2 replies to this topic

[Carl]

I was racing with Champcar  at Gingerman Raceway over the weekend.  We were using a friend's 1999 Spec Miata.  Temperatures were in 50's all day Saturday.  At the end of the third stint I noticed a significant whoosh of pressure as I removed the gas cap.  Seemed very odd, but not a problem.  About 3/4 of the way through the 4th stint our driver got black flagged and we decided to top up the tank to make sure he made the full 2 hours to finish the race.  When I removed the gas cap, there was a louder whoosh and a cloud of vapor came out at high velocity.  A few seconds later the vapor turned into a full-on geyser, about 5' high, of liquid gasoline.  The geyser lasted for 20-30 seconds, so not an insignificant amount.    

 

The car had the stock evaporative emissions systems in place, except for the two small tubes connecting the filler neck to the fuel tank and charcoal canister.  Those were jumpered as described/recommended elsewhere in this forum.  We spent the rest of Saturday and all of Sunday trying to understand how the system works and bypassing various parts of the system.  We would make changes, go out for 5-10 laps and come back in to check pressurization of the tank.  The final configuration was a gutted rollover valve connected directly to the liquid/vapor separator in the engine compartment with the separator vented directly to atmosphere.  Even with that configuration we had pressurized vapor coming out for 10-ish minutes after driving about 5 laps.  

 

At some point we recognized that there was definitely significant fuel boiling going on in the tank during the geyser episode.  It was also noticeable under a couple other scenarios.  After quite a bit of reflection we realized that the difference between the whoosh after the 3rd stint and the whoosh/geyser in the 4th stint, was that the tank was essentially completely empty after the third, but still 1/3 to 1/2 full in the 4th.  When the remaining fuel started boiling when the cap was loosened (and pressure somewhat reduced) during the 4th stint, the violently bubbly fuel was carried up into the fuel neck by the rapidly escaping vapor.  

 

The exhaust system seems to be the standard "Springfield" configuration, so not really very close to the fuel tank.  Obviously some line of sight and heat transfer from exhaust to tank.  It should also be noted that this car was previously raced in a couple of enduros in spring/summer time frame without similar problems.  At this point, I am feeling that the most likely problem was the winter-blend fuel that was likely still being used in Michigan.  The initial boiling point of winter-blend fuel in that area (as opposed to Southern California where I normally race)  is around 100 degF.  Normal summer blend has an initial boiling point around 140 degF.  

 

So my question is whether anyone else has experienced severe fuel tank pressurization using winter-blend fuel, especially in "northern climes".

 

Any other thoughts on the cause are welcome as well, as I could very well be off in the weeds.  

 

Cheers,

 

Carl

[Carl]

Problem probably solved.  An exhaust leak, 1.5" x 0.75", was found directly blowing on the gas tank.  I believe winter blend fuel contributed to the severity, but the exhaust leak seems pretty egregious.  Because the leak was after the muffler, it was not distinguishable from the normal exhaust sound and the hole was high enough that it would have entailed risking burned hand/forearm to search for it otherwise.  Anyway, another day at the track, another lesson.

 

Carl 

[Jeff Wasilko]

Glad you found it!