89 635 Gas Tank Brain Teaser - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 07-09-2008, 02:34 PM
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89 635 Gas Tank Brain Teaser

Yesterday here in Las Vegas it was hot (108F) so I was travelling across town with the air conditioning on in afternoon rush hour traffic -- lots of stop/start traffic. Wanting gas, I stopped at a service station, turned off the air conditioning first and then the car. Opened the refueling door and as I unscrewed the gas cap, pressure from the tank pushed the cap away from the filler tube. Looking down the filler tube the gas was bubbling (like it was cooking or boiling) and you could feel pressure comng out of the tube. It continued to do this for a good ten minutes. When it stopped bubbling I elected not to fill the tank, replaced the cap, drove the five minutes to the house, parked it in the driveway, opened the gas cap and hood and let it sit for an hour while I called my mechanic with 28 years of Beemer experience. He said that he never heard of such a thing but will check through his contacts. I, in turn, thought I'd try this forum to see if anyone else ever experienced this.

To Recap: outside temp was 108F; air conditioning was on; heavy rush hour traffic with numerous stop/start situations; semi aggressive driver; gas tank was half full at the time; no indications on any of the gauges that anything was going wrong; when unscrewed gas tank pressure forced the cap away from the filler neck; bubbling gas was observed in the filler tube and lasted about ten minutes.

Anyone got a clue???????

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#2 (permalink) Old 07-09-2008, 04:57 PM
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re-check your EVAP line routing... you may have crossed them with vacuum lines.
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#3 (permalink) Old 07-24-2008, 05:02 PM
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Been away for a while but finally back at the boards.
First -- Thanks for the heads up Darkhorse but that wasn't it.


The car went in the same day I posted the original and I finally got a call to pick it up yesterday. My guy with 28 years went through every possible pump, check valve, cleaned all gas and vaccum lines. Refilled tank to same level as when the problem developed drove it several times when the temperature neared or was the same as the problem time. Dropped the gas tank, checked the innereds and the lines that ran inside the trunk under rear shelf under the rear window. Checked fuel pressures and all were within tolerances. Finally noted that the vapor expansion tank in the trunk would expand a little more then normal. When it expanded the bubbling would occur indicating gas trying to return to the tank. When tested the tank was found in tolerance even when expanded. The only other possible cause as explained to me is that the new additives being added to the fuel now could be the prime contributing factor. I looked at my Pro questioningly but since every fuel, feeding, and vaccum system was found to be in working order I couldn't argue with him. I'm going to be watching it in the future against all weather conditions.
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#4 (permalink) Old 02-18-2009, 02:37 AM
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d3ity will become famous soon enough d3ity will become famous soon enough
Mine did something similar to this for a while. In hot temperatures the car would stall out when at idle. Turned out to be a vaccum-lock. When you took the gas cap off you could hear a hiss as air rushed into the tank. Mine would actually stall out when driving when temperature went over 95*F. Putting the cap on a little looser fixed the problem... but what you're describing seems rather extreme. Then again, rarely is it that I run into 108*F Temperatures.

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