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Author Topic: Excessive Gas Tank Pressure
Muda Verified Driver Made Donation to Website
ComingToAMirrorNearYou

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Year : 1991
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During my first test day I developed excessive pressure in the gas tank. A small amount of gas collected in the area around the cap. Gas smell in car. Large release of pressure when I slowly removed the cap. Did this at the end of a 20 min. session.

Tank was 1/3rd full. It was a cold day, high temp 43F.

Replaced gas cap. Removed, checked & cleaned rollover valve and vent valve (the metal one next to it). Checked charcoal canister relay. All seem to be OK.

I can't recreate the conditions until the next event so I'd like to be sure it is fixed.

Anything else I should check/replace? Could it have been bad gas/too much ethanol?

--------------------
Muda Motorsports
"We're all here 'cause we're not all there."

B Wilson Verified Driver Series Champ
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Sounds like you are super heating your tank with the exhaust. Very dangerous!

Ken Sutherland lost his tailpipe during a race and super heated the tank. The resulting pressure ruptured the fill hose, which immediately filled the car with fumes.

After that someone posted a photo of a T/A car with the same issue that had caught fire.

If you tried to install the SD exhaust with the clamp, get rid of it and get it welded.

-bw

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Bruce Wilson
2010 Oregon Region Champ
2010 Monte Shelton Driver of the Year
2010 25 Hours of Thunderhill E3 and Under 2 liter Overall Champion
Oregon Region SM Class Advisor

Muda Verified Driver Made Donation to Website
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Hummm, I did swap in an SD over the winter.
Installed with clamp but slit the outer pipe and used two band clamps. It's not going anywhere but it's easy enough to weld it. Also had a Supertrapp on the end but have run it like that with the old system.

Will check it for leaks.

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Muda Motorsports
"We're all here 'cause we're not all there."

Ken SM-94 Verified Driver
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You should get it welded. If you drop wheels and the muffler hits the track, the muffler will get pulled off the end of the pipe. What's left of the exhaust is pointing a the tank. You'll get about two laps before the rubber fuel filler hose in the trunk blows apart from the pressure. Not fun!

Muda Verified Driver Made Donation to Website
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OK, y'all convinced me. Will weld tonight and check to see if leaks caused the tank to heat up. Don't think that was the problem as the exhaust was very quiet.

Can a faulty fuel pump add pressure to the tank?
Are the rollover and vent valves the only outlet for tank pressure?

--------------------
Muda Motorsports
"We're all here 'cause we're not all there."

Motor City Hamilton
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I am having the same problem, but do have my SD exhaust welded.

Excessive pressure and my fuel gage drops to empy about half-way through the 20 min race. Any other thoughts?

Dr.Dan Verified Driver Made Donation to Website
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Does your supertrap have the baffle plates on the end? If so, A) they are very restrictive, B) they send the exhaust gases out almost 90 deg to the tailpipe. so if you exhaust end to short, those exhaust gases will be circulating under the car, potentially causing your issue. I'm guessing of course. you said your car was quiet, so I was assuming you might.

--------------------
Daniel Mairani DDS...still here, just faster.
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Keith in WA Verified Driver Made Donation to Website
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Since rear section has a lot of variability in the way it can be installed, it could be the exhaust is riding too high and the pipe itself is heating the tank. I've haven't heard of it with a Miata but I have with an ITA CRX.

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Keith Novak
(Will work for tires)

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I'll be adding some more reflective adhesive-backed material tonight to the bottom of my gas tank. I've already got it lining the bottom of the trans tunnel and it keeps the interior very cool.

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-Cy
Supported by LTD Racing & Speed Shack - New England's Premier Auto Accessory Store
Rt1 AutoMile - Norwood, MA
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Ken SM-94 Verified Driver
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quote:
OK, y'all convinced me. Will weld tonight and check to see if leaks caused the tank to heat up. Don't think that was the problem as the exhaust was very quiet.


I agree it's not your problem. I was just piling on the get it welded for safety thing. [Smile]

Keith's idea of the pipe being too close to the tank would be my best guess for excessive tank pressure.

Good luck!

Muda Verified Driver Made Donation to Website
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quote:
Originally posted by Dr. Dan:
Does your supertrap have the baffle plates on the end?

Dan, Yes it does and yes it is. Been running Supertrapps at Lime Rock for years as it's 89DB there. Kind of like stuffing a banana up your tail pipe. Good thing LRP is not exactly a HP track.

I did, however, remove the directional baffle at the back of the disk pack which would have kept gases from being directed at the trunk. Still, this is well behind the axles and shouldn't affect the tank temp.

But I did enough under there with the exhaust system to blame it rather than a failure of the emission components. I think I will put the old system back on for the test day next week.

