How to Eliminate Miata Gas Fumes (I hope…)

Thermo-Tec 3' exhaust heat shield.

Thermo-Tec 3′ exhaust heat shield.

Gas fumes are a problem. Ask nearly any first gen Miata owner and they will have a gas fume story. Some are less smelly than others. Some rarely stink. Some always stink. But it’s just a fact of life. If you have an early Miata, you’ve smelled gas in your trunk. Hell, sometimes it happens to NBs too.

Fortunately, there’s a few things you can do about it. Unfortunately, there’s a lot of possible causes. This post will address a couple and their possible solutions.

1: Spare tire

This is what the forums always talk about. The spare tire can out-gas and cause a fuel-like smell. The fix is to either remove it or wrap it in a plastic trash bag and seal it up.

In 12 years of ownership, I’ve never had a spare in Sharka. Bucky has always carried a spare, since 2001, and his trunk has no fuel smell. The tire is not my problem. But it’s always discussed in forums, so I had to include it first.

2: The big tank fill hose in the trunk.

This is the other low hanging fruit. Behind the metal cover on the left hand side of the trunk (port side, US driver’s side, etc) is a huge hose. It is sealed to the tank on one side and the metal filler pipe on the other by hose clamps. Make sure these clamps are tight.

I tried this fix about 3 months ago. I unbolted the panel and inspected stuff. My hoses were all healthy and the clamps were tight. I didn’t think to take a photo for this blog post. It’s just a hose with some clamps anyway. No photo necessary.

3: Exhaust heating up the tank.

Like a built in fuel warmer.

Like a built in fuel warmer.

This is the main point of my blog post. Heat. Heat and the management of it in relation to the gas tank.

Check out that exhaust. It runs right under the fuel tank. There’s actually some space between them… maybe 2″ or so. It doesn’t TOUCH the tank, but it gets quite close. And there’s no shielding anywhere. I think the factory exhaust includes some shielding on the pipe. Aftermarket ones? Nope. No chance.

When Geoff (stoly) and Steve (r2limited) were here in NM visiting me, I started to form a theory. While driving around in the mountains, we were all plagued with fuel smells and a lot of pressure in our tanks. A lot. All three cars. All three were NAs, but all three were rather different. However they all had aftermarket exhausts…

Heat + gas tank = tank pressure? Hmmm. A theory. It formed.

Quite the heat shield.

Quite the heat shield.

So, I researched my theory. It turned out many V8 conversion guys were having crazy problems with fuel tank pressure and fumes. The fumes were forcing most to refuel after only a half tank or even 100 miles of driving. A fuller tank has more fuel mass and heats up more slowly. And two exhaust pipes and V8 heat levels will indeed warm things up. A few of them recommended this particular heat shield. I hit Jegs and picked one up.

Thermotec sells several of these. They do a 1′, a 2′, and a 3′. I bought the 3′ after measuring the midpipe. It seemed like it would probably be mostly covered by a 3′ and I thought I could always cut it down if I needed.

Storm Shadow is impressed.

Storm Shadow is impressed.

Quilted heat shielding. Yes!

Quilted heat shielding. Yes!

I didn’t expect this shield to be quite this nice. Fiberglass cloth with nice metal brackets. And on the other side is a reflective barrier that’s sort of quilted onto the fiberglass. Really really nice. I guess I expected just some stick on stuff that resisted heat. In fact, I’d already tried some stuff like that.

Also, this shield will fit damn near any pipe you can throw at it. Sharka has 2.5″ pipe. I think a 4″ pipe would be no problem. They ship huge hose clamps with this shield.

Place on top of pipe...

Place on top of pipe…

... add hose clamps...

… add hose clamps…

... and fasten!

… and fasten!

Really not much to it. The hose clamps are, as previously mentioned, huge. I used a power driver to fasten the hose clamps down. I started out with a screwdriver and my forearm was burning after just one clamp.

One beefy shield.

One beefy shield.

