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loss of Fuel pressure issue

Shark

Shark

Well Known Member
Looking for some help from somebody smarter than me, I have been chasing a loss of fuel pressure for years now and I am at a loss, Here is what I can tell you:
  • O-320 E2D carbureted RV9A (higher time)
  • Flown approximately 150-200 hours per year
  • holds fuel pressure with the Boost pump on
  • I would suspect just a sending unit but it actually quits running when it hits 0-psi
  • it has happened on take off (too many times) and also just idling on the ramp (more than once)
  • replaced the boost pump (new Facet)
  • Replaced the Mechanical pump (new Tempest)
  • replaced the viton o-rings in the fuel selector
  • Checked and cleaned gascolator
  • drained the whole system and ran scope inside all of the lines I can
  • checked the vent lines for blockage
  • checked the Mechanical pump vent line for blockage
  • rechecked all connections
  • Made sure all the lanes are away from the exhaust
  • checked and cleaned carburetor screen
  • For years I have been able to fly it without ever touching the boost pump (I know, use it on take off)
Any help would be much appreciated, I won't lie, I'm a bit nervous posting this on here so go easy on me please
 
I don't see anything about a filter on the list. I had an inline one with a sintered bronze element that became clogged and you couldn't see that is was dirty.
 
What you've written there forms a pretty good picture however there are still some questions to be asked.

The fuel pressure you should be seeing is not very high - I would expect your normal operating pressure should be between 2.0 to 5.0 PSI. Check to be sure that the fuel pressure is being measured from the OUTLET side of the mechanical pump. If it is not being measured from this location then change your plumbing so that it is. Some people take the fuel pressure reading from the input to the carburettor and this is fine, but it usually requires longer hoses and hoses are not cheap so typically the pressure reading is taken from the outlet to the mechanical pump. I've seen people try to take the fuel pressure from the firewall or the inlet to the mechnical pump and for obvious reasons that doesn't work so good. Imention this as its important to be sure you're getting correct information before you start the rest of the troubleshooting.

Given the long list of work you have done to date, from there I would move to the carburettor itself. How old is it? Is there are chance the float valve is sinking and flooding the engine?
 
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I’ve had 5 carbureted RV’s and all of them exhibit similar problems to what you have - EXCEPT - my engines have never quit. Usually in a full power climb with the electric boost pump off, my fuel pressure will gradually drop to zero. Nothing changes - the power stays the same, EGT’s don’t change, fuel flow remains constant. I usually will turn on the boost pump to get the warning light to go out, but if I don’t do that, the engine continues to run smoothly. After I level of at altitude, if I didn’t turn the boost pump on, the fuel pressure gradually increases to normal for cruise, about 2.5 psi. I’ve changed everything I could think of in the past (fuel pumps, pressure sender, etc) but none of that made a difference. The fact that your engine quits running when the pressure hits zero is much more serious than what I’ve experienced. It seems like your fuel pressure indication might be giving you good information. It sounds like your carb bowl isn’t getting filled at a zero psi indication, and it sounds like mine is. There have been numerous posts over the years about this kind of indication, but this is the first one that I can remember where the engine actually quits - from fuel exhaustion I assume. Does it start back up as soon as you get pressure?
 
planenutz said:
What you've written there forms a pretty good picture however there are still some questions to be asked.

The fuel pressure you should be seeing is not very high - I would expect your normal operating pressure should be between 2.0 to 5.0 PSI. Check to be sure that the fuel pressure is being measured from the OUTLET side of the mechanical pump. If it is not being measured from this location then change your plumbing so that it is. Some people take the fuel pressure reading from the input to the carburettor and this is fine, but it usually requires longer hoses and hoses are not cheap so typically the pressure reading is taken from the outlet to the mechanical pump. I've seen people try to take the fuel pressure from the firewall or the inlet to the mechnical pump and for obvious reasons that doesn't work so good. Imention this as its important to be sure you're getting correct information before you start the rest of the troubleshooting.

