Fuel pressure mystery

[skippydiesel]

Aircraft; New Sonex, with new Rotax 912ULS. Dynon Skyview engine instruments.

 

Mystery;

Fuel pressure with & without boost pump, meets Rotax fuel pressure standards, while ground running, taxi run up etc. Fuel pressure drops (Dynon alarms), during climb out (when boost turned off) and must be maintained with boost pump on for duration of flight. Pressure returns on landing. Engine has never shown signs of fuel starvation (loss of power/hesitator, etc).

 

Have checked & rechecked:

• Fuel sensor security  (attached to firewall)-appears to be okay.
• Gascolator screen for contamination - all good
• Aircraft has two wings tanks - makes no difference which one is in use.

 

Wondering;

Faulty sensor reading but why only in flight?

If cowling air pressure  (through drain hose) could be influencing the pump diaphragm.

 

Ideas?

 

 

 

[facthunter]

Engine pump might not be stroking the full stroke. Too thick a gasket?  Or the pump diaphragm screws were tightened before the Pump was stroked  preventing full rate being achieved.   Nev

[Blueadventures]

Blocked or partially blocked tank vents?

[Thruster88]

I had an almost identical situation with my new to me RV6a a few years ago with low 1 psi on just the mechanical pump. I talked with the previous owner and was told it has always been like this. A few months later on a takeoff there was an alarm in the headset, had that boost pump that I had forgotten, on in about 1 nanosecond, yes I am a bad pilot. So I decided to change the lycoming mechanical engine driven fuel pump, only $650. Problem solved. Disassembled old pump, could not see any defects with valves or diaphragm. It is a mystery. Works correctly now.

 

 

[onetrack]

There could be a possibility of the fuel line length and sizing (and in particular unions and elbows internal hole sizing) that is creating some flow resistance - so that everything works fine on the ground - but the additional force applied by engine thrust when in the air, is adding to the already high flow resistance, and making the fuel pump struggle, when flying.

[IBob]

Skippy, with a mechanical gauge, this is exactly the result you get when the builder has forgotten to take the little shipping plug out of the body of the gauge: so the gauge internals stay at ground pressure, while the fuel pressure does actually fall with altitude.

So: whatever the fuel pressure sensor is, it should have fuel line on one side, and be open to atmosphere on the other.

That's where i would be looking.

[facthunter]

No harm in checking but I don't think that amount of pressure change will happen with usual height changes we are talking of.   Nev

[IBob]

You  might not think, Nev..........but I've now seen this twice with 912 installations.

[facthunter]

I know for a fact that at 17,000 feet a pressure of 8.6 Lbs/square inch puts the cabin on the ground. Aren't we talking of circuit heights ?  5 millibars Nev

Edited December 28, 2022 by facthunter

[IBob]

Come at it another way:

912 min fuel pressure 2PSI
More normally it is between 2 and 4PSI, I think.
The pressure gauge is differential, fuel pressure one side , atmosphere the other.
If the atmosphere side is blanked off in some way, that side remains at ground pressure.
If you then take the aircraft to 5000', the outside air pressure...and the actual fuel pressure....is reduced by approx 2.5 PSI.
So your fuel pressure will appear to have dropped by that much.

[facthunter]

Doesn't it go to OK when the boost pump is on? If the gauge is the problem it's the same gauge. Are we talking of going to 5000 feet? Nev

[IBob]

The boost pump raises the pressure further, so certainly will improve the reading.
I'm not saying the gauge is the problem, I'm saying it's the way it is hooked up: it's a differential gauge, close off the atmosphere side, and this is what you get.
And nobody so far mentioned altitudes or circuits. I just worked an example.

[skippydiesel]

Good stuff gents - will check out the pressure sensor tomorrow .

 

Not sure if its relevant but there was a small amount of engine oil in the-fuel pump drain bottle - may be 2-5 ml

[Blueadventures]

20 minutes ago, skippydiesel said:

Good stuff gents - will check out the pressure sensor tomorrow .

 

Not sure if its relevant but there was a small amount of engine oil in the-fuel pump drain bottle - may be 2-5 ml

That sounds like a leak.  There is a lip seal on the plunger for engine oil side and the diaphragm if failing will allow fuel to exit the drain.  Need to really look if engine oil or fuel mixture or both.  May need replacing of fuel pump, not real dear.  May be reason for low fuel pressure.  Worth a check on Rotax owner if you're a current member.   (I'm not at the moment)

Edited December 28, 2022 by Blueadventures

[IBob]

Skippy, reading back over this, you stated that fuel pressure meets the Rotax spec on the ground.
That is quite a wide range, I believe.
FYI I normally see approx 4.5PSI at fast idle (booster pump off).

 

 

[IBob]

To explain the gauge/sender/transducer thing further:
All of these devices have some sort of diaphragm (or similar setup) normally open to atmosphere on one side and connected to fuel/oil/whatever on the other. They work by measuring how much the diaphragm is flexed or deflected.

 

So, on the ground, engine off, there is equal atmospheric pressure on either side of the diaphragm, pressure reading zero.

 

Start the engine and there is atmospheric pressure on one side and atmospheric pressure plus fuel pump pressure on the other side. The atmospheric pressures cancel out, so you get fuel pressure.

Climb to altitude and the atmospheric pressure is reduced, but equally on both sides, so it still cancels out.

But if the atmosphere side of the diaphragm is not open to atmosphere (the shipping bung is left in the gauge) the atmospheric pressure reduces on one side of the diaphragm as the aircraft climbs, but not the other. And the result is what looks like a loss of fuel pressure.
I have seen this twice with steam gauges, and the fix is simply to pop out the rubber shipping bung on the top of the gauge body.

If this is the problem, then the setup will read okay on the ground, but the the fuel pressure reading will appear less the higher you go.
(This probably needs to be checked with the aircraft level in cruise, as a hard climb may show a reduction in fuel pressure, confusing the picture some.)

 

In a perfect world, the atmosphere side of the transducer would be hooked to the static port setup. But a gauge mounted in the cockpit (and so accessing cockpit atmospheric pressure) is accurate enough for practical purposes.

 

A sender or transducer mounted on the front of the firewall is accessing atmospheric pressure there: if mounted in a high or low pressure part of the airflow, this would also affect the reading. In this case I would expect to see the effect immediately after takeoff, rather than progressively as altitude increased.

 

[Blueadventures]

1 hour ago, IBob said:

Skippy, reading back over this, you stated that fuel pressure meets the Rotax spec on the ground.
That is quite a wide range, I believe.
FYI I normally see approx 4.5PSI at fast idle (booster pump off).

 

 

Then on a later post he added there is up to 5ml oil from fuel pump drain collector bottle.

[IBob]

I see that, Bluedaventures and it does need investigating.
But we need more info:
Is this a bottle just for the fuel pump vent, or is it also for oil reservoir vent. If so, which is the oil coming from?
Also, is oil still coming?
 

I was taught to troubleshoot by formulating a theory, based on my understanding of the machine,  that matched the symptoms...then formulating tests to test the theory. And doing all that before actually picking up any tools.
In Skippy's case here, I am very interested to know more precisely when the problem appears and also to what degree. Does it appear immediately on takeoff, or does it increase with altitude?

 

I have also learnt to simplify wherever possible: remove extraneous stuff rather than adding to it, in order to focus on one specific thing.

[Blueadventures]

26 minutes ago, IBob said:

I see that, Bluedaventures and it does need investigating.
But we need more info:
Is this a bottle just for the fuel pump vent, or is it also for oil reservoir vent. If so, which is the oil coming from?
Also, is oil still coming?
 

I was taught to troubleshoot by formulating a theory, based on my understanding of the machine,  that matched the symptoms...then formulating tests to test the theory. And doing all that before actually picking up any tools.
In Skippy's case here, I am very interested to know more precisely when the problem appears and also to what degree. Does it appear immediately on takeoff, or does it increase with altitude?

 

I have also learnt to simplify wherever possible: remove extraneous stuff rather than adding to it, in order to focus on one specific thing.

Be helpful to know the engine history; year built, TTIS, hours since last run if not new etc  Cheers.

[facthunter]

The problem is only evident when the engine driven pump is working by itself. It's not easy to check engine pump flow rate as the engine must be running to do it.. The engine will consume  fuel at the fastest flow rate at low density altitudes at full power. I sincerely suggest checking the Engine fuel pump would be the first place to start. Unused fuel pumps have a shelf life. Once they've had fuel in them it's even worse.  Nev

[skippydiesel]

1 hour ago, Blueadventures said:

Be helpful to know the engine history; year built, TTIS, hours since last run if not new etc  Cheers.

As stated - new - 11 hrs split fairly evenly between flight & ground test.

Engine purchased new late 2019 - first run earlier this year.

Edited December 29, 2022 by skippydiesel

[Blueadventures]

15 minutes ago, skippydiesel said:

As stated - new - 11 hrs split fairly evenly between flight & ground test.

Engine purchased new late 2019 - first run earlier this year.

Thanks just confirming detail.  Pump seal and diaphragm should be good then.

[skippydiesel]

2 hours ago, IBob said:

I see that, Bluedaventures and it does need investigating.
But we need more info:
Is this a bottle just for the fuel pump vent, or is it also for oil reservoir vent. If so, which is the oil coming from?
Also, is oil still coming?
 

I was taught to troubleshoot by formulating a theory, based on my understanding of the machine,  that matched the symptoms...then formulating tests to test the theory. And doing all that before actually picking up any tools.
In Skippy's case here, I am very interested to know more precisely when the problem appears and also to what degree. Does it appear immediately on takeoff, or does it increase with altitude?

 

I have also learnt to simplify wherever possible: remove extraneous stuff rather than adding to it, in order to focus on one specific thing.

First - I hope I can go for a test fight later today so that I might be able to be a tad more precise in describing some of the symptoms.

 

Second - Bottles/catch can- I had ( past tense) two bottles - one for engine oil breather and one for fuel pump/carburettor air box. The oil (I assume no fuel but hard to be definite) was caught in the fuel pump/air box bottle. I have now separated all into 3 small bottles but have yet to fly.

[skippydiesel]

6 minutes ago, Blueadventures said:

Thanks just confirming detail.  Pump seal and diaphragm should be good then.

yes - but I have already had to replace the starter motor under warranty, so no guarantees that the fuel pump is okay.

Edited December 29, 2022 by skippydiesel

[skippydiesel]

Reporting in after .9 hr test flight -

 

Before TO, checked fuel pressure sensor has a small (about 1.5mm) hole in casing, to atmosphere - appeared to be clear.

 

After leveling out

Turned boost pump off, fuel pressure dropped to below 2 psi  - fuel pressure then returned to 4.5 psi & stayed there. This suggests to me that the pressure sensor may be taking time to "catch" up with change in air pressure.

 

(FYI - Rotax Forum - seems quite a few pilots have the same/similar problem and never find the exact cause - most just live with it)

 

After landing

Noted fuel pressure at 3500 rpm, about 4.5 -5psi

 

After shut down

Checked sample bottle on end of air box breather - about 15ml fuel. Seem I have a flooding/overflowing float chamber - will investigate tomorrow.

Checked sample bottle end of pump drain - bit hard to see but might be empty  - will investigate tomorrow.