Home » I Discovered What This Bolt In My Jeep’s Air Filter Housing Is And I Bet You’ll Never Guess It

I Discovered What This Bolt In My Jeep’s Air Filter Housing Is And I Bet You’ll Never Guess It

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One of the very first things I noticed when I popped the hood of my new 1991 Jeep Wrangler YJ (the square-headlight “Jurassic park Jeep,” to those of you who aren’t Jeep nuts) was a bolt in the air filter housing lid. I realize that’s such a small thing to notice, but when you’ve seen literally hundreds of 4.0-liter Jeep engines, you know what to expect and what not to expect. And that bolt-head atop that black air filter housing didn’t belong. So yesterday, while doing some maintenance, I checked my air filter (which was dirty) and finally figured out what that mystery bolt was all about. And boy was it a surprise.

I’ve been enjoying the hell out of my 1991 Jeep Wrangler YJ. It’s such a joy to drive, with its engaging Aisin AX-15 five-speed, its torquey 4.0-liter straight six, and its only 3,100 pound-ish curb weight. I haven’t even installed the half doors and soft top yet, and it’s still a hoot to drive; it will be the ultimate California summer-mobile, and after selling the hardtop and doors and installing new tires, I’ll probably only be in it $6,000, which is awesome for a machine so rust-free. Anyway, anytime I buy a new vehicle, the first thing I do is replace all the oils, so that’s what I was up to yesterday. Sadly, all was not well:

Vidframe Min Top
Vidframe Min Bottom

That’s right, when I removed the transfer case drain plug, no fluid flowed out. I mean zero; it was bone dry. What long term effects that did to my transfer case’s bearings, chain, and sprockets is unknown at this point, but it’s not ideal. A couple of quarts of fresh ATF, though, and everything seems to be working fine. The transmission oil was a little dark, but it was mostly okay; I threw in some “Driven GL4,” which is apparently non-corrosive to brass synchronizers (this oil was recommended to me by a seasoned manual transmission rebuilder). The engine oil wasn’t too bad, either, and I was encouraged by the use of a decent filter. Still, that transfer case oil — yikes! I have yet to drain my differentials; I’m hoping I don’t get any surprises, there, and I pray that somehow my rear limited slip differential still works.

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Anyway, after pouring six quarts of dirt-cheap NAPA Auto Parts 10W-30 conventional oil into my engine, it was time to change out the air filter. This involved removing the lid, which features that weird bolt I’d noticed earlier:

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Removing the lid, I was greeted by an air filter that really wasn’t as clean as I’d have liked. I’m replacing it immediately with a nice Purolator filter:

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As for that mystery bolt? Well, I looked at the underside of that air filter housing lid, and here’s what I found:

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Keys!

Yes, the previous owner cleverly kept a spare set of keys in the vehicle’s air filter housing! That’s brilliant!

The Jeep Wrangler’s hood does not feature a release mechanism, so one can access it from the outside without issue. So if you were to, say, accidentally lock the keys in the car or lose them while out and about, you’ve got a spare set right there under the lid — where nobody would ever look! Plus, it appears there are house keys and gas-cap-keys bolted to the air filter housing lid, as well.

I dig it, though I will be replacing that lid, since I do not want to risk 1. A key getting sucked into the intake. This is extremely unlikely given how much a key weighs, but it’s not entirely impossible. But 2. My bigger concern is potential dust-intrusion through that bolt hole. Obviously, whatever gap there is between the bolt and its washer, and between the washer and the housing lid is very minimal, but dust — especially out in the California desert — can be fine, and I’m just far too particular about my engines to risk that. This is has got to go.

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With new oil in my engine, transmission, transfer case, and axles, as well as a new air filter and greased chassis parts (tie rod ends, ball joints, driveshaft U-joints), I’ll be read to whip this thing around LA without worry. Okay, so I first need to get some new tires (the ones on this Jeep are 13 years old) and do some more suspension checks (I suspect i need to replace a few leaf spring bushings), but then this thing will be ready to take on California’s beautiful off-road trails.

So far, I dig this machine. I don’t love that a previous owner drove it with a bone-dry transfer case, but I can appreciate that owner’s clever spare-key solution. And I appreciate the incredible shape they keep this machine in; it’s remarkable how gorgeous this machine’s body is.

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Mr Sarcastic
Mr Sarcastic
1 year ago

I am surprised checking the level and condition of all fluids isn’t something you do as a pre-purchase inspection. There level, condition, color, and yes smell can tell you so much so easily.

