Fuel Pressure Issue

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ekimneirbo

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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.:)
 

Kyle Boatright

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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.:)
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.
 

Dan Thomas

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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.
 

Midniteoyl

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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.
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.. ;)
 
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Rockiedog2

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>>> 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
 

Midniteoyl

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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 backup 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...
And most likely the slowly clogging filter making the pump work harder to suck the fuel contributed to the early failure...
 

Midniteoyl

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Messages
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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.:)

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.
 

ekimneirbo

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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.
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

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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.:)
Well, getting 5psi with the boost pump only means the boost pump is working ;)
 

Kyle Boatright

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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?
 
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Toobuilder

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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.
 

ekimneirbo

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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.
 

Kyle Boatright

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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.
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).
 

ekimneirbo

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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).
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.:)
 

Kyle Boatright

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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...
 

don january

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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
 

ekimneirbo

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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.
 

Kyle Boatright

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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.
Very logical idea, but I'm hesitant to knock a(nother) hole in the firewall for the line for a mechanical gauge.
 
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