Coming over from a Toyota off-road forum. I've seen a few posts on here about getting a little MPG improvement and some HP gain.
One thing that was popular there that I did with good results was an K&N and AIR BOX DECK PLATE MOD. I searched a bit on here to see if anyone was doing it. But instead of dropping a few hundred $$$ on an air intake we would just do this.
I haven't gotten to look at mine yet to see if it's even possible since I'm going to buy my 02 4x4 this weekend. But thought I'd share if anyone was interested.
1. Go to a boat shop or auto shop that would sell a deck plate drain for a boat. I think I used a 6" round one in my Tacoma.
2. Get a K&N filter to fit stock air box
3. Pull out stock air box
4. trace the inner circle of the deck plate housing on the air box. (on the Tacoma we did it on the side with the most open area by it)
5. Cut out the circle you traced on stock air box.
6. Take frame of deck plate and (should be an inside and outside) Silicone both sides onto the air box and then bolt together.
7. Once dry, deck plate should snap into place locking on the frame completely sealing off the box to water and dust.
If driving around town or on Hwy take the plate off and put it somewhere safe. Pretty much opens up the side of the box bringing in more air like the K&N or Air-aid intakes providing a better airflow.
When you go off road, just snap the plate in to seal out water and dirt.
I don't remember exact numbers but I saw a noticeable increase in gas mileage and even a little bit of throttle response. Plus it made the stock exhaust a little deeper.
Not sure how possible it is on the Pathfinder but I'll look at it and try it if it seems like it will work. I think it's $10 for the plate and $40 or so for the filter if you don't already have one. More people on the site (www.ttora.com) swore by it and did their own testing. If I do it, I'l try to do a write up and post on here. Sure beats spending $200-$300.
Hopefully this is helpful