Jason Smith
Jason Smith
1 year ago
Reply to  Mr Sarcastic

I just made the observation on the post where he documented the purchase process that he didn’t even test drive the thing. But DT did point-out that he happens to have a spare engine and can rebuild a transmission in his sleep (well documented), so I guess just making sure it wasn’t 50% iron oxide kind of made sense. You know, in a David Tracy kind of way…

Speedway Sammy
Speedway Sammy
1 year ago
Reply to  David Tracy

Two quarts leaking out over time makes a pretty big mess. My old S-10 4.3 has a rear main seal leak with no perceptible oil consumption yet it looks very wet under there with noticeable drops on the ground. And since you said farther down it’s dry underneath, this is an interesting mystery.

OverlandingSprinter
OverlandingSprinter
1 year ago

I realize who I’m talking to here, but I’ll say this anyway: Check the axle breathers. If they’re sound and the ends are placed correctly up high, changing your diff fluids are not an urgent matter. But, speaking from experience, if your breather hoses are cracked or non-existent you may have trouble with your diff fluid.

Looks like one key is for a Club anti-theft device.

Andy Individual
Andy Individual
1 year ago

That’s pretty clever. If I had a Hyundai, I’d go and hide a USB key in my air filter right now.

In the old days we would replace our radio antennas with coat hangers in case we locked ourselves out.

CSRoad
CSRoad
1 year ago

Well at least you didn’t find it was part of the hidden NO2 system. (-;

Dumb Shadetree
Dumb Shadetree
1 year ago
Reply to  CSRoad

Honestly, this is what I expected it was. Obviously there’s no bottle, but I expected the bolt was plugging the hole from an old nitrous fitting.

Idiot_with_a_garage
Idiot_with_a_garage
1 year ago

I am not sure what is worse David; not having anything come out of the transfer case or the water and liquid feces that came out of the FC when that was drained. As someone who has locked his keys in an XJ 3 times now I may steal this idea, but if you are already locked out of the car how would I open the hood?

Mr Sarcastic
Mr Sarcastic
1 year ago

Did you know the old Fiat 124 Spiders have a emergency back up hood release?

FUCK YOU
FUCK YOU
1 year ago

You could always have just thrown a rubber washer under that bolt. ¯\_(ツ)_/¯

Arrest-me Red
Arrest-me Red
1 year ago

I though that was the rust attractor.

StayPutReachJump
StayPutReachJump
1 year ago
Reply to  Arrest-me Red

I think the correct term is ‘sacrificial anode’.

Chris Stevenson
Chris Stevenson
1 year ago

I just remembered another form of secret backup keys from the 80s and early 90s: Ford’s plastic credit card keys! Did anyone ever use these?

https://www.ebay.com/itm/185791611226

TXJeepGuy
TXJeepGuy
1 year ago

I remember AAA used to cut these for members at the Los Angeles Auto show as well. Had one for my Chevy s10

Jason Roth
Jason Roth
1 year ago

Wow, I totally forgot my Saturn had one of those! I wonder if it’s still sitting in some box somewhere.

Jalop Gold
Jalop Gold
1 year ago
Reply to  Jason Roth

Loved my Saturn one! Used it twice.

PlatinumZJ
PlatinumZJ
1 year ago

I think I remember something similar came with one of my dad’s trucks…that would have been a Chevy. I remember wondering how you were supposed to store them in your wallet, since the little vinyl slipcase was more bulky than a credit card. (Dad kept them in the glove box with the manual.)

Slow Joe Crow
Slow Joe Crow
1 year ago

My pickup came with a key that opened the doors but wasn’t coded to the ignition so I have it stashed under one of the stake pocket tie downs. So far I have used it once in 3 years when I thought I locked the keys in the cab but discovered that for some inexplicable reason I had put my keys in my right pocket instead of my left pocket. Apropos of nothing I have used the emergency start when the fob battery is dead procedure on my Mazda more times than an emergency key

Bizness Comma Nunya
Bizness Comma Nunya
1 year ago

I gotta admit, that hidden key location is pretty clever. Very strange…. but still clever.

Col Lingus
Col Lingus
1 year ago

Well you can take the shit show out of Michigan, but it just continues in LA.
Bolted keys? No fluids? When it comes to janky shit, I believe you have met your match California style. Several Forrest Gump expressions come to mind here. God help you man/child. Jesus. How about a new column? “Stupid shit DT does and buys.” It could be a daily thing….

Jason Smith
Jason Smith
1 year ago
Reply to  Col Lingus

At this point doesn’t you brain just kind of process any title as “Stupid shit DT does and buys.” whenever you read “by David Tracy”?