The heat shield runs the length of the midpipe. Almost from the cat all the way back to the muffler. It provides shielding not only for the exhaust, but also for the differential. And that’s GOTTA help. It’s just gotta.

I’ve only driven a handful of miles since installing this piece. I’ll update this post when I have some concrete data on the effectiveness of this mod.

-Update: 10/2014-

I have no definite proof that this shield is the magic bullet fix, but I’ve had ZERO problems since installing it. Well, until recently. I’ve started to have a bit of fumes in the trunk and excessive tank pressure after a long drive. BUT! I’ve not cleaned my tank valves (as seen below) since posting this article. Probably about time for a cleaning.

The heat shield has been a great addition. I highly recommend picking one up for anyone with a tank pressure or fume problem.


4: The valves on top of the fuel tank.

Stuff designed to keep fuel in the tank.

Stuff designed to keep fuel in the tank.

This is another possible problem area. The gas tank has these little valves in the top. One is designed to keep fuel from splashing out in the event of a roll. The other is a sort of one-way check valve designed to let pressure from the tank out to the EVAP system to eventually burn though the motor. That’s the idea anyway. After 15+ years, how many of these valves on the NA Miatas are still functional?

Sharka’s is actually functional. At least it was a few years ago. I’ve already had this apart to clean once. I didn’t blog about it back then, but it worked great. This time, I brought my camera.

1: Roll back the carpet.

1: Roll back the carpet.

If you have a soft top, this isn’t the easiest job in the world. You have to put the top up to get enough room to roll back the carpet. You also have to sort of crouch behind the seats. It sucks. Having no top or a bikini top makes life really nice.

2: Unscrew this small panel.

2: Unscrew this small panel.

If you ever wondered, this is how you change the fuel pump as well. It’s the larger item beside the valves in the top photo.

3: Remove valves.

3: Remove valves.

There’s really nothing to it. The anti-roll valve is held in place by four small screws. The check valve just has hose clamps holding it in place.

By the way, manual transmission Miatas have one check valve. Automatic cars have two. And I think CA-spec cars have a network of three valves? Very strange. I once saw a photo of one. Looked like a maze designed to keep fuel vapor from ever leaving the tank.

4: Clean out the valve.

4: Clean out the valve.

Fill both sides with carb cleaner and let sit.

Fill both sides with carb cleaner and let sit.

The little valve works exactly like a PCV valve. It should rattle when you shake it. You should also be able to blow through it in one direction but not the other. When I removed Sharka’s it didn’t rattle. I could also blow through it both directions.

A little carb cleaner fixes it right up. I just sprayed a small bit in each side and then shook it up a little bit and drained the cleaner onto the rocks in my front yard. Then I filled up each side of the valve and let it sit propped up in the corner of my garage for maybe 10 minutes. Nothing to it.

After letting it sit, the valve once again rattled when I shook it. It also let me blow though in only one direction. Success!

The anti-rollover valve should also rattle when you shake it. Mine seemed fine. Still, I gave it a tiny cleaning with the carb cleaner while I was waiting for the check valve to un-stick.

5: ????

I’m sure there are other ways to deal with gas fumes and excess pressure in the fuel tank, but I’m not remembering any off the top of my head. Anyone out there reading along know of any? If you do, please share. I’ll either update this post with them or make a whole new one.

One more thing….

Way better than carpet buttons.

Way better than carpet buttons.

The plastic carpet pins that hold the carpet in place on top of the rear package tray are the devil. They suck. They will magically evaporate while you’re trying to remove them, regardless of your skill or what awesome specialty tool you’re using.

The things I’m holding in my hands are small plastic fasteners designed for keeping the plastic interior panels attached to their metal bodywork frames. They are AWESOME replacements for the evil, crumbly, awful carpet pins. I’ve had these holding my rear package tray carpet in place for at least 5 years. They work perfectly. And I can remove and install them as many times as I want and they just keep on working.

If you ever have to roll back your carpet for any reason, I highly suggest swapping out your broken carpet pins for these bad boys.