Given the long list of work you have done to date, from there I would move to the carburettor itself. How old is it? Is there are chance the float valve is sinking and flooding the engine?
Click to expand...
Thank you, I should have mentioned it, the first time it died on take off I bought a new carburetor thinking that it “has to be it”
Fortunately the gauge is plumbed correctly and I originally thought maybe it was a bad sending unit so I replaced that also, I now trust the gauge since it quits running shortly after it hits 0- psi
 
Scott Hersha said:
I’ve had 5 carbureted RV’s and all of them exhibit similar problems to what you have - EXCEPT - my engines have never quit. Usually in a full power climb with the electric boost pump off, my fuel pressure will gradually drop to zero. Nothing changes - the power stays the same, EGT’s don’t change, fuel flow remains constant. I usually will turn on the boost pump to get the warning light to go out, but if I don’t do that, the engine continues to run smoothly. After I level of at altitude, if I didn’t turn the boost pump on, the fuel pressure gradually increases to normal for cruise, about 2.5 psi. I’ve changed everything I could think of in the past (fuel pumps, pressure sender, etc) but none of that made a difference. The fact that your engine quits running when the pressure hits zero is much more serious than what I’ve experienced. It seems like your fuel pressure indication might be giving you good information. It sounds like your carb bowl isn’t getting filled at a zero psi indication, and it sounds like mine is. There have been numerous posts over the years about this kind of indication, but this is the first one that I can remember where the engine actually quits - from fuel exhaustion I assume. Does it start back up as soon as you get pressure?
Click to expand...
It does start up again when I fire up the boost pump, Thankfully I’ve had people say “ just run the boost pump all of the time” great but that doesn’t fix the problem. I am a little suspicious of the Gascolator? Do you think it’s sucking air around the o-ring?
 
Shark said:
It does start up again when I fire up the boost pump, Thankfully I’ve had people say “ just run the boost pump all of the time” great but that doesn’t fix the problem. I am a little suspicious of the Gascolator? Do you think it’s sucking air around the o-ring?
Click to expand...
Possibly. If your plumbing is like mine with the gascolator between the electric boost pump and the engine driven fuel pump, a leaking seal on the gascolator could be sucking air with the boost pump off. Just start out by eliminating the gascolator via modified plumbing for test flights. This would be going into a Phase 1, so clear it with your FSDO, and make sure your engine driven pump, and boost pump are producing required pressure. Maybe plumb in an inline filter since there’s some plumbing changes to be made and now you don’t have a filter. If this corrects your problem, get a different gascolator, or incorporate a plumbing change to include an inline filter that you can inspect annually.

It’s a process, but I wouldn’t want to depend on a single backup boost pump as your sole source of fuel pressure in a low wing airplane.
 
Scott Hersha said:
Possibly. If your plumbing is like mine with the gascolator between the electric boost pump and the engine driven fuel pump, a leaking seal on the gascolator could be sucking air with the boost pump off. Just start out by eliminating the gascolator via modified plumbing for test flights. This would be going into a Phase 1, so clear it with your FSDO, and make sure your engine driven pump, and boost pump are producing required pressure. Maybe plumb in an inline filter since there’s some plumbing changes to be made and now you don’t have a filter. If this corrects your problem, get a different gascolator, or incorporate a plumbing change to include an inline filter that you can inspect annually.

It’s a process, but I wouldn’t want to depend on a single backup boost pump as your sole source of fuel pressure in a low wing airplane.
Click to expand...
I like the way you think, I have been thinking the exact same thing except to just bypass the Gascolator long enough to see if that’s the problem and then purchase a new one if it is
 
Shark said:
Looking for some help from somebody smarter than me, I have been chasing a loss of fuel pressure for years now and I am at a loss, Here is what I can tell you:
  • O-320 E2D carbureted RV9A (higher time)
  • Flown approximately 150-200 hours per year
  • holds fuel pressure with the Boost pump on
  • I would suspect just a sending unit but it actually quits running when it hits 0-psi
  • it has happened on take off (too many times) and also just idling on the ramp (more than once)
  • replaced the boost pump (new Facet)
  • Replaced the Mechanical pump (new Tempest)
  • replaced the viton o-rings in the fuel selector
  • Checked and cleaned gascolator
  • drained the whole system and ran scope inside all of the lines I can
  • checked the vent lines for blockage
  • checked the Mechanical pump vent line for blockage
  • rechecked all connections
  • Made sure all the lanes are away from the exhaust
  • checked and cleaned carburetor screen
  • For years I have been able to fly it without ever touching the boost pump (I know, use it on take off)
Any help would be much appreciated, I won't lie, I'm a bit nervous posting this on here so go easy on me please
Click to expand...
Just maybe the mechanical pump is not pumping -- been known to buy new airplane parts that don't work. The new fuel pump might not have been installed correctly -- there is a specific procedure for installing a new mechanical fuel pump (must line-up properly with internal engine drive). There are several different types of mechanical fuel pumps, too. I would consult your specific Lycoming service manual to answer these questions. It really sounds like a mechanical fuel pump issue to me, and I would keep looking there.
 
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