--------------------
Muda Motorsports
"We're all here 'cause we're not all there."

Greg Bush Verified Driver
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Gas gauges don't work in a race car at speed.

They all end up saying "E" as the fuel spends most of its time sloshed to the side of the tank.

Put enough gas in before the race and ignore the gauge, you have faster moving things to worry about.

Motor City Hamilton
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quote:
Originally posted by Keith in WA:
Since rear section has a lot of variability in the way it can be installed, it could be the exhaust is riding too high and the pipe itself is heating the tank. I've haven't heard of it with a Miata but I have with an ITA CRX.

What is the fix for this within the rules. Exhaust wrap would be the easiest, but that is no longer a grey area. Adding reflective tape or insulation also isn't mentioned in the rules, so wouldn't be legal. My Springfield Dyno exhaust seems to hang right where it should and I'm getting tank pressure, enough to blow fuel past the filler cap.

Champ
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This may seem obvious, but BE CAREFUL WELDING NEAR YOUR GAS TANK!!!!

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'99 Miata (1.8l) purchased and dedicated to SM in '09

chrisp993 Verified Driver
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Just wanted to add to this - last weekend at Putnam, when I attached my fuel test hose (to drain fuel) about a cup of gas spurted out as I completed the connection - car wasn't running, fuel pump wasn't on. Hmmmm. Odd, couldn't figure it out.

Next session I noticed a noise from the exhaust - found that the horizontal hangar just forward of the rear axle had broken and left a hole atop the exhaust, pointing toward the fuel tank. Yowza! Lesson is always root cause I guess .....

B Wilson Verified Driver Series Champ
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I saw one of our cars do that squirt a couple weeks ago too. We all commented on it, but didn't check anything -- too busy. Might be a good way to tell something's amiss.

Thanks!

-b

--------------------
Bruce Wilson
2010 Oregon Region Champ
2010 Monte Shelton Driver of the Year
2010 25 Hours of Thunderhill E3 and Under 2 liter Overall Champion
Oregon Region SM Class Advisor

Brandon F. Verified Driver
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quote:
Originally posted by Motor City Hamilton:
quote:
Originally posted by Keith in WA:
Since rear section has a lot of variability in the way it can be installed, it could be the exhaust is riding too high and the pipe itself is heating the tank. I've haven't heard of it with a Miata but I have with an ITA CRX.

What is the fix for this within the rules. Exhaust wrap would be the easiest, but that is no longer a grey area. Adding reflective tape or insulation also isn't mentioned in the rules, so wouldn't be legal. My Springfield Dyno exhaust seems to hang right where it should and I'm getting tank pressure, enough to blow fuel past the filler cap.
Though I see no explicit allowance (the old "no say can, then can't) I believe the approach would be to shield the gas tank since, as you note, wrapping the exhaust is verboten now per GCR. IMnsHlO (not so humble legal), it's allowed as a safety item to specifically protect the fuel from exhaust heat (corrected) exposure.

I guess it would come down to a protest, appeal, then rule clarification for it to properly shake out in the end...

[ 05-18-2010, 07:59 PM: Message edited by: Brandon F. ]

Irondragon
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Loosen the fuel cap!!

Motor City Hamilton
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quote:
Originally posted by Irondragon:
Loosen the fuel cap!!

And spill even more fuel out the back of the car. Even with my cap tight, fuel get's pushed past the cap seal.

B Wilson Verified Driver Series Champ
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is it pressure caused by pump or ?
Or is it pressure caused by heat? Mabe take the temp of your fuel after a session using the fuel sample port and compare to others.

-bw

--------------------
Bruce Wilson
2010 Oregon Region Champ
2010 Monte Shelton Driver of the Year
2010 25 Hours of Thunderhill E3 and Under 2 liter Overall Champion
Oregon Region SM Class Advisor

Keith in WA Verified Driver Made Donation to Website
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Or point an infrared pyrometer up at the gas tank. That will give you the temp of the part of the fuel system with the most volume. Wherever you have the most volume, will contribute most to the pressure if everything is the same temp. (It's been more than 15 years but I guess I did learn something in Thermodynamics after all!)

--------------------
Keith Novak
(Will work for tires)

Roger Caddell Verified Driver Made Donation to Website
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quote:
Originally posted by Ken SM-94:
You should get it welded. If you drop wheels and the muffler hits the track, the muffler will get pulled off the end of the pipe. What's left of the exhaust is pointing a the tank. You'll get about two laps before the rubber fuel filler hose in the trunk blows apart from the pressure. Not fun!

Ken, I remember that race at Pacific when it happened to you... scared me and I was not in the car!!

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Thanks!

Roger Caddell

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