Update!!! 2/1019

After way too many years, I’ve finally solved the fuel tank pressure problem and fuel smell. Check out the blog post and scrutinize your white check valve that I mentioned cleaning above. For me, it was the culprit.

I chose to swap it out with a piece of straight hose. A new valve from your Mazda dealer might be a better choice. However, anything that keeps that pressure from building up has gotta be an improvement.

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  • Elliot says:

    Holla! Could you link which carpet pins you purchased? I recently did my softop (nice writeup by the way) and destroyed a few of them. 🙁

    • Chris says:

      I’m sure any hardware store (HD, Lowes, Ace, etc.) would carry them. I need to replace mine as well.

      • revlimiter says:

        Every time I hit a hardware store looking for car parts, I end up disappointed. ACE does indeed carry a lot of stuff, but it’s kinda hit or miss for car fasteners.

        The local O’reilly’s has a good selection of these little pins. And there’s always McMaster-Carr online.

    • revlimiter says:

      Sorry, but I can’t. The ones pictured in my hand are from my dead 97. I saved all the plastic fasteners and as many bolts as I could. It’s kind of awesome having a whole car worth of spare hardware.

      That said, just about any auto parts store should have these. They have the regular carpet pins too. The kind that breaks after one removal.

  • revlimiter says:

    I just looked up the part number in one of my Mazda parts catalogues. It seems to be G032-68-865 for that little plastic pin.

  • Harry says:

    Mitsubishi uses those fasteners as well, 1994-2001 ish. You can probably find 7 of them easy by hitting a breaker yard and popping a few hoods. They seemed to use them to fasten the top of the bumper skin to the radiator support obracket.

  • mxRev says:

    You and your site have me as a new NA Miata owner REALLY EXCITED….You sir are AWESOME. Internet high five! Also, I really want to try the exhaust wrap. What’s the product name, so that I may look it up. Thank you!

  • […] Inside gas fumes?…-fumes-i-hope/ There ya go. There's a few things you can try in here. Probably cleaning the valve on top of the […]

  • ken says:

    Just wondering how your gas fume situation has been, now that it’s been over a year.

    Do you still have gas fume smell or has it been eliminated?

    • revlimiter says:

      The fume smell comes and goes, but it’s much better than the previous year. I think it really does stem from the valve on the top of the tank, at least in my case. A yearly cleaning is probably what I need.

  • WabaamJam says:

    I am having excesskve tank pressure and was unable to find the heat shield so I wrapped the entire mid pipe with header tape. I have not had time to test my set up yet.
    How has it affected the tank pressure issue?

    • revlimiter says:

      Now this is one I can answer. (unlike the above where I don’t have a good answer)

      Problem solved. There’s no more tank pressure issue. I’m SO pleased about this.

  • Tom says:

    I was pretty amazed – I popped the cover off of the top of the gas tank and the fumes I’d been smelling just came wafting up, and the little white valve was definitely not clicking. Easy fix. The toughest part was getting the surprisingly rusted coverplate off. Apparently my car has spent more time in the elements than Sharka…

  • Alex says:

    Thanks for the write up, the fumes have been killing me lately. I’m going to try some of these methods when i put my roll bar in this weekend. also i’ve heard getting a new tank cover helps (mine was so rusty it had holes in it) seal off the area better.

    Unrelated, but what seats are those?

  • Alex says:

    So I cleaned my valves (got the heat shield kit on order) my car is manual but has 2 valves. I took on out because if they came with one I’m sure that’s fine. Plus one less thing to cause poor venting.

  • Montalvo says:

    Any updates??? I just tried this. Let’s hope it works. Thanks for your help.

    • Adam says:


      It was in the 110s here last week. That’s uncommon. Hot for New Mexico is usually 100s. 115 is a neighborhood unknown to us for the most part.

      Bucky had a hell of a time. Gas fumes like crazy. A tire melted into a parking lot while parked in the hot hot sun for a couple hours. No fun at all. And Bucky is an NB with the factory exhaust.

      Sharka, on the other hand, had no problems through the heatwave. No fuel fumes. No bad behavior. Even decent intake temps thanks to the intercooler. Really, Sharka just shrugged off the heat like it was nothing.

