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Old 07-23-2015, 02:54 PM   #1
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Default NB2 Fuel Smell after track use in high ambient temps

Guys,

I have a stock 2001 NB with the exception of brakes and tires/ wheels. I just tracked it at Summit Point Friday - Sunday. On Sat/Sun after 3 sessions there would be a strong fuel smell, from the trunk / right rear wheel well.

I have never had the issue driving the car on the street only after track use where the ambient temps were in the 90's.

After speaking with Emilio from 949 who has not experienced the issue he advised me to post here. I have already gone through Miata.net with no firm conclusions.

For the record the gas cap was tight, I do not fill the tank past the pump clicking off and the fuel level was below half tank.


Any help would be appreciated as I go back to Summit in early August.

If there happens to be this exact thread on here and I missed it I am prepared for the ensuing flame fest
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Old 07-23-2015, 03:42 PM   #2
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Two thing I know of that can cause that:

1. No check valve on EVAP hose connected to intake manifold, allowing the boost to pressurize the gas tank.
2. No heat shielding on exhaust near gas tank, causing it to heat the fuel tank under high load.

But I'll be watching this thread to see what fixes it! Mine does this sometimes too, and I'm almost certain I just need to shield the exhaust better as it only happens after shutdown on realllyyyy hot days.
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Old 07-23-2015, 04:01 PM   #3
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I can confirm that exhaust heat can pressurize the tank and cause a strong gas smell. An extreme example of this happened to me when my exhaust broke. However, most of that smell was coming from the gas cap venting...
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Old 07-23-2015, 04:10 PM   #4
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Quote:
Originally Posted by patsmx5 View Post
Two thing I know of that can cause that:

1. No check valve on EVAP hose connected to intake manifold, allowing the boost to pressurize the gas tank.
2. No heat shielding on exhaust near gas tank, causing it to heat the fuel tank under high load.

But I'll be watching this thread to see what fixes it! Mine does this sometimes too, and I'm almost certain I just need to shield the exhaust better as it only happens after shutdown on realllyyyy hot days.
I have the stock exhaust, is the heat shielding not sufficient ? The car is completely stock other then brakes and tires? I looked under the car on a lift and there was shielding I thought that was an issue for aftermarket exhaust that had none.
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Old 07-23-2015, 04:18 PM   #5
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My NA8 builds pressure in the tank on hot days but isn't connected to the intake manifold.
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Old 07-23-2015, 04:29 PM   #6
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Originally Posted by sixshooter View Post
My NA8 builds pressure in the tank on hot days but isn't connected to the intake manifold.
I mean should I be more concerned or just keep driving it. Last event the car smelled super strong even after being shut off to cool, and stayed like that until I turned it back on and let it idle for 5 to 10 minutes.
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Old 07-23-2015, 05:46 PM   #7
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I don't like it, so I'm going to shield my exhaust better as I believe it's the cause of this on my car. It hasn't exploded or burned down, but it doesn't seem ideal, and I think I can make it go away with better shielding. I don't have any shielding currently back there....
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Old 07-23-2015, 05:52 PM   #8
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I just finished modifying my gas cap to try and alleviate all the pressure I build since deleting and capping the EVAP purge line.

The early NA6 caps have a two stage check valve on the top half, and a spring and ball bearing weight on the other to lock the cap up in case of a rollover.

I removed the check valve, or at least the plunger of the valve but left the rollover spring and ball bearing in place and verified it kinda sorta blocked flow a little bit when upside down. I suspect, or hope I guess, vacuum on the tank will keep it from dripping at all. I didn't modify that half of the cap, so that was as good as it'd ever be.

I'm kinda worried the tank pressure might seal the rollover vent valve shut since it's spring is pathetically weak, but there's only one way to find out, right?
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Old 07-24-2015, 10:25 PM   #9
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The only cars we had fuel odor with had extensively modified fuel systems and over filled (enduro). We ended up retaining the OEM charcoal canister overflow line that dumps in the right side of the front subframe. Charcoal can gone on the race cars of course. Our theory is any potential fuel drip at speed will be well evaporated by the time it hits the back of the car and it's away from the hot exhaust. Doesn't shed any light on the OP"s issue though, sorry.
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Old 07-26-2015, 10:47 PM   #10
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<p>considering the age of these cars and possible shape, it doesn't seem unlikely you may have a crack or leak in one of the evap/charcoal lines. &nbsp;there is two canisters if I am not mistaken, take a look at both. &nbsp;Right rear wheel well is pretty close to the fuel filter and fuel tank charcoal canister. &nbsp;Just because you got it &quot;stock&quot;, doesn't mean it IS stock (or in the same shape as stock).</p><p>&nbsp;</p><p>And yea, any exhaust leaks under the gas tank will cause you a bad day. &nbsp;We had a SM/PTE car lose/damage his&nbsp;exhaust earlier in the day and literally blew the rubber fill hose into pieces while waiting in pits. &nbsp;It then promptly emptied its contents in the trunk.</p>
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Old 08-02-2015, 04:17 PM   #11
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The only time I ran into this with my NB1 was after I pinched the gasket on top of the fuel tank while installing a new fuel pump. New gasket solved my problem. If you have not had it apart then that is probably not the issue but its something to consider.
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Old 08-02-2015, 10:06 PM   #12
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With my 97 NA, I get pressure build up in the tank on hot days. This is with the OEM tank/filler/rear lines, roll over check valve, EVAP, etc. However, the EVAP system in the engine compartment is MIA and I have the vent line routed into the front subframe. Without the engine applying vacuum to the EVAP system, is there some sort of valve which isn't opening to relieve pressure?
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Old 08-02-2015, 10:36 PM   #13
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You should just modify the gas cap, something in the lines is probably clogged.

I suspect you are right though, it's probably a rollover valve or something. If I'm not mistaken that vent line comes from the top of the tank. I remember reading somewhere someone blew their tank up through that charcoal vent hose, then the pressure got stuck till he unscrewed the cap. Check valve or defective rollover valve, doesn't really matter?

My car seems to run better and stink a **** ton less since I modified the cap.
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Old 08-03-2015, 03:42 PM   #14
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My 90 builds pressure on hot days as well. Smells a bit until I vent it by opening the cap and drive away.
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Old 08-03-2015, 03:49 PM   #15
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This thread is making me nervous. I also experience this gas smell in my stock powered NA8 and the symptoms are exactly the same. It is much stronger towards the passenger side of the car though I can't really tell whether it is from the front or rear. I don't ever notice it from the drivers side until I get out and walk around or a passenger points it out.

As the OP mentioned earlier, turning the car on and letting it idle for a minute seems to remove most of the smell. Next time out I will remove the gas cap and see if that removes the smell as well.
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Old 08-12-2015, 08:54 AM   #16
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So I just finished another 3 day event at Summit Point for Porschefest. The temps were only in the mid 80's and I never let the tank go below half. I had no fuel issues this time, but at one point when filling up some gas and taking the cap off there was a nice pressure release and a little bit of gas came out.

So I will keep monitoring it and let you know.
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