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Thread: Fuel Flow Problem

  1. #1

    Join Date
    Mar 2010
    Location
    TX
    Posts
    112

    Default Fuel Flow Problem

    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!

  2. #2
    Senior Member Peteohms's Avatar
    Join Date
    Sep 2008
    Location
    Leander, TX Hangared @ KGTU
    Posts
    272

    Default Re: Fuel Flow Problem

    Hank,

    Part of your problem is you didn't invite me to fly along with you. That being said, my Kitfox III left tank drains faster than my right tank also. Your problem seems worse though. I believe if you get your caps to seal the problem will get better. You'll have positive pressure from the vent tubes. I made my own gaskets out of some rubber gasket material that was very thin so I had to use 3 gaskets. The problem mostly went away and is only an issue when I'm running on fumes after a 6 hour leg. You need to stop and smell the flowers anyway and when you land the tanks can equalize.
    Pete
    Leander, TX
    Model III SN 1000
    912
    Grove

  3. #3

    Join Date
    Mar 2010
    Location
    TX
    Posts
    112

    Default Re: Fuel Flow Problem

    I can't get a vertical picture to upload properly. It ends up being displayed horizontally. If I can figure out how to fix that I'll post it.

  4. #4
    Senior Member
    Join Date
    Feb 2011
    Location
    wilmington nc
    Posts
    201

    Default Re: Fuel Flow Problem

    this also could be a problem with the vent hose on the right tank.
    we have fuel pressure on the g3x screen and after about 2.5 hr we would see the fuel pressure start to drop. after swinging the right wing back I found a tye wrap holding the vent hose to the rib was to tight, all was well for about 100 hr,s until our trip to Lakeland this year.
    Lakeland is about 5 hr,s each way for us. the trip down was no problem but on the way back at about 3 hr,s we got a low fuel pressure warning.
    I pulled some power off and the pressure stopped dropping. then I reached over to the fuel lines inside the cabin from the right tank and moved them around and held up on them. after a bit the pressure started to rise and so we came on home. after taking the fabric cover off the lines in the cabin and folding the wing we found the vent hose had worked it,s way under the fuel line and wires for the lights on the wing.
    that vent line was made of soft tigon tubing and is easy to flatten out.
    I replaced it with 5/16 gates fuel hose after cutting off the first two barbs on the vent hose barb fitting in the tank to have room to make the turn along the rib going down to the header tank. hope this helps.

  5. #5
    Senior Member jiott's Avatar
    Join Date
    Jun 2010
    Location
    Portland, OR
    Posts
    2,960

    Default Re: Fuel Flow Problem

    Jamesmill-That is very good information. Thanks for posting.

    Jim

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