I was inspired by the air filter design of my vintage Honda riding mower. It uses a paper filter with a foam wrap. For years I have taken off the foam everyone once in a while, bathed it, and oiled it with a squirt of engine oil. It sure does collect a lot of dirt and the paper filter never seems to show any accumulation.
I wanted to try a similar approach with the 4Runner. After a dusty ~50mi trail ride last year my new-ish paper filter was shot. When I installed a new OEM one I added an oiled piece of Uni foam trimmed to fit. The filter cartridge just kind of jams it into place in the bottom of the airbox. This foam flows pretty well so I was not concerned about power loss.
Brand new foam next to the new paper filter
After oiling with Uni spray oil
After this point I drove the truck approximately 4000 miles. There were a few hundred miles of offroad driving, 200 of which were in the desert, but at no point was it particularly dusty.
Here's the filter taken off showing the inner surface that is up against the bottom of the paper cartridge
Flipped over to see the business end -- this is where the dirty air meets the foam filter. Clearly it has captured some stuff and the paper filter has remained very clean! Because the foam is jammed against the inlet tube you can see there are some dead spots but overall the surface is pretty evenly dirty. This means the air is able to flow pretty well underneath it.
After a wash in solvent, soapy water, and rinse with clean water
I'll pop it back in after reoiling it and try again in a few thousand miles.
My hope is that this foam can:
- Improve the overall efficiency of the air intake system (less dirt coming in)
- Protect the paper cartridge during super dusty events. The foam can just be replaced on a frequent basis during this type of service. Possibly could have two or three so a clean on is always handy.
I drove a lot of long hills and stuff in northern CA during this 4000 interval. I noticed no power loss. That's saying a lot since I only have 150HP at best to begin with! I figure if I lost even a couple HP I'd be able to notice.
So... yeah!

arty: