I'm assuming here cause I don't know the exact configuration of the C150e.  I'm assuming that it is similar to a C172M, i.e. there is no fuel overflow, the fuel tank caps relieves pressure due to vacuum and positive pressure.  Your description sounds like you are talking about the tank vent that has a check valve, which seems, in your case, to not be sealing closed preventing fuel from leaking out.

As the tank pressure increases due to fuel tank heating, the check valve in the fuel vent should close tighter preventing leakage and allowing the pressure to rise to the point that the fuel cap vents relieve pressure in the tank(s).  So I am guessing with all these assumptions that the problem is in the check valve in the tank vent and not the fuel caps...

Herb:  You suggested to search this site for fuel tank venting.  What site?
Will

JimJ, 
     No, it is not on a slope, but I wish it was, that would explain everything.

Thanks, I'll check the caps again, and reverse them to see what happens.  As I mentioned, I didn't see any rubber flap indicating a vented cap on either one.  It is not leaning.  And, finally, I checked the POH, but it doesn't even show the overflow valve in the fuel system schematic, much less a valve for the overflow, or any dialogue about the gas caps.

Sorry for the confusion and terminology.  The fuel is coming out of the fuel vent under the left wing.  I thought the correct name for it was the fuel overflow tube.

Thanks for the help.  Are these special caps, or can I use vented caps from an auto parts store?

Tony; Where do you get your caps at?

   Dave

The condition you are having is most likely NOT from fuel caps.The caps can cause problems,but I doubt if replacing them will correct the problem.
The vent valve is located on a "dogleg" tube on the upper front portion of your pilots side tank.This valve can sometimes inadvertently get twisted,and subsequently put the valve in the fuel itself.proper alignment is to have someone observe the valve with a flashlight,and extension mirror to see if it is indeed in the"air" portion of the tank(basically touching the top of the tank) this is designed to keep the valve in the"open air" area of the tank.If it is not in the upper area,an adjustment of the outlet fitting is required.Pressure inside the tank is supposed to be relieved by this valve,but if it is below the upper fuel level it will vent fuel instead of vapor.This valve is about 100.00,and a real bugger to replace.A much cheaper soultion is to refrain from completely filling the pilots side tank.Cross feed will effect the fuel level,but it rarely wil make both tanks exactly even.Were talking maybe 3 gallons less than you normally would,and your "leakage" should subside.

Will,
I suggest that you don't fly the airplane with the caps switched  unless you can determine that the switched positions are the correct positions as recommended by Cessna.  You don't want a tank to implode as you descend due to the lack of air to equalize pressure (or have fuel exhaustion due to a vacuum in a tank)! Don't depend on cross-feeds between tanks to keep you out of trouble.   I know that for C172's, Cessna no longer makes a non-vented cap; if you order a replacement cap, there is only one---vented.  You might want to check into having 2 vented caps instead of one.

Well, here's the latest scoop.
When I went back to the airport on Friday, I didn't see any fresh fuel on the ground after switching the caps.  And, the levels were even, unlike normally when the vented cap was on the right, which was always lower in level than the left.  When I went back on Sunday to fly, there was very little difference.  I flew to the Mid-Atlantic fly in at Lumberton, NC from Lexington, NC, and back, totaling 2.3 hrs. of flight.  Thank goodness that the tanks didn't implode.  If I would have gotten the message from Bob prior to going, I would not have left the caps switched.  But, everything seems to be fine.
I know it doesn't make sense, but the level didn't drastically change since switching the caps, and I'll keep watching it to determine my next course of action.

Will,

Here is the website for Yingling Aviation in Wichita,KS.  I bought my Cessna vented fuel caps from them for around $21.00 each, and got them within a day or so.

http://www.yinglingaviation.com/parts/index.html

Bob,
    Thanks for the info.  The Lumberton fly in was really good on Sunday when we went.  What the pilots and planes did was beyond my belief.  There was a lot of airobatics, military planes, and the master of disaster truck racing and beating a high speed bi plane.  The food and concessions were varied, good, and fairly priced.
There were not many vendors there, to my dissatisfaction.  I plan on going back next year if they have it again, which I think they will.
Will