll be stuck with the plastic stink for a few weeks if you don't remove it quickly.
4) oil leaks. Just make sure there isn't any oil leaking from someplace and dripping on the manifolds or cats.

Have you change your gear oil recently? Gear oil overflow could happen. Burned gear oil on exhaust doesn't smell very good... The odor stains remains in the air ducs and cabin for a long long time

But if it's from exhaust fumes, you may smell some of the other chems from the exhaust with it.

Still, you're correct. If I'm driving a car that was snowed in for a minute, I tend to crack the windows for a while and freeze my butt off, just in case.

maybe your evap hose is clogged?

I kind of get this too. I've got barely 6600 miles on the Z but I rarely ever use the fans. It only smells when the thermostat is either at "75" (from coldest to the middle) and the A/C is either on or off. It's a moldy kind of smell, and it disappears after a couple minutes of operation. I've never gotten that smell with heat though.

i would say clear your A/C filter and it should be fine.

lmao...

Smell
 

OK, let me clarify. The smell is very random. It happens intermittently. Lately, it happened once every two days. And when it happens, it stops as quickly as it starts. So I think we can rule out something being stuck in the air filter, or molds. And I don't think it has anything to do with the A/C. The smell never happened with the A/C on even if its drawing air from the outside mode.