Gewf631
Gewf631
1 year ago

Used to embed a spare key in silicone caulk, placed in a clean spot behind a bumper or some other out-of-the way spot. Using a dark caulk, spreading it around (feathering if you will), it kept the key clean and dry, and was easy to peel away from the key.

TOSSABL
TOSSABL
1 year ago
Reply to  Gewf631

When my daughter went to college some 3 hours away, I taped a spare to her front strut with black gaff tape. Couldn’t see it easily, but could find it by feel. Worked for her-twice as I recall. Don’t use duct tape: it will get the key-then lock-all gummy, and fall off within a year anyway.

I like your caulk idea. May use that on my car

Icouldntfindaclevername
Icouldntfindaclevername
1 year ago

You’re worried about dust via the bolt hole Pre-Filter, not Post-Filter?
Granted, I would remove the keys/nut too. However, the dust will be stopped by the filter

My Goat Ate My Homework
My Goat Ate My Homework
1 year ago

that top cover is actually on the side that goes to the throttle. The bottom of the air cleaner is the part that is connected to the supply air.

Duffman
Duffman
1 year ago

I did that in my old TJ, but instead of bolted they were just sitting under the filter… DID until my little sister found out about that and took it on a joyride, followed by me screaming for a few hours.

My Goat Ate My Homework
My Goat Ate My Homework
1 year ago

So I guess you have to have access to a wrench, too? Imagine being stuck and not being able to get the bolt off.

Andy Individual
Andy Individual
1 year ago

He swallowed a fly, I don’t know why…

and so on.

Alan Christensen
Alan Christensen
1 year ago

Yeah, doh! The wrenches are locked inside! Or is there a pair of wrenches bolted to the frame somewhere with wing nuts?

Last edited 1 year ago by Alan Christensen
BigThingsComin
BigThingsComin
1 year ago

I’m assuming you just twist the key and unscrew it from the bottom. No need to remove the bolt.

Flyingstitch
Flyingstitch
1 year ago

Watching California David emerge, I hear Circle of Life on repeat in my head…

From the day we arrive on the planet

And, blinking, step into the sun…

sentinelTk
sentinelTk
1 year ago

I’d be more worried about #3…..Getting my “new” car stolen because I just posted an article about where the spare keys are to my website.

A. Barth
A. Barth
1 year ago
Reply to  sentinelTk

DT addressed that:

I dig it, though I will be replacing that lid, since I do not want to risk 1. A key getting sucked into the intake. This is extremely unlikely given how much a key weighs, but it’s not entirely impossible. But 2. My bigger concern is potential dust-intrusion through that bolt hole.

He explicitly expressed concern about the keys being there, so I think there’s a near-zero chance that he simply left the keys in place. Give the man some credit. 🙂

TheHairyNug
TheHairyNug
1 year ago
Reply to  A. Barth

what?

sentinelTk
sentinelTk
1 year ago
Reply to  A. Barth

My comment

————-

Your head

A. Barth
A. Barth
1 year ago
Reply to  sentinelTk

Uh huh. You expressed concern about the keys being there and the knowledge being made public; I replied (using too many words) that DT indicated he is not leaving the keys there.

There might be a disconnect, but it’s not on this end. 🙂

sentinelTk
sentinelTk
1 year ago
Reply to  A. Barth

“You know, I wouldn’t want to be hurled into the sun because it would ruin this pair of jeans I’m wearing.”

“Uh, what about that whole incinerating to death thing? Wouldn’t that be more concerning?”

“I already told you…..losing these jeans would suck.”

4jim
4jim
1 year ago

I thought that the transmission fluid would just leak into the t-case and keep it lubed up ???? ( i have owned a few old jeeps)

Chronometric
Chronometric
1 year ago

Jeep Security Secret the Manufacturer Doesn’t Want You to Know!

A. Barth
A. Barth
1 year ago
Reply to  Chronometric

Steal a Jeep with this one weird trick!

Sam Hoffman
Sam Hoffman
1 year ago

So if you lose your keys then you need a wrench to be able to get the spares? That is a nylock right?, usually pretty impossible to undo by hand. I guess you better look for wrenches…

A. Barth
A. Barth
1 year ago
Reply to  Sam Hoffman

I was thinking that myself. A nylock wingnut would be a good solution here.

On my old dual-purpose Yamaha there is a small compartment for the tool kit; the lid to the compartment is secured with a bolt that has a wingnut (rather than hex) head. The designer realized that a person who needs the onboard toolkit might not have tools to hand. 🙂

Col Lingus
Col Lingus
1 year ago
Reply to  A. Barth

This key shit is what God invented duct tape for. Why make shit so much work?

