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Installing a Uni foam filter inside the airbox

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Okay, here’s my first “Do It Yourself” posting for the Comanche forum. It has to do with relocating the valve cover breather hose from the airbox, and installing a Uni dual stage foam filter inside the air box.


Here’s a picture of the parts I used to relocate the valve cover breather hose. It’s a box of rubber caps, and a breather filter that goes on the end of the VC breather hose:


This is the stock location of everything, with the VC breather hose connected to the airbox lid on the driver’s side of the engine compartment:


I unplugged the VC breather hose from the airbox lid, rotated it 180 degrees towards the battery side of the engine compartment, and found a nice place for it right by the rear battery holddown. The trick to getting the breather hose and this particular breather filter to work is you have to remove the rubber neck at the bottom of the breather filter. The end of the VC breather hose fits right inside:


Once the VC breather hose is relocated, I plugged the inlet on the airbox lid with one of the rubber caps:


To do this Uni filter install, I used these two parts. It’s a Uni dual stage foam filter, part number 6300AST, which means it’s 6” long, has a 3” inlet diameter, and has a slightly angled neck. It’s dual stage because it has a coarse outer sock, and a denser inner sock. The adapter I got from Home Depot, in the HVAC section. It’s one of those three piece heater duct gizmos that has rotating pieces:


You have to remove the inner “neck” from the airbox lid outlet hose connection to the throttle body. This is what it looks like as seen from inside the airbox lid:


Flip the airbox lid over, and you’ll see this small hole in the neck with a tab right underneath it. There’s also an inverted triangle that serves as an alignment mark for the inner neck. I used a small screwdriver, stuck it inside the small hole, and moved the tab forward to unseat the inner neck. Be careful doing this part, because the inner neck could be stuck in there pretty good, so don’t try to muscle it out, or you’ll end up breaking it:


On second thought, that inner neck, once it’s removed, doesn’t really seem to serve any purpose, other than to restrict the size of the opening, so have at it!


You have to cut the adapter in order to make it fit inside the airbox. On the rippled end, I made a cut down to the first section, in order to make it easier to install this end into the airbox lid. The cut ends will slightly overlap when you push it into the airbox lid, but it’s still wedged in there pretty good. The second cut was to remove the first section at the other end. Compare the two pics and you’ll see what I mean. Make sure to cut above the “flared” section, to get the narrow diameter, which goes into the Uni foam filter:


I sprayed some ArmorAll into the neck and pushed the adapter in there and turned it around to where I thought it had to be, in order to put the air filter on the end, and checked to see that it cleared the edges, and was angled down towards the bottom of the airbox. Once I had an idea of how the adapter had to go in, I took it out, mounted the Uni filter on the end, then put the adapter in. It’s much easier to put the filter on the adapter, then put the assembly into the airbox lid, than trying to put the filter on the adapter once it’s snugged down into the airbox lid. This is what it looks like when the airbox lid is ready to be put back together with the airbox body. Even though it looks like the red outer sock will get caught on the airbox edge, these Uni filters are somewhat flexible, so you just have to give it a nudge and it will fit easily inside:


Before I put the lid back down on the airbox body, I cut a 2” hole in the bottom of the airbox, and thru the metal underneath, to give the airbox a better source of colder outside air, in addition to the stock inlet source which is right behind the driver’s side headlight. I also used clear fingernail polish on the metal edges I just cut, to help prevent rust:


So how’s everything work? I road tested these two mods for a few miles, and here are my initial observations. Relocating the VC breather hose like I did has no affect on how the MJ runs, and I didn’t see any oily residue at its new location. I’ll keep an eye on that though, and if there starts to be some oily residue dripping down from that new breather filter, I’ll go to Plan B.


As for the Uni filter in the airbox, I noticed a couple of things. First, the intake sound now seems to drown out the exhaust tone from my Thrush all-welded chambered muffler, but in a good way, lol! Second, it seems to pull stronger in the mid-range. Didn’t really notice any change at idle or lower rpms, but during normal driving it seems to pull noticeably stronger in the mid-range, from about 1800 rpms on up.


Total cost for everything was as follows: rubber caps ($3), breather filter ($9), Uni filter 6300AST ($25, but I already had one on my shelf), adapter ($4). So give or take, about $40 to do these two mods. Oh, one other thing. If you buy one of these Uni filters, you don’t need to buy the “special” filter treatment to oil it up. Just buy a can of regular cooking oil spray (NOT olive oil, it’s WAY too oily, lol), get the red outer sock damp, then massage the oil into the inner sock. Once that’s done, roll the filter back and forth on a paper towel to remove any excess.

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Nice write up !! Is yours a high output motor !! Is the only difference , how many vacuum lines there are ?


Thanks, but I have the 2.5 four-cylinder. As for vacuum lines, not sure what you're referring to. But on my airbox lid, there was one connector for the valve cover breather hose, and another connection that I'm not sure what it does, so I left it alone. The connector for this mystery line was right at the outlet tube that goes to the throttle body.


Interesting note: I was at the pick and pull this morning, looking for a "test" airbox lid, and the one I ended up buying came off a 6-cyl. They had an '86 4-cyl XJ, but the airbox lid was narrower than the one on my '88 4-cyl MJ, and it had "2.5L" stamped on the top, where mine just has a few raised ridges. The test airbox lid still had two hose connections, but they weren't in the exact same places as the ones on the stock MJ airbox lid. Also, and this was a BIG issue, the big hose outlet to the TB was located in the corner of the airbox lid on the 6-cyl test lid, where it's closer to the middle on my 4-cyl lid, so I ended up just using mine. But I think this procedure will go much easier on a 6-cyl, because the fact that the adapter would be in the corner of the airbox lid will give the filter more room inside the airbox.

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