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Thread: Low Fuel Pressure

  1. #1

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    Default Low Fuel Pressure

    Today I tested my KF 4, 912S by climbing to 15,500'. OAT was -3C at this altitude. Everything performed great but I quit climbing because I was cold. I then started my descent and 30 mile return to my home airport when my Dynon flashed a red "low fuel pressure" warning. Needless to say, I forget that I was cold. I think I was down to about 8-10,000'. I turned on the electric fuel pump and the warning disappeared. I then tested by turning off the elec. pump and the warning returned immediately. After turning on the elec pump, I toggled the Dynon into the page showing the actual fuel pressure, and repeated the test. The fuel pressure immediately started dropping when I turned off the elec. pump. I suspected that the mech. pump had failed. I returned home on the electric pump, landed, but then could not replicate the low fuel pressure warning when on the mech pump only. Full power run up on the ground, on the mech pump only, ran great. Any suggestions? Did the cold temps cause a problem with my 100ll av gas? I sumped the tanks and gascolator but did not find any water. Bruce Estes

  2. #2
    Senior Member Dorsal's Avatar
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    Default Re: Low Fuel Pressure

    I have had issues twice with low fuel pressure, in both cases for me it was the sender. What pressure did you get with the electric and was it lower than normal? This all may not be related but I had my sender tapped into my fuel system over the engine, moving it back to the firewall with a restrictor tube has worked great. Also what have you set for your low pressure limit?
    Dorsal ~~^~~
    Series 7 - Tri-Gear
    912 ULS Warp Drive

  3. #3

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    Default Re: Low Fuel Pressure

    If it's a pressure sensor failure, why is it OK on electrical pump ?

  4. #4
    Senior Member Dave S's Avatar
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    Default Re: Low Fuel Pressure

    Bruce,

    A couple thoughts if it may help:

    1) As has already been commented - sorting out what is a sensor issue and what is a pump issue has to be determined. For a low pressure indication - if it is simply a light and not an actual pressure reading it can be a bit of a red herring - i.e. the difference between light coming on and not coming on may only be a fraction of a PSI - the conditions at altitude which can affect the sensor (temperature for instance) may be small but sufficient to cause the sensor to trigger under the conditions at altitude but not on the ground or lower altitudes. If this is the first time the plane has been operated under this specific set of conditions - it is entirely possible the low pressure indication would not have shown up any other time.

    2) Was the engine running just fine when the pressure indicator came on? If it was - apparently the engine was getting adequate fuel. On the other hand if the power started to drop with the low pressure indication - a person pretty well would know there is a pressure/fuel supply problem. There is a tolerance/range for fuel pressure from low to high which is deemed acceptable by Rotax - Consider that the "set point" for the light may be at a different pressure than the low tolerance as defined by Rotax. Another point to consider - the mechanical pump and the electric pump probably have different pressure values - then there is the issue that in series with both pumps on it would be likely the total pressure may be a bit higher than with either pump operating separately. Theoretically no low pressure light should be coming on with either or both pumps working.

    3) There was an SB on certain Rotax Fuel Pumps and pump eccentrics back in 2006 - here is where to find it to see if this applies or doesn't apply to your engine:

    http://legacy.rotax-owner.com/si_tb_...-912-049ul.pdf

    Basically the SB applies to a reduction in pump stroke due to wear caused by a specific out of spec part - reduced pump stroke results in reduced fuel flow - at high flow demand - the next thing that happens is reduced fuel pressure - it's sort of like an accelerated fuel pump wear out.

    4) THE ONLY REAL STANDARD for determining the actual fuel operating pressure is to plumb in an accurate fuel pressure gauge and operate the engine - a full throttle ground run should be telling.

    It's not a bad idea to have fuel pressure determined and logged so a person knows if any changes have occurred over time.

    5) Beware of the possibility of a fuel line restriction somewhere - filters? line swelling? line kinking? - some of this can add up slowly till a sensor is finally tripped.

    Not a bad idea to do periodic gravity fuel flow tests and log for so a person knows if any changes have occurred over time.

    Let us know what you find out.
    Sincerely,

    Dave S
    KF7 trigear
    912ULS - warp drive

  5. #5

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    Default Re: Low Fuel Pressure

    Yesterday after the flight, I checked everything and found nothing amiss. Today, I test flew, but did not climb to the same altitude and temperature as yesterday, and today, everything went perfect, as it has in the past. Yesterday when I got the low fuel pressure reading, I immediately started the electric pump so the engine never had a chance to stumble. Today, with full throttle, at 3,000', I got pressure readings on the Dynon of 3.7 with the mech. pump only, and 4.0 with the elect. back up pump. jAt 1800 rpm, I got 4.6 with the mech. and 5.3 with the elect back up. I think that yesterday, I must have had some moisture that froze in the small, brass screen in my gascolator that caused a partial blockage. I'm open to any thoughts anyone might have. thanks,Bruce

  6. #6
    Senior Member Dorsal's Avatar
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    Default Re: Low Fuel Pressure

    Do you recall what you got for numbers during the flight with the warnings?
    Dorsal ~~^~~
    Series 7 - Tri-Gear
    912 ULS Warp Drive

  7. #7

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    Default Re: Low Fuel Pressure

    4.0 with the electrical pump, less with the mechanical, but that red "LOW FUEL PRESSURE" warning, when running only on the mechanical pump had me pretty concerned so I never let the fuel pressure decline very much, once I saw that the pressure was indeed declining when only under the mechanical pump. As I had to reduce my altitude to get under the SFO Class B airspace that my home airport is located under, I quit fooling around with testing as I did not want to risk killing the engine. Bruce

  8. #8
    Senior Member Dave S's Avatar
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    Default Re: Low Fuel Pressure

    Bruce,

    Quick question - what kind of pressure reading do you get with the Dynon under the following condition - on the ground with the engine not running but the electric fuel pump running?

    Dave S

  9. #9

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    Default Re: Low Fuel Pressure

    Not sure but I'll check tomorrow. bruce

  10. #10

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    Default Re: Low Fuel Pressure

    O/K Answers to questions above. Without the engine running, but with the elect pump on, pressure max's at 4.9. The alarm (LOW FUEL PRESSURE) warning comes on at 2.2#, which is set by myself per the Rotax's min. fuel pressure. I think what happened is that I got some ice in the gascolator filter screen. The gascolator is mounted on the firewall down low, and my engine temps were declining because of the OAT at 0deg.C The elect pump forced fuel past the gascolator. The filter screen is a fairly small cylinder, approx 1" high x 3/4" diameter, with a small brass screen. The warning came on after my descent, which was faster airspeed than my climb, which combined with the wind chill factor, would have really chilled the gascolator. Thoughts?

    Bruce Estes

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