Michael Beranek
Michael Beranek
1 year ago
Reply to  Sam Hoffman

Probably the kind of guy who wears a Leatherman all the time, even while making love to his wife.

Chump Change
Chump Change
1 year ago

Wait. Is this wrong?

Phantom Pedal Syndrome
Phantom Pedal Syndrome
1 year ago

You can never be too handy.

Last edited 1 year ago by Phantom Pedal Syndrome
Andy Individual
Andy Individual
1 year ago

Hey, this motor oil is still good!

BigThingsComin
BigThingsComin
1 year ago
Reply to  Sam Hoffman

Just unscrew the keys from the bottom. No need to remove the bolt.

Chris Stevenson
Chris Stevenson
1 year ago

I get the idea for hiding a spare set of keys in a Wrangler engine bay, but I’d rather use a strong magnet to attach a keyring to the firewall (behind the battery, maybe, some spot that’s hard to see but easy to reach) instead of drilling a hole through the air filter cover.

Sid Bridge
Sid Bridge
1 year ago

That is crazy and genius at the same time! Also, where do you leave the Jeep parked and what are some convenient times when it will be unattended? Asking for a friend.

Crank Shaft
Crank Shaft
1 year ago

That is fucking brilliant! Zero thieves would have guessed correctly.

Mr. Canoehead
Mr. Canoehead
1 year ago
Reply to  Crank Shaft

It’s a manual transmission which pretty much already makes it theft proof these days….

Crank Shaft
Crank Shaft
1 year ago
Reply to  Mr. Canoehead

Indeed!

Jblues
Jblues
1 year ago

This also gives me the opportunity to re-state that you are under-valuing that khaki colored hard top. You need to list that sucker for $2k minimum. It’s rare.

Jblues
Jblues
1 year ago
Reply to  David Tracy

It is for the LJ. The hard tops for them go for $2K starting out but khaki ones are practically priceless. I’m assuming khaki is just as rare for YJs.

Jblues
Jblues
1 year ago

Most people probably don’t even realize the t-case has oil in it in the first place. Many that do know it, probably think it’s not something you ever need to check. What’s puzzling is that if there’s no evidence of a leak, how did it get dry?

JDE
JDE
1 year ago
Reply to  David Tracy

probably from flat towing it 100k miles. keep an eye on the out put shaft seals as you bomb around in it. one of them probably leak or seeps more with temperature increases.

Unclewolverine
Unclewolverine
1 year ago
Reply to  David Tracy

I dont know about 231s, but my 242 has a rubber plug hidden on the side obscured by the front drive shaft. Make sure that sucker is still there!

Unclewolverine
Unclewolverine
1 year ago
Reply to  David Tracy

Yes, it’s a small hole, I think to access a set screw or something. I think I knocked it out cleaning mine off, smelled hot atf while running the mail route, looked underneath and saw where it was running from. My d35 does have the large rubber fill plug and I love it. BTW, if you have the same limited slip my xj does they only work for about 50k miles.

Unclewolverine
Unclewolverine
1 year ago
Reply to  Unclewolverine

Are we talking about the same hole? The one I’m talking about is on the front, driver side of the case behind the double cardian joint/ front output flange. It’s too thin and close to the u joint to be any kind of sensor bung. Also I thought the 242 was a longer case, I know the drive shafters are different lengths.

Unclewolverine
Unclewolverine
1 year ago
Reply to  Unclewolverine

Not sure how it’s a pressure port, I just looked it up and it’s access port for a roll pin.

Bill Garcia
Bill Garcia
1 year ago
Reply to  Unclewolverine

Loving this deep dive into stuff I will never ever see – it’s the kind of useless (to me) knowledge I most enjoy acquiring.

Thank you fellow Autopians!

TXJeepGuy
TXJeepGuy
1 year ago
Reply to  David Tracy

Based on the year this doesn’t have the sealed output… the driveshaft does the sealing. Do you see any evidence of it flinging fluid around underneath?

10001010
10001010
1 year ago
Reply to  TXJeepGuy

I was gonna mention the same thing. My AWD Eagle (not a Jeep but sold on the same lot) Talon had this issue. The driveshaft yoke had a weepy seal and would slowly let the transfer case drain itself one small drip at a time. Never enough to leave a puddle but you could tell by looking at how clean the undercarriage was on either side of the yoke. It was a common enough issue back in the day that we convinced the NHTSA to force Mitsubishi to recall it.

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