We've got one parking spot on our ramp that kind of slopes to the left a little, and if you leave the fuel selector set to both, it will crossfeed due to gravity. Leave it overnight and in the morning, the right wing will be half empty while the left wing will be full because the fuel is leaking from the fuel vent(due to gravity) and will be continually replenished from the other wing. I don't know if this is the problem you are having but you might try turning the fuel selector to either left or right, or even off, when parking for extended periods and see if it does the trick.
I train in 172s but if I'm not mistaken there is an fuel shutoff valve in the 152, you just can't select the left or right tanks exclusively. Turning it to the off position should prevent the crossfeed as well.
Buddy of mine was flying a 172 with a BENT fuel drain. It was bent from behind the strut, so it was getting ram air like a pitot tube. He just happened to look behind him in flight and saw the fuel streaming out from the vents in the top. (I'm guessing that's where the fuel was coming from. He doesn't seem the type to forget the caps)
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The aircraft I am training in is a C-152... it doesn't have a fuel selector.
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Try the fuel shutoff valve. Although it is not a "selector", it is an on-off valve. It has been awhile since I have been in a 152, so I'm not sure where that valve sits in the fuel system...check your POH and see if shutting the fuel off would solve a cross-feed problem if the a/c is sitting on a slope.
I was standing on the ramp at INT the other day dropping passengers and witnessed a King Air depart trailing fuel vapor behind him ... looked like somebody forgot a fuel cap!
Had it happen to me in a 172 once ... didn't forget the cap, but the vented fuel cap was leaking on one side ... leaving a nice mist cloud behind us. Noticed it shortly after leaving the pattern, returned safely, and replaced the fuel cap.
Last month, my friend and I were about to depart on an early morning flight out to Asheville, and after I tested the fuel on the right side, the fuel just kept on dripping out (at a steady 10-20 drops per seconds....
). We ended up waiting about 35 minutes for the frost to melt enough to close the valve....