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Unfeasibly high fuel pressure

Sportquattro

Well Known Member
Hi folks, I had my final inspection done today by the LAA inspector and the only snag was fuel pressure reading.

Engine is a Lycoming O320-D2A carburettor fed. Cockpit is fitted with Garmin G3x touch, and the fuel pressure sensor is a Garmin 15psi sensor for carburettor engines.

All the Garmin kit was purchased new at the same time from Bournemouth Avionics.

With the engine not running but the Vans supplied electric fuel pump running the G3x display shows 39psi. No fuel leaking from the carburettor or other obvious issues. (As aircraft still apart for inspection not able to test with running engine at the moment)

I’m sure it’s a incorrect reading as if it really was 39 psi I’d expect to see fuel pouring from the carburettor as that pressure I imagine would overcome the float valve.

Any suggestions as to the cause and solution would be welcome as I’m stumped, my only thought is have I gat a mislabelled sensor for the injection version of the engine and if so would that even give a massive high reading spuriously?.

Thanks in advance before I start replacing expensive sensors that may not be the issue.

Nige
 
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Usually always a pressure sender issue. No garmin in my plane but when i have wonky readings on pressure, i check/replace senders.
 
Hi folks, I had my final inspection done today by the LAA inspector and the only snag was fuel pressure reading.

Engine is a Lycoming O320-D2A carburettor fed. Cockpit is fitted with Garmin G3x touch, and the fuel pressure sensor is a Garmin 15psi sensor for carburettor engines.

All the Garmin kit was purchased new at the same time from Bournemouth Avionics.

With the engine not running but the Vans supplied electric fuel pump running the G3x display shows 39psi. No fuel leaking from the carburettor or other obvious issues. (As aircraft still apart for inspection not able to test with running engine at the moment)

I’m sure it’s a incorrect reading as if it really was 39 psi I’d expect to see fuel pouring from the carburettor as that pressure I imagine would overcome the float valve.

And suggestions as to the cause and solution would be welcome as I’m stumped, my only thought is have I gat a mislabelled sensor for the injection version of the engine and if so would that even give a massive high reading spuriously?.

Thanks in advance before I start replacing expensive sensors that may not be the issue.

Nige

IIRC the pressure necessary to overcome to float valve is approximately 10 PSI. I’d say it’s almost certainly a sensor issue since you didn’t observe any fuel pouring out.
 
I would confirm that sensor is wired correctly. I had an issue with oil pressure reading pressure with the engine not running. It ended up that the panel maker had reversed 2 wires. I forget the details but one wire supplies low voltage and it was connected to the wrong lead. I have the G3X Garmin and sensors.
 
Put a manual gauge on the line and confirm it is really 39 psi then decide which branch of the diagnostic tree you follow.

2 cents - - typically if the G3X has a 15 psi range sensor selected then it will show over range or a dash if the range is greatly exceeded. It is odd that it reads any pressure this high. I think you missed something in configuration- something easier to fix than swapping parts. Think about it - the sensor has a 5 volt signal basically zero volts is zero pressure, 5 volts (4.8) is full range, 15 in this case. Gotta be the configuration. again -2 cents.
 
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Put a manual gauge on the line and confirm it is really 39 psi then decide which branch of the diagnostic tree you follow.

2 cents - - typically if the G3X has a 15 psi range sensor selected then it will show over range or a dash if the range is greatly exceeded. It is odd that it reads any pressure this high. I think you missed something in configuration- something easier to fix than swapping parts. Think about it - the sensor has a 5 volt signal basically zero volts is zero pressure, 5 volts (4.8) is full range, 15 in this case. Gotta be the configuration. again -2 cents.

+1

With the proper configuration and hardware pair, the G3X cannot show anything greater than 15 PSI. Transducers (3 wires sensors) do not provide a reading higher than 4.5 volts under any circumstance (range is .5 - 4.5V, with 1=.5V and 15=4.5V) and when configured correctly, that should give a display of 15 PSI. So if the display is showing something higher than 15, it must be configured incorrectly. If a 15 PSI transducer sees 100 PSI, it still sends 4.5V.

Note that there are both transducer and resistive options in the config. Guessing you picked a resistive setting with a physical transducer or some other error.

Sure you didn't send 12 volts to the sensor? That might send back a voltage that is too high and the G3X keeps on going higher after 4.5 volts (it may leave out error correction, as the sender is responsible for capping the output at 4.5V. I don't think the tiny regulator in the transducer can properly step down 12 volts and likely sends out higher voltages.

Larry
 
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