That mushroom filter vent you mention....if that is the crankcase vent your talking about....it was originally routed to the air intake for a reason...to take engine fumes into combustion chamber...and out exhaust

 

Thanks everyone for the replies. Problem has been fixed:

It was the crankcase vent. During an inspection of the engine bay, i noticed oil splats on the driver side hood and firewall. The PO had a cold air intake and did not re-route the vent line back to the intake, and instead used the mushroom filter. I've had this truck for almost 10 years and did nothing to this filter except clean it. And never had a problem. But I guess over time it has lead to this fume smell.

Well, I re routed it into the intake (step drill, 5/8" barbed fitting, some gasket maker) and no smell since the fix.

Also, my particular truck does not have the cabin filter option

 

Awesome. At least it was an easy fix. Think I would be going after the PCV system though, to make sure it is working properly. There shouldn't be pressure in the crank case. (and thus, blowing out the mushroom filter, and you smell it.....) I think yours is just a fixed-orifice tube, so, maybe it is clogging up?

 

I try to avoid getting vehicles with cold air intakes, "special" high flow filters or audio systems that have been removed. These are usually done by inexperienced people who often shorten the life of a vehicle. I had a Furd Powerstroke that just didn't want to run. I ended up taking it to an auto electrical place. 3 hours to find the problem and 3 minutes to fix it. ($100 an hour shop rate.) The truck was originally a California truck and a kid had put a performance chip in the computer. The chip was for Federal emissions and eventually the computer just didn't want to talk to the rest f the systems. They unplugged the chip and everything started running again.

 

I will check that. i think it's located on the throttle body or there abouts