Fuel Pressure Issue

Discussion in 'Firewall Forward / Props / Fuel system' started by Kyle Boatright, Jan 3, 2016.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Jan 3, 2016 #21

    ekimneirbo

    ekimneirbo

    ekimneirbo

    Banned

    Joined:
    Oct 31, 2014
    Messages:
    1,009
    Likes Received:
    324
    Location:
    Deep South
    OK here are my thoughts.....If I read this thread correctly, you get 5 lbs of pressure with the mechanical pump when cranking the engine, but you
    do not get 5 lbs when the engine is actually running and the electric pump is not in use. The engine does appear to run normally
    in that situation. Question 1 Did you run the engine rpms up and hold them there to see if the engine stumbled or was erratic after a period of
    time at the elevated rpm while using only the mechanical pump? Did CHT temps increase from a possible lean condition even if the engine wasn't erratic?

    Apparently the engine runs fine when the electric booster pump is employed. The thing that changes at that time is that you have fuel being forced
    under pressure greater than gravity, so it might overcome some type of blockage in the line or push the fuel thru the mechanical pump at the higher
    pressure. My conclusion is that the mechanical pump is probably OK because it does push 5 lbs when cranking. When cranking, less actual flow to
    the engine is needed than when actually running so even with a restricted flow the pump could show 5 lbs. When running more flow volume is needed,
    so if the flow getting to the mechanical pump is restricted ......the pump does not now have enough volume available to create 5lbs....but it could still
    be flowing just enough to run the engine. Using a container with a hose connected to the pump and providing gravity flow should tell you if the problem
    you are having is from the pump and after the pump . If you then have 5lbs of pressure the problem has to be in the components prior to the pump.
    If that happens then I would check your tank vent and blow some air through the fuel line from the tank.

    Don't really know if thats whats happening, but from what I understand about the situation, thats my best guess.:)
     
  2. Jan 3, 2016 #22

    Kyle Boatright

    Kyle Boatright

    Kyle Boatright

    Well-Known Member

    Joined:
    Nov 12, 2012
    Messages:
    851
    Likes Received:
    474
    Location:
    Marietta, GA
    The engine ran fine in all situations. I did not perform a run-up at a higher power setting, but it is on "the list".

    A fuel vent blockage is unlikely, as the low pressure event was a minute after start-up, and my fuel caps don't seal that well in the first place. But that's on the list too, as is checking the gascolator and checking all of the fittings in the system to preclude cracked flares and vacuum leaks. Another concern might be a blocked fuel pick-up tube in the tank, which would be a huge pain in the arse to fix - BTDT with an earlier service bulletin.
     
  3. Jan 3, 2016 #23

    Dan Thomas

    Dan Thomas

    Dan Thomas

    Well-Known Member

    Joined:
    Sep 18, 2008
    Messages:
    4,945
    Likes Received:
    2,119
    That fuel pump has a spring in it that does the driving-out of the fuel. It determines the fuel pressure. It can break and lower the pressure output but the pressure should be low whether running or cranking.
     
  4. Jan 3, 2016 #24

    Midniteoyl

    Midniteoyl

    Midniteoyl

    Well-Known Member

    Joined:
    Sep 4, 2003
    Messages:
    2,406
    Likes Received:
    500
    Location:
    Indiana
    Oh, I know, and I would fully test it to be sure, but ;) if I couldn't get the fault to happen again (as in your case), there is no way I would trust that it wasnt the pump.

    As you said, its highly unlikely to be a grounding or sensor issue as it showed proper readings with the boost pump, though I would clean and inspect the grounds and any other connections anyways.

    However, that also doesn't mean it couldn't be which is why I asked about the pressure differences. A failing sensor could read zero at 5psi, but read correctly at 5.5 or 6psi. Kinda like when your fuel sensor has a dead spot and wont read correctly below 1/4 tank. Or, better yet, a dirty oil pressure gauge that wont read below 30psi, but rev a little and suddenly its reading fine at 40psi (just had this happen as a matter of fact)..

    But I see that you tested by spinning the engine on the starter, so again, possibly unlikely.. Which brings us back to the fuel pump.. ;)
     
    Last edited: Jan 3, 2016
  5. Jan 3, 2016 #25

    Rockiedog2

    Rockiedog2

    Rockiedog2

    Well-Known Member

    Joined:
    Dec 12, 2012
    Messages:
    2,313
    Likes Received:
    1,630
    >>> if I read this thread correctly, you get 5 lbs of pressure with the mechanical pump when cranking the engine, but you
    do not get 5 lbs when the engine is actually running<<<

    did it say that? I missed that about 5 psi when cranking
     
  6. Jan 3, 2016 #26

    Midniteoyl

    Midniteoyl

    Midniteoyl

    Well-Known Member

    Joined:
    Sep 4, 2003
    Messages:
    2,406
    Likes Received:
    500
    Location:
    Indiana
    And most likely the slowly clogging filter making the pump work harder to suck the fuel contributed to the early failure...
     
