G3X Touch - Fuel pressure low

I noticed today that my fuel pressure was very low to nonexistent during a test flight. The pressure ranged from a high of about 1.5 to a low fluctuating between 0.2 to 0.0, ahem. The plane was set up in cruise at about 2300 rpm, fuel flow about 9 gph, with IAS around 135kts. The engine is an O-360. The fuel flow sensor is a Kavlico P4055-15G that was supplied as part of the G3X system when purchased from Stein. The sensor is mounted on a Van's sensor manifold, using a Van's braided line that connects to a restrictor mounted to a tee off the mechanical fuel pump (the other branch goes to the Red Cube, then on to the carb.) The G3X configuration seems to be correct - if the only thing that needs to be done is set the sensor type and gauge markings. The display range is configured 0-10 psi

So, what pressure should I be seeing? I was thinking it should be at least 3 psi. Is the restrictor messing up the pressure reading? Is the mech pump failing? Could the sensor be bad?

Carb Fuel Pressure

Hello Charley,

You should be able to pretty quickly troubleshoot the wiring and sensor since the sensor outputs 0.5 Vdc at 0 psig and 4.5 Vdc at 15 psig.

I would start with a ground test with the engine off and boost pump on. I pulled the SD card flight data log from a recent flight in our G3X Touch carbureted RV-7A and see that the electric boost pump was producing ~7 psig by itself before engine start.

On climb out at 2350 rpm with both pumps running, the fuel pressure was 5.5 psig. Single pump operation in cruise flight was about 5 psig and the pressure increased to around 6.5 psig in the descent to land with both pumps running.

Let us know if we can help.

Thanks,
Steve

g3xpert said:

Hello Charley,

You should be able to pretty quickly troubleshoot the wiring and sensor since the sensor outputs 0.5 Vdc at 0 psig and 4.5 Vdc at 15 psig.

I would start with a ground test with the engine off and boost pump on. I pulled the SD card flight data log from a recent flight in our G3X Touch carbureted RV-7A and see that the electric boost pump was producing ~7 psig by itself before engine start.

On climb out at 2350 rpm with both pumps running, the fuel pressure was 5.5 psig. Single pump operation in cruise flight was about 5 psig and the pressure increased to around 6.5 psig in the descent to land with both pumps running.

Let us know if we can help.

Thanks,
Steve

Click to expand...

I ran the EFP on the ground / engine off today - showed about 3.5psi. Did another test flight, and the FP gradually decreased to 0 psi during transition to cruise after turning off the EFP. After the flight, we made sure there was no air in the manifold line, and ran the EFP again. This time, we were unable to get the FP above 1.2 psi. I'm getting 5.04v from the sensor harness. The green line meters ~ 0.4v when the gauge shows 0 psi, and ~ 0.8v when the gauge shows 1.2psi (when the EFP is on.) I talked to a couple guys at Garmin, and it appears to be a sensor problem based on voltages the sensor is outputting and the pressure the EFP / MFP should be suppling. They are sending out a replacement. Meanwhile this evening, I'm looking at the data logs. The first few test flights show FP consistently at about 5psi. The past few test flights show a variable psi roughly between 1.5 to 3 psi at the beginning and end of flights, with long periods of 0 psi in the middle. Weird.

Is your fuel pressure dropping as you climb and going up as you descend? If the Kavlico sensors get sealed too well they become absolute sensors not gauge sensors and can act this way. Especially noticeable with low pressures like fuel on a carb.

Dynon has an advisory on this, and we actually switched the exact Kavlico sensors we use to ones that are less susceptible to the issue we identified (which may or may not be your issue, of course):

http://www.dynonavionics.com/docs/support_bulletin_120414.html

-Ian

dynonsupport said:

Is your fuel pressure dropping as you climb and going up as you descend? If the Kavlico sensors get sealed too well they become absolute sensors not gauge sensors and can act this way. Especially noticeable with low pressures like fuel on a carb.

Dynon has an advisory on this:

http://www.dynonavionics.com/docs/support_bulletin_120414.html

-Ian

Click to expand...

I'll look at the data later today, thanks. Would that explain a zero reading for level flight @ 2500 - 3000? Anyway, we put a mechanical pressure tester on the line today which indicated almost 4 psi while running the efp on the ground. I'm waiting for help to do the same test with the engine running.

UPDATE: Help came. MFP (engine running on ground) put out about 5.5 psi according to the mechanical pressure tester.

First, here is FP and BALT data, graphed courtesy of SavvyAnalysis (blue is FP, green is BALT):
1st test flight:


Latest test flight:


I also checked my sensor against the Dynon service bulletin. The pressure sensor is a Kavlico P4055-15G, with a date code of F2514 - which works out to be June 25, 2014, and falls into the range of affected/possibly defective sensors.

An update on this thread: I received a replacement Kavilco sensor from Garmin - newer than the date range mentioned in the Dynon letter - and installed. This one reads 0 to .5 PSI (depending on which test I did - let's say there was more than one) while running the EFP on the ground, while the mechanical fuel pressure tester reads about 3 PSI when attached to the same point in the fuel system. JFTHOI, I attached my air compressor to the sensor, and it reads initial pressures above 4.5 PSI. I'd love to set up a test bench with proper air pressure gauges, power supplies, etc.... Anyway, I'm beginning to think that the sensor has trouble translating low pressures into voltages. Or maybe the calibrations internal to the G3X need refinement?

The other course of action is to replace the sensor with another type/manufacturer. UMA makes a 0-7 PSI sensor that is supported by the G3X system - and is also ~$130! Comments?

A capacitor will only help prevent erratic fluctuations. If the problem is steady but incorrect fuel pressure, a capacitor is unlikely to help. Observe correct polarity of the capacitor. As long as the capacitor is connected to the correct two wires, its physical location should not matter. Many capacitors have solid leads, not stranded. Solid leads should be well supported to prevent breaking due to vibration.