OK I'm new here, I just registered today in fact because I don't know where else to go. No one knows my issue and I'm at my wits end.

Truck is a 2003 F150 5.4L XLT 4X4 with a K&N intake

Basically my issue is, whenever I use the panel vents/floor vents/defrost vents there's a foul odor coming out whenever I use them. I could have the setting on heat or A/C and the smell remains the same. Now here's the weird part, when I'm driving highway speed there's no smell, it only occurrs whenever I'm stopped or slowing down (15mph and below) and when I speed back up the smell goes away just as quickly as it occurred.
The only way I can describe the smell is, imagine you're driving through an old neighborhood and you catch a whiff of heating gas, housing gas or furnace gas that's the only thing I can closely relate the smell to. Recently while I was looking around to find the source of the smell I spotted sunflower seed shells under the fiberglass blanket on the underside of the hood. I pulled that down and was greeted with hundreds of seeds, mouse droppings and what looked to be a mouses burrow all dug into the fiberglass blanket which I gladly threw away. That made me think the smell was rodent urine, so I power washed the engine bay thoroughly along with the intake cowl. (vent intake at the base of the windshield) The next day I followed that up the with an entire can of Lysol sprayed into cowl and into the vents inside the truck. No change.
Next thing I checked was the battery because I've heard of the battery overcharging causing the water to boil and release a sulfur-like smell. So I pulled the battery covers off, no sign of anything wrong, used a multi-meter on it and while the truck was off it read 12.6 and when the truck is running it read a consistent 14.2. So the battery and charging system is fine.

I have no idea what else could be causing this smell or if I missed something entirely.
I'm hoping you guys can help me with this issue or I just may go insane trying to find a solution.

 

They make an algae killer that helps remove the smell. It comes in a green and white can and is in Fred Meyers/AutoZone, etc type places. It runs about 10 a can but it will help immensely get rid of the smell. Now if you have a dead rodent in the truck, you are on your own, but for AC smell, this stuff really works well.

BG also makes it. They call it Frigid Fresh and it is the best I've ever found. It's just hard to track down our local distributor when you want something for some reason. I'm still waiting for the 44K I wanted a month ago. lol Of course that was only 6-7 days ago, but it was in May and now it's June. lol

 

Thanks for the quick reply
I'll give the BG Frigi-Fresh a shot (Amazon appears to be the only one who can get it to me in a timely manner)
As far as the odor goes though, it's not just concentrated into the vents it also seems to be coming from the engine bay itself. I'm pretty sure it's not a dead rodent because I've had a cat die and it's carcass rot in my back yard and let me tell you, that smell is nowhere near as strong as this. It's also intermittent, like the smell moves around, with the engine running or not running it comes and goes very vaguely. Like if you hold your head above certain points in the engine bay, you'll catch very brief whiffs of it then it disappears. Nothing that lingers but you know it's there when you smell it.
Did you have any idea as far as what could cause the smell in the engine bay?

 

are all of your fluid levels good? Not exactly sure but if it's at slower speeds, it could be something dripping and burning off the engine And when you get up to speed it's getting blown off before it can burn off. Idk just a suggestion, make sure nothing is dripping anywhere. Windshield wiper fluid, antifreeze, oil, etc. easy thing to check and mark off your list of things you know it's not

 

That was what my first thought was, figured maybe burning some kind of fluid would cause a bad odor, but all the levels are correct. It has plenty of pick up, no CEL, starts up like new.

As far as my list I've compiled and done:
Shampoo carpet, clean and vacuum interior
Check fluids
Check battery voltage
Check for dragging brake(causing smell)
Power wash engine bay
Spray cleaner into cowl
Change air filter
Throw in some Techron fuel system cleaner
Change oil
Use Meguiars Air Re-Fresher
Yet the stench persists

 

Look under the Glove Compartment if your vehicle is equipped with
Cabin Air Filter.

 

To my knowledge the 97-03 F150s didn't come equipped with cabin air filters from the factory.

 

There shouldn't be a cabin air filter unless someone added one in the past. I did experience a smell that kinda smelled like you described in my bosses old Silverado. The brake cylinder was leaking and the brake fluid was smoking when the brakes got hot (sketchy I know) and it kinda smelled like natural gas. It would only smell when I was stopped bc a) they were hot after stopping and b) the smell would dissipate with the moving air. It was a very nauseating smell. Check to make sure there's no brake fluid leaking and getting on anything hot enough to make it smoke.

 

Not all F150 have Cabin Filter. It's optional, depends upon the dealer.

 

I just went out and took that plastic lookin cover off that's right above where the passengers feet would be and pulled the glovebox down and didn't see anything that looked serviceable. I can only assume that mine doesn't have a Cabin Air Filter.