Low fuel pressure ?

I have asked before and got no response. My fuel pressure reading (carb) is OK on the ground but drops significantly flying. Example, at startup/run-up, mechanical may be near green and in the green on boost pump. But flying, both pressures drop proportionately to about half way between o and green. High Altitude (8k) and temperature affect it a little, but the same trends remain.

If the sender referenced local pressure, why wouldn’t the fuel pressure go up, proportional to altitude . If the sensor is a strain gauge, wouldn't’ it be indifferent to altitude and a little sensitive to temperature ?

I know the absolute reading could be a calibration issue with Vans steam gauge, but drop at altitude is puzzling. Ideas appreciated.

Two atoms walking down the street.

Are your fuel vents free flowing? are they facing forward so you have positive pitot pressure with airspeed? Or are they in a location where there is slight negative pressure? -( like inside the cockpit perhaps)

There are two atoms walking down the street and the one gets an un settled look on his face. He declares to the other that he thinks he has lost an electron.

The other atom responds "Well, are you certain?"

The response " Yes, actually I am positive"

Two Rocket vents facing forward.

Thanks F1, thats a good observation. Both face forward, but I will contnue to ponder with your point in mind.

As I understand the OP, the indicated fuel pressure is low when flying, even when the boost pump is on. Is that correct? I think I'd be suspicious of the gauge indication.

One could temporarily add a parallel mechanical gauge like this to the plane: https://www.aircraftspruce.com/catalog/topages/fuelpmpvactst.php?clickkey=27206

A test flight would help isolate the problem to a true fuel pressure issue vs a gauge issue.

Also, be aware that some Van's designs (the stock RV-6 for example) used an automotive electrical boost pump without a bypass. That pump reportedly has a failure mode that has apparently brought at least one RV down: http://www.vansairforce.com/community/showthread.php?t=44901&highlight=fuel+pump

Thanks Tim

Yes, at altitude, both mechanical and boost are lower. The boost pump is facet.

One idea: Assume that you have one of the the 4-7 psi or 4-6.5 psi Facet "cube" pumps, and the low pressure (4-6 psi) engine-driven fuel pumps. Both of these devices are designed to produce that much gauge pressure, i.e. delta between output and ambient. I do not know which pressure sender you have, but they make those in both absolute and gauge types. If you have an absolute-type sender, the measured pressure would drop as the ambient pressure where the sender is mounted drops and the pumps maintain relative delta-P.
Second idea: Facet makes many variations of the cube pumps for many different pressures and many different flow rates. If by accident you got a lower-flow version, the output pressure might drop as the flow exceeded the rated value. This is much less likely, however, because the engine driven pumps can easily provide all the flow you need, even for a 540. But if you put the sender between the electric and the mechanical pump, it could happen.
FWIW,
Andy

More questions

Yesterday I noticed fuel pressure down almost to the white mark ( just above red)on the Vans logo?d gauge. Boost pump picked it up to bottom of green arc. Since the difference , with boost is about the same, I conclude the deviation source is the sensor / gauge. Since there is always pressure, just indication variance, it is apparently OK but very UNCOMFORTABLE.

I can build a test apparatus to validate what 5psi shows on the gauge. But I don?t find data to show sensor voltage output vs pressure nor voltage input vs gauge reading for units available on ACS. I could just buy new UMA gauge and sensor and live with the results, but what if 5psi in does not indicate 5 psi on the gauge ?

Then there is the question DIFF Sensor vs Other shown in the ACS catalog.

I will call UMA today. Any comments to help guide me to resolution would be appreciated.

Larry DeCamp said:

Yesterday I noticed fuel pressure down almost to the white mark ( just above red)on the Vans logo?d gauge. Boost pump picked it up to bottom of green arc. Since the difference , with boost is about the same, I conclude the deviation source is the sensor / gauge. Since there is always pressure, just indication variance, it is apparently OK but very UNCOMFORTABLE.

I can build a test apparatus to validate what 5psi shows on the gauge. But I don?t find data to show sensor voltage output vs pressure nor voltage input vs gauge reading for units available on ACS. I could just buy new UMA gauge and sensor and live with the results, but what if 5psi in does not indicate 5 psi on the gauge ?

Then there is the question DIFF Sensor vs Other shown in the ACS catalog.

I will call UMA today. Any comments to help guide me to resolution would be appreciated.

Click to expand...

I suggest you call Lycoming (or the engine manufacturer) as well. Normal fuel pressure for a carbed engine can be surprisingly low.

From a Lyc manual.

O-360 Series (Except -A1C, -C2B,
-C2D); HO-360-A, -C Series

Inlet to carburetor
Max: 8.0
Desired: 3.0
Minimum: 0.5