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Sudden fuel pressure drop

in 912 / 914 Technical Questions 14 years ago
Maybe someone can help me figure this one out. Since about two months ago my Rotax 912ULS on my Zenith 701 occasionally gets an alarming drop in fuel pressure on application of full power, i.e, on takeoff or go around. Turning on the electric pump will make it recover. I've started using my Facet electric pump during T.O. & landing. Today after takeoff when I reached safe altitude and turned off the fuel pump, the pressure plummeted down from 5.4 to around 2.5 before recovering within 15 seconds or so on it's own. The rest of the flight it stayed steady. I am using mogas with ethanol. My field elevation is 4700'. OAT was around 60F. I have a fuel return line installed with the .014 orifice.

So far I have replaced the engine driven fuel pump and replaced the fuel filter, both to no avail. Tanks are venting fine. Any ideas or insights would be appreciated. Thanks!

  • Re: Sudden fuel pressure drop

    by » 14 years ago

    Hi Paul,

    Without the electric pump on at all what is the fuel pressure to start and what does it drop to? If you do not engage the electric pump when the fuel pressure drops does it recover and how quickly?

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Sudden fuel pressure drop

    by » 14 years ago

    It runs about 5.4 without the electric pump on. I'm not sure how low it drops, because I've turned on the electric when the warning light on my EIS goes on. Today, however, at safe altitude, I let it drop just to see and it dropped to 2.5 or a little less, maybe 2.0, then began climbing back up to normal. I'm guessing it took 15-20 seconds to fully recover.
      
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  • Re: Sudden fuel pressure drop

    by » 14 years ago

    Hi Paul,

    I'm not 100% sure if it is really a mechanical fuel pressure issue or an electrical one. Here is the best way to know for sure and it will help with the diagnosis. Go to the hardware store and pick up a gauge that reads from 0-10 psi. Pick up some 1/16th" copper tubing (about 6') and some compression fittings and a Tee fitting.(about $20) You need to disconnect the fuel line at your pressure sender. Put this new temporary setup in line with a tee fitting so you can not only see the fuel pressure electronically, but mechanically. You need to set this up so you can see each at the same time. You need to rule out electrical or mechanical. For those who have oil pressure problems this is a good way to see where the problem lies, but use a gauge that goes up to 100 psi.

    p.s. Paul,
    Does your fuel pressure sender come off right on top of the carb balance tube at the fuel distribution point?

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: Sudden fuel pressure drop

    by » 14 years ago

    Thanks, Roger. I'll try that. I don't have the Rotax fuel distribution setup. My fuel pressure line comes from a T fitting I put off the line that feeds both carbs. I mounted the sending unit on the firewall, and they're connected by a 15" or so fuel hose.

    I appreciate your answering me. This has been a scary issue. The first time it surfaced I was on go around to a back country canyon strip and the fuel pressure dropped to 0 indicated before I got the electric pump on, and it slowly climbed back up. The engine never missed a beat, so maybe it is an electrical issue.
      
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  • Re: Sudden fuel pressure drop

    by » 14 years ago

    Hi Paul,
    I'm leaning towards electrical, but you need to make sure.
    Maybe I know the problem.
    Do you use 91 oct and does the sender hang upside down on the firewall?

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report
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