Fuel tank pressurized

[dmac156]

I wonder if any of you have experienced this. Sometimes when I go to fill up my car, the gas tank will have some pressure in it. I'll slowly open the cap and let it escape. I'm not sure if it affects anything, but it also doesn't seem right. Has anyone else experienced this and/or know what is causing it?

[bkats]

Mine also does this; particularly closer to empty.

[DesktopDave]

My old Euro E23 did that all the time. Sometimes it would also hold a vacuum. The importer had installed a one-way breather valve by the fuel tank to purge fuel vapor, but it had failed. I sold the car before I figured it out. My vote is for a stuck purge valve on the US model, or a bit of hackery on the Euro car.

[86 shark]

I live in Colorado and when driving thru Rocky Mtn Park and stopping at rest areas at high elevation ( 12,000 ft ) 20-40 years ago I would always see a few cars that wouldn't start.
Remedy was to undo gas cap and relieve pressure that fuel pump was fighting.
Newer cars shouldn't have this problem.

[rlkrlk]

Yeah my euro does that also. I believe the euro tanks were vented to atmosphere, something that is a no-no here. There was a past thread, alluding to an exploding gas tank being narrowly averted or some such drama but I didn't glean the specific answer from it (at least that's what my memory is telling me). I think Dave is on the right track; purge valve and charcoal canister circuit. Hopefully others with actual useful info can chime in.

Cheers,

[tschultz]

Would be great to see your car.

Ever have a chance to make it to any of the local Sonic gatherings? There is also a CO BMW group on facebook you should look up...



Regarding the fuel pressure. My 635 did this years ago and I found out that during federalization it was a requirement to plug up the fuel tank expansion valve. My euro and my brothers 745i both had these lines plugged. The valve was still in place via the expansion tank in the trunk, but the pressure could only escape when the gas cap was removed. It didn't feel safe when I opened the tank and had vapors exhausting onto me.

The exploding gas tank was caused by the same issue. If you can't find the system plug, it may be wise to get a self-ventilating one way valve gas cap.

[dmac156]

The valve was still in place via the expansion tank in the trunk, but the pressure could only escape when the gas cap was removed.

Where exactly was the valve or plug located?

[tschultz]

The valve is part of the fuel tank expansion. Usually a grey tank in the trunk behind the carpeting.

The plug on both of our cars was on the bottom of the venting line which runs out to under the rear valence. I seem to recall that it was coming from the expansion tank which is how I found it. The end of the tube was somewhat near my exhaust pipe and pretty obvious when I found it.

[DesktopDave]

My car's modification (hack) looked like a plumbing elbow in a rubber boot, stuck into a length of fuel hose dangling in front of the fuel tank. I guessed it was some type of a one-way valve. It seemed to be connected to the breather tank and didn't have any electrical component. It was definitely done after importation - that elbow was probably connected to a mysterious vinyl line (yup, hard vinyl) that went to the front of the car and wound up plumbed into the intake manifold.

I knew I had a picture! It's hard to tell what's going on here, but the valve in question is the black thing on the center right that's not connected to anything:
http://i729.photobucket.com/albums/ww299/desktopdave/BMW/1984%20Alpineweiss%20745i/2014%2007/th_20140626_201920_zpskqjnoo6y.jpg (http://s729.photobucket.com/user/desktopdave/media/BMW/1984%20Alpineweiss%20745i/2014%2007/20140626_201920_zpskqjnoo6y.jpg.html)
The picture is taken from the driver's side above the rear axle. Ignore the flared fitting, I think that's part of the deleted 745i rear SLS system AFAIK.
There is a grey plastic breather tank in the trunk, with four breather hoses.

Here's the rear driver's side, with the breather hose heading up the frame rail:
http://i729.photobucket.com/albums/ww299/desktopdave/BMW/1984%20Alpineweiss%20745i/th_2014-03-28183540_zps6c0b67ad.jpg (http://s729.photobucket.com/user/desktopdave/media/BMW/1984%20Alpineweiss%20745i/2014-03-28183540_zps6c0b67ad.jpg.html)

I never got around to fixing it...I wanted to do it properly with a full US-type purge valve and charcoal canister. I sold it and advised the new owner to leave the fuel cap loose until he got around to fixing it.