I had an unusual fuel system problem the day before yesterday. I think I know what happened but am interested in what you all think of my analysis.

I flew three legs of 300 NM on Tuesday. The first two were uneventful and fuel burn was as expected. At the second stop due to the slope of the ramp the right tank was completely full. I have never filled to overflowing before and it was not my intent to do so this time but after filling my tanks, during preflight when i cracked the cap on the right tank some flowed out. I did not fill it to this level but if flowed down from the higher left tank. Both lids do leak slightly which is one reason I do not fill to the brim.

So, I took off and after about an hour I noticed the left tank level was at about 3/4 but the right was full. As I flew on the left tank level kept coming down but the right tank stayed full. I even flew in a slip for awhile to see if I could get some fuel into the left tank. As I neared my destination it was nearly empty and then . . . the low fuel warning light (sensor in header tank) came on. That got my attention. To conserve the fuel in the header tank I went to idle and was able to glide to my destination (I was at 7.5K feet) with no problem as it was only about 5-6 miles away.

After I landed the low fuel light went out and after I shut down I saw that there was now some fuel in the left tank.

Here is what I think happened. The low pressure area on the top of the wing slowly sucked fuel out of the full right tank and this caused the left tank to drain. Then it sucked some fuel out of the header tank, enough to cause the sensor to come on. After I landed, the suction was gone and the fuel was able to seek level. The real evidence is on the top of the right wing, a VERY LARGE fan shaped blue stain from the right fuel cap to the trailing edge. I fueled to my normal level and based on past fuel burns, about 3-4 gallons has gone missing.

I looked on the forum and have ordered the mercedes fuel cap seals that have been recommended. The next day I flew a 300 mile leg after making sure that the tanks were not completely full had no issues. I will post a picture of the fuel stain shortly.

All comments welcome!