  7. Jan 3, 2016 #27

    ekimneirbo

    ekimneirbo

    ekimneirbo

    Banned

    Joined:
    Oct 31, 2014
    Messages:
    1,009
    Likes Received:
    324
    Location:
    Deep South
     
  8. Jan 3, 2016 #28

    Midniteoyl

    Midniteoyl

    Midniteoyl

    Well-Known Member

    Joined:
    Sep 4, 2003
    Messages:
    2,406
    Likes Received:
    500
    Location:
    Indiana

    A failing pump could, in fact, develop 5psi while cranking and not at a higher RPM. Its all about flow (as you say). While cranking, you are basically building pressure against the needle valve/float combo, which will be closed or darn near so. At higher RPMs, the higher flow required means the needle is unseated and thus there is nothing for the pump to create pressure against. It might still flow, but it cant develop pressure.

    But yes, check that there is free flowing fuel to the pump.
     
  9. Jan 3, 2016 #29

    ekimneirbo

    ekimneirbo

    ekimneirbo

    Banned

    Joined:
    Oct 31, 2014
    Messages:
    1,009
    Likes Received:
    324
    Location:
    Deep South
    I agree with you that a failing pump might still be the problem, but I think there would still be some pressure
    once the needle is unseated.As Kyle mentioned, he still reads 5 psi with the engine running on the electric pump....and thats with the needle off the seat. The thing that seems
    most logical to me is that if the pump is capable of producing 5 psi when cranking, it most likely is capable of producing 5 psi all the time and some other factor (shortage of
    supplied fuel)would be where to look first. At any rate its all ways best to check the repairs that don't cost money ...first.:)
     
    Midniteoyl likes this.
  10. Jan 3, 2016 #30

    Midniteoyl

    Midniteoyl

    Midniteoyl

    Well-Known Member

    Joined:
    Sep 4, 2003
    Messages:
    2,406
    Likes Received:
    500
    Location:
    Indiana
    Well, getting 5psi with the boost pump only means the boost pump is working ;)
     
    SVSUSteve likes this.
  11. Jan 3, 2016 #31

    Rockiedog2

    Rockiedog2

    Rockiedog2

    Well-Known Member

    Joined:
    Dec 12, 2012
    Messages:
    2,313
    Likes Received:
    1,630
    yeh, OK I missed that statement by Kyle about the cranking thanks. Not unusual around here anymore.
     
  12. Jan 4, 2016 #32

    Kyle Boatright

    Kyle Boatright

    Kyle Boatright

    Well-Known Member

    Joined:
    Nov 12, 2012
    Messages:
    851
    Likes Received:
    474
    Location:
    Marietta, GA
    Today's update:

    - Pulled the gascolator and inspected the screen. No debris or blockage was evident.
    - Pulled the facet electric pump. No debris or blockage was evident and the flares in the attached lines looked good.
    - Pulled the fuel line from the mechanical pump to the carb. No problems noted.
    - Performed fuel flow tests. 11 GPH on the mechanical pump with the starter spinning the engine (~300 rpm). Approximately 35 GPH on the electric pump.
    - Confirmed (by fuel valve position) that last Saturday's problem was on the left tank. That gives me a trail to follow if I need to chase the problem upstream of the fuel valve.

    Next steps:

    -Check vent operation on left tank.
    -Verify sender ground.
    -Re-cowl and run the engine, trying to repeat the problem. If it occurs, see if the engine continues to run properly for a lengthy run-up. If so, it might indicate a gauge problem.

    What other things should I look at before I re-cowl the engine?
     
    Last edited: Jan 4, 2016
  13. Jan 4, 2016 #33

    Toobuilder

    Toobuilder

    Toobuilder

    Well-Known Member

    Joined:
    Jan 20, 2010
    Messages:
    4,542
    Likes Received:
    3,385
    Location:
    Mojave, Ca
    You might plan to do an aborted takeoff or two and watch pressures, listen for abnormalities, etc. even if it looks good on initial roll, plan to shut it down and taxi back.
     
    SVSUSteve likes this.
  14. Jan 4, 2016 #34

    ekimneirbo

    ekimneirbo

    ekimneirbo

    Banned

    Joined:
    Oct 31, 2014
    Messages:
    1,009
    Likes Received:
    324
    Location:
    Deep South
    I think you are proceeding the right way. There is only one other thing that I can think of at the moment.
    When only the mechanical pump is drawing fuel, is the electric pump in series where fuel must pass
    thru the electric pump before it arrives at the mechanical pump? Also you might look at the gravity flow
    to the mechanical pump with nothing pumping, and then remove your gas filler cap and see if flow
    increases any when there is a larger vent opening.
     
  15. Jan 4, 2016 #35

    Kyle Boatright

    Kyle Boatright

    Kyle Boatright

    Well-Known Member

    Joined:
    Nov 12, 2012
    Messages:
    851
    Likes Received:
    474
    Location:
    Marietta, GA
    The electric pump is in series with the mechanical pump.

    There really isn't any gravity flow from the fuel tanks in an RV to the fuel pump. It is an uphill path. Maybe I don't understand your suggestion (which I bolded).
     
  16. Jan 4, 2016 #36

    ekimneirbo

    ekimneirbo

    ekimneirbo

    Banned

    Joined:
    Oct 31, 2014
    Messages:
    1,009
    Likes Received:
    324
    Location:
    Deep South
    OK, I didn't realize it was a low wing airplane. What I was alluding to was that the electric pump might be impeding normal
    flow or allowing a path for a loss of suction when only the mechanical pump is working (drawing fuel). A small leak in the
    electric pump could cause slight loss of flow drawn by the low volume mechanical pump, but be small enough to have no
    real effect when the higher volume electrical pump is running. That is just a "shot in the dark" guess on my part, but since
    the problem is hard to diagnose, I thought I'd toss it out there.:)
     
  17. Jan 25, 2016 #37

    Kyle Boatright

    Kyle Boatright

    Kyle Boatright

    Well-Known Member

    Joined:
    Nov 12, 2012
    Messages:
    851
    Likes Received:
    474
    Location:
    Marietta, GA
    Update...

    I was down for a weekend due to the plague or some other malady, but got to take another pass at the problem last weekend when a buddy and I checked all electrical connections, tightened all fuel connections, and eyeballed the entire fuel system. We also scratched our heads a bit. I performed a run-up and the fuel pressure behaved. On that note, I went flying and saw no problems during a 15 minute flight.

    This weekend, I repeated the test flight. Upon arriving at altitude, and after turning off the electric fuel pump, the fuel pressure (as measured by the GRT EIS4000) dropped again. It fell to a value of between -0.3 and -0.7 PSI. The engine continued to run fine in that condition for the duration of the flight - about 30 minutes.

    I suspect I have an intermittent measurement problem. Funny enough, if you look at GRT's online troubleshooting guide, it mentions that their VDO senders often fail and become erratic at 200-500 hours, particularly on carbureted engines. On the other hand, the troubleshooting guide also mentioned the erratic pressure tends to happen on the high side, rather than the low indicated pressure I saw.

    So, I guess I'll order a new sender from GRT and see if that resolves the problem Unless one of you has a better idea. If you do, I'd appreciate hearing it...
     
  18. Jan 25, 2016 #38

    don january

    don january

    don january

    Well-Known Member

    Joined:
    Feb 11, 2015
    Messages:
    2,706
    Likes Received:
    1,153
    Location:
    Midwest
    This may be way out there but maybe some food for thought. I have a 6.9 diesel engine in my truck and thought I'd help out the mechanical fuel pump with a electric near the tank's . At first it ran good until I drove for a little will (2 min) and the Diesel foamed up and shut the truck down. Probably not related but that's what hit my mind when reading your Thread. Don
     
  19. Jan 25, 2016 #39

    ekimneirbo

    ekimneirbo

    ekimneirbo

    Banned

    Joined:
    Oct 31, 2014
    Messages:
    1,009
    Likes Received:
    324
    Location:
    Deep South
    I didn't reread the whole thread, so maybe this was already suggested. I'd put a mechanical guage on it in addition to the the electrical guage...just a temporary one that you can see. Then see if they
    both change or just the electrical one.
     
  20. Jan 25, 2016 #40

    Kyle Boatright

    Kyle Boatright

    Kyle Boatright

    Well-Known Member

    Joined:
    Nov 12, 2012
    Messages:
    851
    Likes Received:
    474
    Location:
    Marietta, GA
    Very logical idea, but I'm hesitant to knock a(nother) hole in the firewall for the line for a mechanical gauge.
     
    Last edited: Jan 25, 2016

Share This Page



arrow_white