How To Fix A Broken Vacuum Line Fitting on the Air Box or Air Filter Housing Of A 2008 Toyota Highlander And Other Models - It's Easy and Cheap!

I’m going to leverage my inner Forrest Gump – used cars are like a box of chocolates. You never know what you are going to get. In this case, it was our 2008 Toyota Highlander. I decided to check the air filter just in case and noticed both that the vacuum lines had cracked and that someone had done a “creative” fix on a 4mm vacuum port that had snapped off.

So this vaccum line was just resting in the hole. There’s some white glue – maybe silicone and som odd metal insert.

If there’s one thing I have learned with modern computerized cars – don’t mess with their sensors or vacuum lines. You can get odd random codes thrown, lose performance, fuel efficiency, etc. So, I knew this needed to be fixed. The big problem – the vacuum fitting was cast into the airbox!! By the way, the air box is the car part that holds the air filter in this case – the air filter housing is another way of thinking of the part.

Let me start by telling you the expensive and time consuming way – buy a replacement air box. Yeah, this is going to cost you a bundle. A OEM Toyota air box will run you $275-400, used is about $100-185 and aftermarket tends to be under $60-90 and maybe iffy quality. Then there is the labor to do the actual swap – it’s going to either take your time or you are going to pay a mechanic to do it and the cost is going to go up fast. There is an easier and faster way.

My Recommended Approach

The magic fraction for today is 5/32″. Why? Because 5/32″ is almost exactly 4mm. If we get a small brass, aluminum or stainless barbed 5/32″ hose fitting with a threaded rear, we can easily fix this. I live in a rural area and this isn’t something I can easily walk into a hardware store and find so I did some searching for NPT to 5/32″ hose barb and found both 1/16″ NPT and metric fittings plus some were brass, aluminum and mystery metal. I discounted anything plastic/nylon because I wanted a stronger fitting.

Another reason I went with 1/16″ NPT is that it can fill a pretty big hole all by itself up to about 0.31″ given the taper plus I could then use a 1/16″ to 1/8″ bushing adapter if I needed to go larger.

After much digging around using Google and Amazon, I opted for an Aeromotive #15630 hose fitting made from 6061-T6 aluminum that I did order off Amazon [click here for the Amazon listing]. There was a no-name generic brass one but no spec so I didn’t go with it. By the way, searching for fittings like this really shows the limitations of general search engines to find parts. I spent a ton of time filtering through tons and tons of search results that turned up the wrong products. What a headache.

Aeromotive 15630 fitting that has 1/16″ NPT thread on ne end and a 5/32″ hose barb on the other.

Installation

The first thing to point out is that the molded vacuum fitting is above the air cleaner so you can safely work on the box without removing it from the vehicle! Any debris from drilling or sanding will land on the filter and you can vacuum it out later. If you have a used car, peak inside and make sure the air cleaner is there and intact.

If you have just the old busted remains of the hose fitting to contend with then Dremel or sand the area flat. You want the installed barb to be able to sit flat against the wall of the box.

Next, pick a drill bit that is just the same size as the tapered bottom of the thread. NPT thread is tapered so the bottom has a smaller diameter than the top. If the air box was steel, we would use a letter “C” drill bit to make a 0.242″ hole. Notice how this is slightly smaller than 1/4″ but I am betting most people do not have lettered drill bit sets so you pick a close size and run with it. We do want the hole slightly smaller in order to thread it. Because this is plastic, we are going to push forward with the metal adapter fitting while turning and let the thread on the adapter cut the thread into the plastic. We aren’t going to bother tapping it first. That’s right – don’t buy a tap to do this uness you are a tool junky and perfectionist, which is fine if you are – I get accused of that a lot.

I’d recommend you start with a bit smaller than 1/4″ where the bit is slightly smaller than the bottom of the adapter, drill the hole and see if you can press it in. If not, go to a bigger bit. If you mess up and have a slightly too big hole, all is not lost – read the “Well crap” section below 🙂

Let’s say that everything goes great and you can screw the fitting into the plastic – Once threaded in, it’s done. Just screw it in and quit. Don’t put too much torque or you can strip it. That works just fine unless the previous guy bubba’d it, which takes us to the next part of the story.

Well crap….

