Kitfox Aircraft Stick and Rudder Stein Air Grove Aircraft TCW Technologies Dynon Avionics AeroLED MGL Avionics Leading Edge Airfoils Desser EarthX Batteries Garmin G3X Touch
Page 1 of 2 12 LastLast
Results 1 to 10 of 18

Thread: Fuel Pressure Relative to Manifold pressure

  1. #1
    Senior Member JoeRuscito's Avatar
    Join Date
    Jun 2018
    Location
    Charleston, SC
    Posts
    361

    Default Fuel Pressure Relative to Manifold pressure

    Ive been trying to figure out why Im seeing a pressure drop on my fuel pressure sensor (912is) when I start the engine. I see a full 43-44 psi before start and when running consistently ~38 psi (either fuel pump). Im wondering if it could be due to the fuel pressure regulator trying to hold constant pressure relative to the manifold pressure. I.e. when I start the engine manifold pressure drops so the regulator also drops the pressure slightly and my gauge shows the drop. For you 912is guys does that make sense? Its just a hypothesis. What are yall seeing before engine start and while running? Im using the 70 psi UMA fuel pressure sensor right before the fine filter.

    Thanks as always!

  2. #2
    Administrator DesertFox4's Avatar
    Join Date
    Aug 2008
    Location
    Arizona
    Posts
    3,563

    Default Re: Fuel Pressure Relative to Manifold pressure

    Could it be as simple as when the engine is off, the injectors are all closed so pressure builds as the pumps push fuel into a closed system with nowhere for the fuel to go until the engine starts. I would imagine that is when you would see the highest pressures. As soon as the engine starts, fuel flows thus reducing indicated pressure and also may fluctuate slightly as injectors take turns opening and closing.
    No expert on fuel systems so just a theory.


    DesertFox4
    Admin.
    7 Super Sport
    912 ULS Tri-gear


  3. #3
    Senior Member JoeRuscito's Avatar
    Join Date
    Jun 2018
    Location
    Charleston, SC
    Posts
    361

    Default Re: Fuel Pressure Relative to Manifold pressure

    The logic totally make sense. However Im below the min fuel pressure for the engine. 46.5 psi is min. Im around 39-39 while running.

  4. #4
    Senior Member
    Join Date
    Dec 2017
    Location
    Mapleton,UT
    Posts
    1,224

    Default Re: Fuel Pressure Relative to Manifold pressure

    Does it change when the rpms come up? Can you verify indication is correct? Any way to see the fuel pressure your 912IS is seeing like accessing the ECU?
    Last edited by Shadowrider; 09-29-2019 at 12:45 PM.
    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
    N33TF......FLYING!
    N53TF......FLYING!

  5. #5
    Senior Member JoeRuscito's Avatar
    Join Date
    Jun 2018
    Location
    Charleston, SC
    Posts
    361

    Default Re: Fuel Pressure Relative to Manifold pressure

    Not sure if there is a way to see what the ECU is seeing. Yes it does come up when the rpm is increased. Gets into the 40s.

  6. #6
    Senior Member JoeRuscito's Avatar
    Join Date
    Jun 2018
    Location
    Charleston, SC
    Posts
    361

    Default Re: Fuel Pressure Relative to Manifold pressure

    at 5000 rpm I see 44.9 psi.

  7. #7
    Senior Member efwd's Avatar
    Join Date
    Jun 2015
    Location
    Yorba Linda, CA
    Posts
    2,817

    Default Re: Fuel Pressure Relative to Manifold pressure

    Joe, Im seeing the same. I think mine drops to 40-41 until Im off idle. I believe Im seeing 43 at 4000rpm. Just did my 100hr and the ECU told us nothing. That's a good thing IMHO. Im fixing to go fly so I will verify.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  8. #8
    Senior Member aviator79's Avatar
    Join Date
    Dec 2016
    Location
    Los Alamos, NM
    Posts
    913

    Default Re: Fuel Pressure Relative to Manifold pressure

    Hey Eddie, Do you have your G3X set up to give you differential fuel pressure relative to manifold pressure, or normal "gauge" pressure relative to ambient? I don't have a 912iS or a G3X, but I believe Joe may have have come across the solution to this, and since you have the same engine/EFIS, you can probably help each other out.
    --Brian
    Flying - S7SS

  9. #9
    Senior Member JoeRuscito's Avatar
    Join Date
    Jun 2018
    Location
    Charleston, SC
    Posts
    361

    Default Fuel Pressure Relative to Manifold pressure

    I did find that setting and I’ve been exploring it. The next question is do you set a compensation value in the g3x or leave the manifold and gauge pressure unsynced. The garmin manual doesn’t explain this.

    To clarify I mean the offset in the g3x. With no fuel pressure I see 15 manifold (sea level as
    Expected) and 0 fuel pressure. If I then turn on the pumps I see 30 psi (without manifold compensation I see 44). It seems too coincidental that I’m off by exactly manifold pressure which makes me think I need to add in 15 psi to get back to 45. Likely overthinking it! Thanks for the help.
    Last edited by JoeRuscito; 09-30-2019 at 12:30 PM.

  10. #10

    Join Date
    Apr 2012
    Location
    Oak Harbor,Wa
    Posts
    168

    Default Re: Fuel Pressure Relative to Manifold pressure

    I have 260 hrs. on my 912Is installed in an SS7, and using a Advanced 5600 EFIS. Fuel pressure will show +/- 47 psi when start power is applied and drops to 44 psi when engine starts. (only one pump on for start). It stays within specs until I exceed 10,000 ft when it drops to 40psi. The Advanced 5600 uses the formula;
    Fuel pressure= analog fuel pressure + ambient pressure- MAP pressure. The off-set shows up on the 5600 admin page for fuel pressure settings.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •