Fuel Pressure Issue

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

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  1. Jan 3, 2016 #1

    Kyle Boatright

    Kyle Boatright

    Kyle Boatright

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    This is a bright bunch, so I'll ask for help troubleshooting a problem I'm experiencing with my carbureted, O-320 powered RV-6.

    Last Saturday as I was taxiing out for a flight, the GRT EIS master warning light came on. The digital readout indicated low fuel pressure (in the range of 0 PSI). I was taxiing with the electric fuel pump off and usually see ~5 PSI in those circumstances. I turned the electric pump on and the pressure immediately rose to 5 PSI. Then, I turned off the boost pump and the pressure dropped to 0. Once again, I turned the boost pump on and the pressure came back up to 5 PSI. I turned the pump off again and the indicated pressure dropped back to 0 PSI. I taxied back to the hangar and the reading never changed. During this whole process, the engine never gave any indication of fuel starvation.

    Going through the potential causes, I identified these, and performed some tests trying to isolate the issue:

    -Gauge or sender problem. To try and isolate this problem, I plumbed a mechanical gauge in the system. Today, it and the electronic gauge confirmed pressure both with the boost pump on or the engine (and engine driven pump) operating under power from the starter (with plugs removed). In all cases, the pressure was 5-6 PSI. Both gauges agreed at 0 PSI with the engine stopped and the boost pump off.

    -Failing mechanical pump. The mechanical and electric gauges both indicated appropriate pressure when the engine was spinning today.

    -Vacuum leak in the fuel system allowing air to enter the system. I got out the inspection light and checked all fuel connections for evidence of weeping fuel after pressurizing the system for 5 minutes using the electric pump. No fuel leaks were found.

    -Obstruction in the fuel lines. (Trash, collapsing hose, etc.). I have not done any testing to identify these issues.

    The only remaining tests I can think of are to disassemble the fuel system looking for blockages, cracked flares, or collapsing hoses. Also, to crank the engine and conduct a run-up, looking for the low fuel pressure indication on the mechanical pump. If that indication occurs, continue with the run-up (boost pump OFF) to see if the engine suffers from fuel starvation.

    Given that fuel is flight critical, I'm very interested in clearly identifying the root cause and correcting it. I do not want to shotgun this problem with replacement parts that may or may not fix the problem (or the indication of a problem).

    What are the collective troubleshooting suggestions of the group?
     
  2. Jan 3, 2016 #2

    kent Ashton

    kent Ashton

    kent Ashton

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    Is this a new build? I pulled my hair out over such FP problems before. Turned out it was ground loops--sender and gauge grounded at different points with a tiny difference in ground potential. Moving the grounds to the same point on the airframe (the engine) fixed it for me.
     
  3. Jan 3, 2016 #3

    Kyle Boatright

    Kyle Boatright

    Kyle Boatright

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    <Note for all: Regardless of what suggestions anyone offers, I'm going to play the skeptic. Not to be an a-hole, but to add a bit of rigor to the process.>


    The airplane has been flying in this configuration for years. I added the GRT 2 years (?) ago, and prior to that, I used round gauges for ~13 years.

    I'm disinclined to believe there is a ground issue. Nothing has changed there, AND I can't figure out why turning on the boost pump would cure the issue if it did exist.
     
    Last edited: Jan 3, 2016
  4. Jan 3, 2016 #4

    Joe Fisher

    Joe Fisher

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    I would disconnect the engine driven suction side, put a hose into an open container of fuel and see if you get fuel pressure cranking the engine. I am all ways suspicious of engine driven fuel pumps.
     
  5. Jan 3, 2016 #5

    Kyle Boatright

    Kyle Boatright

    Kyle Boatright

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    What does this tell me that I didn't learn by today's test where I turned the engine over with the starter and the mechanical pump generated 5 PSI?
     
  6. Jan 3, 2016 #6

    TFF

    TFF

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    A friend with his RV7 went through 4-5 sender units until he bought the updated one. I dont remember if the sender was from Vans or Dynon. The new one required an update with the Dynon. His was a IO-360 so it was higher pressure, but it seemed it was the style of sender. Of course the new style was more expensive. Did you do a run up? If it was really at 0 PSI the engine would have quit. At minimum you would see EGT CHT high because of the lean running.
     
  7. Jan 3, 2016 #7

    Kyle Boatright

    Kyle Boatright

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    My fuel pressure measuring system is GRT, not Van's or Dynon. That doesn't preclude a sensor issue, however I cannot figure out why turning on the boost pump would cause a sensor that was measuring 0 PSI to correct to 5 PSI, which was what it should have read with only the mechanical pump running (and with both pumps running).

    I did not do a run-up. That is one additional test I can perform.
     
  8. Jan 3, 2016 #8

    TFF

    TFF

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    The sensors tend to be the same off the shelf stuff. All the mechanical pump failures I have seen were leaks from the overboard. But I will say the pumps had hours but calender times were low.
     
  9. Jan 3, 2016 #9

    Hot Wings

    Hot Wings

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    Junk under one of the mechanical fuel pump valves that kept it from closing fully? The fuel pump then might still suck enough to draw fuel to keep the engine running but not develop any pressure?
     
  10. Jan 3, 2016 #10

    Midniteoyl

    Midniteoyl

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    Does the mechanical and electric pump develop the same PSI, or does the electric provide just a little more? Like 5.5 vs 4.5?

    I would tend to think that it was a failing mechanical pump, however.. Remember you are on a carburetor, so you have a bowl full of fuel that will take a bit to run down at or just above idle. And, it wouldn't take much pressure at all to keep it filled under such low power conditions, maybe low enough the GRT sensor couldn't read it (and you did toggle the boost pump ;))

    You can do a full run up with the boost pump off, but if its acting intermittently it might or might not fail this time, but will the next. I would change it out regardless, especially if its more than a few yrs old.
     
  11. Jan 3, 2016 #11

    gtae07

    gtae07

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    Dad had fuel pressure issues on his RV-6 for a long time; in the end, it was a grounding issue. We'd see pressure drop to zero on occasion before it rebounded. Engine kept running with no hiccups (he uses a throttle-body injector). This was with the old mechanical gauge, and then the Skyview.
     
  12. Jan 3, 2016 #12

    Kyle Boatright

    Kyle Boatright

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    Both pumps generate 5-6 PSI. The difference, if any, is insignificant. As to changing the pump, that's the "parts changer" approach that I'd rather avoid unless there is no way to isolate the problem.
     
  13. Jan 3, 2016 #13

    Rockiedog2

    Rockiedog2

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    Dunno know your system plumbing so this may not be a valid guess
    I was having pressure probs similar to yours...low pressure on engine driven pump and normal when turning the backup electric pump on. Had the common glass bowl fuel filter installed

    UNIVERSAL IN-LINE FUEL FILTER from Aircraft Spruce

    between the electric pump and the mechanical pump and at the lowest point in the fuel system. I would check that filter visually on preflight and it always "looked" clean and open. Not so as I found. That particular filter was skimmed over from the cargas/ethanol/100LL(whatever I could get at the time). I opened it and the skim didn't show atall but was there. Threw the thing in the trash and replaced it with our common gascolator and no probs since. Took it off my other plane and threw it away too. I don't use anything but gascolators anymore. The filters seem too fine to me. This prob may have been a compound problem...the other side has already been mentioned. Yeh, the engine driven pump was failing. The electric pump was able to push the fuel thru the stopped up filter and the failing mechanical pump couldn't suck it. If it hadn't been failed/failing maybe it could have sucked it thru...not sure. Replaced the filter and pump same time.
    Well, the filter info may be an aside but I consider it important in general. If I understood the testing you did correctly, it sure sounds like may be the mechanical pump. If I had a filter I would check it first regardless and then check the mech pump. I dunno your system component sequence (where is the elec pump/mech pump/pressure pickoff etc)so can't come up with a progressive troubleshoot sequence but am sure you can. Easy...

    Oh yeh, I was running a mech gauge so no possibility of some grounding prob. Also if you do as mentioned by somebody and crack the line on the suction side of the mech pump and run the electric pump(assuming it's upstream of the mech pump) and you got full flow then you've pretty well eliminated an obstruction upstream of the mech pump. Depending on your plumbing again...
     
    Last edited: Jan 3, 2016
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  14. Jan 3, 2016 #14

    Kyle Boatright

    Kyle Boatright

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    The only filter in the system is the screen in the gascolator.

    The gross sequence of devices in the fuel system is electric pump > gascolator > mechanical pump > pressure transducer (and carb).
     
  15. Jan 3, 2016 #15

    Rockiedog2

    Rockiedog2

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    yeh that's the way I set mine up.
    I would ck/clean the gascolator screen always first thing regardless. It's free, right?
    Then so if it won't gravity feed first I would do a high power ground run like somebody mentioned enuf to for sure run the bowl dry and see if it quits. When it first starts surging turn on the electric pump and see if it smoothes out. I would prob do than a couple x to confirm, then let it starve just as a final check. For me, that would pretty well confirm the mech pump, enough that I wouldn't feel like I was throwing parts at it. If there's somebody around that would let me borrow their known good pump I might install that and see. I would expect that one to solve the prob then order a pump, if not there's something real weird goin on...

    That's the way it looks from here, as always I mighta missed somethin'...
     
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  16. Jan 3, 2016 #16

    Rockiedog2

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    One thing that enters into it for me that may not be technically correct (but I've developed the assumption over time and always troubleshoot accordingly) is that a push from a gravity flooded pump is more reliable than a suck from a dry pump; particularly if it has to suck thru a partially blocked filter/gascolator to get its prime. Thus my personal procedure for troubleshooting any system setup like yours is to check the filter/screen first. Regardless of the technicalities involved. Your electric pump is likely gravity flooded(I setup the one in my RV8 so it was) and your mech pump likely isn't except for whatever trapped fuel is in the line and that likely drained back into the tank after shurdown. Right?
     
  17. Jan 3, 2016 #17

    Kyle Boatright

    Kyle Boatright

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    On the ground, the electric pump is probably on the same plane as the fuel tanks, so it may or may not be gravity flooded. The mechanical pump and carb are at the standard locations for an O-320.
     
  18. Jan 3, 2016 #18

    Rockiedog2

    Rockiedog2

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    Mine started surging on a couple different flights. Wife(who could sleep thru a train wreck) was asleep in the right seat and Rockie was asleep in the bag compartment. She sat bolt upright on the first surge, and Rockie(can't hear thunder anymore) even raised his head and looked around to see what was going on. You'll find out how well/quick your hand knows where the pump switch is
    You could keep flying it til it surges on you then you won't mind throwing parts at it as much.
     
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  19. Jan 3, 2016 #19

    Toobuilder

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    There are a couple of check valves in the pump and they are susceptible to trash partly blocking their operation. Id check the gascolator screen and pull that pump apart looking for evidence of fuel tank sealant or other trash. Were your tanks sloshed when built? - that was popular back then.
     
  20. Jan 3, 2016 #20

    Kyle Boatright

    Kyle Boatright

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    No slosh in my tanks, thankfully. I plan on checking the gascolator screen as part of the next wave of troubleshooting.
     

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