In my case, I had a box of chocolates moment. The previous owner or a mechanic had drilled the hole out and installed some small metal bushing that was hidden due to the white silicone on it. I have no idea what it was from. Once I discovered and removed it, the hole in the air box turned out to be just a tad bigger than the entire 1/16″ tapered thread. Argh… not what I wanted to deal with.

Dear bubba, thank you for hiding this under the white silicone. You just made my fix a bit more complex but not impossible.

Okay, I wanted to get this job and had three options, go to the hardware store and try and find a 1/16 to 1/8″ bushing, install a 1/16″ NP threaded nut on the back to hold it in position, which I also did not have, or glue the hell out of it and call it even.

Because I am always working on cars and pressure systems due to Ronin’s Grips, did have a tube of black Permatex Optimum Black Gasket Maker that I could use. Being black, it wouldn’t be so glaringly obvious. By the way, I opted not to use epoxy due to the flexible plastic walls and expected vibrations that might break down the bond with time. [2/12/24 Note: I think Permatex dropped the Optimum series and their Black Ultra should work just fine – I’m linking to the Amazon listing]

This is what I had on hand at the time and worked great. I could have used any quality black silicone RTV glue/gasket maker and gotten the same results. I tend to either have Permatex or LocTite brand products that I use the most.

First, I scuffed the surface around the hole with 100 grit sand paper so the glue could get a better grab on the surface. The second thing I did was to spray brake cleaner on the area to remove any trace oils that might prevent a good bond.

I was wearing nitrile gloves and also sprayed brake cleaner on the fitting to make sure it was clean. Gloves both keep your hands clean and also prevent you from getting oils from your skin on the parts — assuming the gloves are clean of course. Having a clean surface makes a HUGE difference in terms of how well any glue is going to stick.

Next, I applied a bead around the top of the NPT thread of the fitting and pushed it into the hole. Since I was wearing nitrile gloves, I just took a finger and smoothed the glue out a bit and let it cure for about an hour or so. You want it cured enough to hold the adapter in place – you don’t need it fully cured yet. With warmer weather it will cure faster and in cold weather it my take a long time and need a hot light or something to warm it up above 70F to get things done. Depending on what I am working on, I try to get somewhere between 60-110F. The warmer it is, the faster it will cure but don’t burn it or melt the plastic either!

This is the first pass. It’s not pretty but it will dry strong enough to hold the fitting in place while the second thicker coat is applied.

I then put a second layer of black gasket maker on and fanned it out to get a good grip and to securely hold the fitting. I then let this all cure overnight. I’ve learned long ago not to rush faster than what the adhesive’s manufacturer recommends or you are liable to ruin an otherwise good job.

Here is the second heavier coat this was meant to reinforce the part.

So, once it was fully cured I then needed to change the cracked vacuum lines which are what I noticed in the first place.

4mm Vacuum Lines

What got me started on all this in the first place was noticing that two 4mm vacuum lines were very cracked where they slid onto their respective hose barbs. This happens as rubber ages and gets brittle so finding them wasn’t surprising.

This is one of the ends that was badly split.

Fixing this is easy. You just need either real 4mm vacuum hose or 5/32 vacuum hose. You can either go with one formulated from rubber or more expensive silicone. The advantage to the latter is that it ought to last longer.

Because we own a number of aging Toyotas, I bought an assortment bag of metric sized silicone vacuum hose of Amazon some time ago. It comes with 4mm x 82″, and the 52″ lengths of 6mm, 8mm, and 12mm. It was expensive but now I have an assortment for when I need to repair small lines such as this case. [2/12/24 the vendor I bought from years ago is gone. Check out these listings of sets on Amazon – go with a vendor that has at least 30 ratings of 4-5 stars]

You can see the two pieces I replaced because the new silicone hose is shiny black and a thicker diameter.

Conclusion

Going this route saved us a ton hundreds of dollars and has held up just fine. I did this repair last fall and have not had a single problem. Yeah, the glue does make it a big of a bubba fix but it is revsersible and the fitting isn’t going to fall out. The black gasket maker has held the little barb in just fine and the hose is very supple and shows no wear at all. I hope this helps you out as well.

2/12/24 Update: My repair is still just fine. A reader pinged me that none of the Amazon links worked so I went through and updated them.

7/28/2023 Update: Everything is holding up just fine and we’ve put quite a few miles on the car since this was written in March of 2020.

4/9/2021 Update: Still holding just fine. Not one problem to report.

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