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:
TFW you remove your Jeep's transfer case drain plug and not a single drop comes out.
— David Tracy (@davidntracy) May 14, 2023
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.
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:
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:
As for that mystery bolt? Well, I looked at the underside of that air filter housing lid, and here’s what I found:
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.
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.
Umm – re the concern about California dust getting into the air-filter housing….. isn’t the air-filter itself going to prevent anything getting into the engine? I mean, this cover is “outside” the air-filter….. isn’t it?
I have never worked on a Jeep, but in a lot of cars the top of the filter box is the clean side. It pulls air up from the bottom.
David, this Jeep desperately needs the early ’90s Islander graphics package to truly be California dreamin’
DT, not like you’ll need the spare key anyway with the soft top. I never used to lock my JK; better that someone doesn’t cut the top to get in (not realizing the windows unzip). The top is way the hell too expensive and annoying to replace.
In my old XJ (not sure if it’s true on my current one, haven’t actually checked) there was a thin metal bar (support?) across the inside of the gas door that had a little gap behind it. It was a perfect spot to leave a key; it would sit between that bar and the door, wouldn’t fall through, and the gas door just pushes open from the outside. Great place to hide a key when at the beach or somewhere similar that it’s hard to have a key in your pocket. I didn’t leave one there all the time, but it was convenient.
Am I going too far when I bench grind the plastic and any excess metal off the copied key, drill a whole in it and epoxy a neodymium disc magnet into that hole then coat the whole thing in Plasti-dip?
You can stick that spare anywhere.
I was kind of, sort of, right. When you first mentioned that bolt I figured that plastic is easy to tap and something was mounted there. I didn’t think that something was mounted under there. It must of been fun to find the keys. Get an adhesive Jeep emblem to cover the hole.
Considering the look both inside and outside it doesnt appear to me to be an after market add on. I was thinking additional air, filter spray, or not likely starter fluid spray. That bump on the underside looks like it is for a reason and owner just used it to hide a key.