      So, I’d say the heat blanket on the exhaust really really helped.

  • Hector Bracamonte says:

    thanks for all the info, I will try it over the weekend

  • Elaine says:

    Thank you, thank you, thank you for all your tips. I couldn’t figure out why I smelled gas all the time and especially in my trunk. I have a 2002 Miata rag top and love it. Wish it was a 6 speed instead of a 5 tho. Always sounds like it needs another gear…Miata dealership advised not topping off my tank when I fill up…I knew it wasn’t that!! Some mechanics think all women are idiots when it comes to cars. Again, Thanks for the Miata Knowledge. Appreciate it!!

  • […] also reminded of what Adam/revlimiter did (for another issue but it can probably be applied here): How to Eliminate Miata Gas Fumes (I hope?) ? googletag.cmd.push(function() { googletag.display('div-gpt-ad-14'); […]

  • Don says:

    I was finally able to try out your tips on my 1997 Miata, manual transmission.
    1. Bought the same heat shield you did and installed it over my stock exhaust,which happens to have its own heat shied. Went for a long drive on a hot day. The pressure in the gas tank was still very high when I cracked open and then removed the gas cap. Still smelled gas which brought me to #2.
    2. I smelled gas at the front of the engine. The outside of the #1 injector seemed cleaner than the other three so I thought I had an O ring leak. I replaced all the O rings on all four injectors. No more gas smell in the engine compartment.

    Went for an hour drive on a warm, but not hot day, and the pressure in the tank seemed reasonable. Not high. No gas smell.

    #3 Even though there didn’t seem to be a gas smell, I did check my check valve over the tank and it didn’t make a sound when I shook it. I couldn’t get air through it in either direction. I cleaned it with carb cleaner. Now it rattles and air goes through it as it should. Of note, per the 1997 Miata service manual, air is supposed to be able to flow in both directions but there is more restriction in one direction than the other. The little arrow on the valve points to “port A.” The other port is “port B.” If you apply a vacuum of at least -15 mmHg to port A, you should get air flow. If you apply a vacuum of at leat -44 mmHg to port B, you should get air flow.

    Went for a two hour drive on warmish day, 80 degrees, and the gas tank pressure was not all that high and no gas smell anywhere.

    I am thinking that the elevated pressure in the tank was caused, in combination, from the exhaust heating the tank and the inability of the excess pressure to be releaved by the stuck check valve.

    I am caustiously optimistic that the problem is solved. That is very good news as my spouse will now allow me to keep the car!

    Thanks for keeping up your webpage! Love your tips and ideas.

  • Ken says:

    I can’t believe it’s been over 5 years. Just wondering if you still have gas fumes?

    My Miata’s been sitting for over 12 years now so I haven’t had a chance to do this…but it will rise again…soon.

  • Mikki says:

    I have a 1999 Mazda Miata automatic
    The gas fumes are so bad my eyes water and I get an instant headache!!
    I have 187 thousand miles. Always do maintenance on the car. I don’t live near a Mazda dealership and my mechanic in a small town probably has it worked on a lot of Miatas!
    Any advice?

  • Dave Reiter says:

    I am doing the modified setup and running a line directly from vapor outlet to the catchcan sitting behind rear tire. Q: Do I need to cap the old line (that one that runs up to the charcoal canister in engine compartment)? I have not capped it as of now and car seems to be running okay, but maybe I’m just dull. Thank you so much for your help! I am excited about this fix!

  • Burnice Bauch says:

    Thank you for sharing such an insightful article. Hope to read more content just like this in the future.

  • Dylan says:

    So far I’ve cleaned up the filler neck on my 2002 Miata and replaced the O-ring on the gas cap. The hise on the filler neck is prone to rusting and getting loose over time. My O-ring was cracked and dry and a new one with some silicone paste seemed to fix filling my garage with fumes after getting gas. The O-ring size is 50mmx40mmx5mm. Makes tightening/loosening the cap a chore however.

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