rotten smell

[ARCDenver]

So about 2 months ago I noticed a rotten smell coming from the vents of my car just after start up and after running for a few minutes the smell would go away. Just today I replaced the transmission fluid and noticed the fluid I drained smelled almost just like the smell I've been smelling, I added the normal HCF-2 fluid the vehicle calls for and was just wondering if this may be the smell I've been experiencing. Some input would be great.

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[With_extra_salt]

Maybe change the cabin filter?

 

[ARCDenver]

Maybe change the cabin filter?

just did didnt find any odd in the filter other than pine needles

 

[TriangleHeat]

So about 2 months ago I noticed a rotten smell coming from the vents of my car just after start up and after running for a few minutes the smell would go away. Just today I replaced the transmission fluid and noticed the fluid I drained smelled almost just like the smell I've been smelling, I added the normal HCF-2 fluid the vehicle calls for and was just wondering if this may be the smell I've been experiencing. Some input would be great.

What's "rotten"? There needs to be a better description than that. Does it smell like something specific?

You said "normal HCF-2" so I'm guessing you used a non-Honda CVT fluid just prior to this? Two automotive fluids that smell awful are:
1. Certain aftermarket AT and CVT fluids: Especially Valvoline, I don't know what additive package they use but it smells like fish it's disgusting.
2. Gear oil: The gear oils with sulfur-based EP additives smell like rotten eggs.

Sometimes after a bit of driving and things warming up, if I open the hood I can smell trace amounts of heated transmission fluid, not burning but as if its heated up on a hot plate or something. Been like that on every Civic I owned. Honda's AT and CVT fluid smells good though with a bitter and high-pitched scent, drastically different from fishy Valvoline.

The transmission is vented through a rubber plug on the top of the CVT around where the CVT mates to the back of the engine (it's near the radiator, just under the intake tubing), so I just assume the scent of heated oil is coming from the vent. If you used an aftermarket smelly CVT fluid I would assume heating that up inside the CVT would produce a smelly odor from the vent...

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