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Thread: Fuel pressure variation on FX-3

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    Default Fuel pressure variation on FX-3

    Has anyone noted variable fuel pressure on the FX-3 (see attachment). I have the added restrictor orifice in place. The fuel flow is steady, but the pressure fluctuates up to 12 psi.
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    Default Re: Fuel pressure variation on FX-3

    Quote Originally Posted by KCKS View Post
    Has anyone noted variable fuel pressure on the FX-3 (see attachment). I have the added restrictor orifice in place. The fuel flow is steady, but the pressure fluctuates up to 12 psi.

    What is the “restrictor orifice” you refer to? I’ve not heard about that. I’m not sure why the pressure would fluctuate and hopefully someone at CC will respond. I do know that the fuel filter needs to be cleaned regularly.

    The regular fuel pump is a mechanical, engine driven pump. The electric fuel pump is adjustable and should be set to be as close to the same pressure that the mechanical one is at if possible. There is a very small set screw on the pump used to adjust the pressure. Turning out (counter clockwise) lowers the pressure but the pumps are higher rated pumps and have to be adjusted almost down to their lowest setting to get close to the mechanical pressures.

    Maybe see the pressure with the electric pump off and then flip the fuel pump on and see the difference. Also does the pressure vary only with the pump off? Maybe see if turning the pump on when it varies and see if it stabilizes. If the filter is clogged maybe thats the problem? I have heard some stories of really clogged filters. I think they are very small micron filters and catch maybe too small of particles and too much of smaller particles that would normally pass through?

    Just some thoughts to consider. I flew someone else’s FX3 a couple of months ago across half the country. Fuel pressure stayed pretty constant on all the legs but one. I refueled and then while flying noticed that on this leg the fuel pressure had some pretty big variations and were fairly rapid. After the next engine shutdown for refueling for the next leg it did not do it again. My theory was that perhaps the fuel filter was dirty and maybe some particles got stuck and restricted fuel flow and when the engine/pressure stopped, the particles fell out and didn’t clog on the restart????
    Dave Embry
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    Senior Member Andy's Avatar
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    Default Re: Fuel pressure variation on FX-3

    Interesting topic as I'm currently seeing unusual fuel pressure variation on my PA28-180. I though that issue would go away when I got the FX-3.

    What is the source of the posted fuel pressure and flow plots? Is that data from a CGR-30P or from a Garmin G3X?

    Andy

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    Default Re: Fuel pressure variation on FX-3

    - They put an orifice upstream of the electric fuel pump to dampen out the fluctuations they were seeing. Apparently it worked for some, but not for mine. Sometimes it's better than the plot that I showed- variations of +/- 5 psi, but usually it's 5-10 psi.

    - I took the pressures off of the csv file from the G3X.

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    Default Re: Fuel pressure variation on FX-3

    Also they have changed the fuel filter used from a 10 micron to a 62 micron filter now. No doubt the 62 is much larger and won't catch as many small particles but it "probably" still smaller than the max size allowed through the fuel servo???? If the smaller filter gets clogged like I mentioned before....and maybe would sometimes let pressure fluctuate ....then that could alleviate the problem with that and of having to clean them every 50 hours as recommended I think.

    Quote Originally Posted by KCKS View Post
    - They put an orifice upstream of the electric fuel pump to dampen out the fluctuations they were seeing. Apparently it worked for some, but not for mine. Sometimes it's better than the plot that I showed- variations of +/- 5 psi, but usually it's 5-10 psi.

    - I took the pressures off of the csv file from the G3X.
    Dave Embry
    "You only live once.......but if you do it right.........once is enough."..

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    Default Re: Fuel pressure variation on FX-3

    I have the 62 micron filter and it is clean- clogging isn't the reason for the fluctuation. Since no one else has responded, apparently I'm the only one with the problem or the only one that has plotted the data. Thanks for your responses Dave

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    Default Re: Fuel pressure variation on FX-3

    It must be related to the mechanical pump. Perhaps some air is trapped in the system and the pump can't get it out (like a vapor lock of sorts). Have you tried turning on the electric fuel pump when you see the fluctuation or is too fast to react to?

    Quote Originally Posted by KCKS View Post
    I have the 62 micron filter and it is clean- clogging isn't the reason for the fluctuation. Since no one else has responded, apparently I'm the only one with the problem or the only one that has plotted the data. Thanks for your responses Dave
    Dave Embry
    "You only live once.......but if you do it right.........once is enough."..

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    Default Re: Fuel pressure variation on FX-3

    Quote Originally Posted by Daveembry View Post
    It must be related to the mechanical pump. Perhaps some air is trapped in the system and the pump can't get it out (like a vapor lock of sorts). Have you tried turning on the electric fuel pump when you see the fluctuation or is too fast to react to?
    It might be in the fuel hose going to the pressure sensor it runs up hill from fuel pump you might be able to blead it at sensor with useing electric pump

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    Default Re: Fuel pressure variation on FX-3

    Thanks, I'll give that a try

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    Default Re: Fuel pressure variation on FX-3

    I have a similar issue with my FX-3. Please let us know what you find.

    I don't know anything about the orifice you mentioned, but ill see variations in fuel psi of 1-2 psi at lower power settings, 23/2300. At a higher power setting, 25/2500, is when I see larger swings in psi that are rapid and constant. Any where from 3-6 psi while the fuel flow stays consistent down to the tenth.

    For example at a power setting of 25/2500 without the electric fuel pump on, fuel psi varies between 19-23 psi but will periodically have more drastic drops or spikes. Same with the electric fuel pump on, except the pressures are bouncing more between 25-30 psi. Again, in both of these scenarios fuel flow stays constant.

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