What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Problem Fuel Pressure too High

design4p

Active Member
Hi,

Just completed my RV14A and test flown the aircraft.

I am concerned that my fuel pressure is in the high 40's (48psi). I have a full Garmin avionics setup G3X, GEA24 etc and the sensors are all from the Garmin sensor kit for 4 cylinder with a Lycoming (390 engine).
The G3X fuel pressure sensor I have installed is the 494-30004-02. I am assuming, but can be wrong, that the sensor is a Kavlico P4055-50G. I therefore, configured the G3X for that particular sensor.

The connection to the GEA24 is as suggested in the Garmin Installation manual. Using pins 1, 2 ,4 on J243 - I will double check these connections on my next visit to the airfield.

I would be most grateful for assistance on solving my problem.

Regards John
 
Give us details of your system fuel configuration e.g what kind of FI system do you have; is it full flow with back pressure regulator, etc?

Apply Occam's Razor. (could be something as simple as a back pressure reg setting)

Have you verified your observation with a trusted mechanical gage?

Excluding myself, there's some smart guys on the forum. Help us help you.
 
Hi,

Just completed my RV14A and test flown the aircraft.

I am concerned that my fuel pressure is in the high 40's (48psi). I have a full Garmin avionics setup G3X, GEA24 etc and the sensors are all from the Garmin sensor kit for 4 cylinder with a Lycoming (390 engine).
The G3X fuel pressure sensor I have installed is the 494-30004-02. I am assuming, but can be wrong, that the sensor is a Kavlico P4055-50G. I therefore, configured the G3X for that particular sensor.

The connection to the GEA24 is as suggested in the Garmin Installation manual. Using pins 1, 2 ,4 on J243 - I will double check these connections on my next visit to the airfield.

I would be most grateful for assistance on solving my problem.

Regards John

May or may-not be related to your problem, but for background, the Garmin 494-30004-02 is a 50 psi sensor that's been discontinued in favor of the Garmin 011-04202-20 / KAVLICO P4055-75G....75 psi sensor.

To complicate matters a little (not sure if this relates to you) the Kavlico P4055-75G is the subject of a Garmin Service Bulletin (2069 Rev A), released about a year ago. Not sure where Garmin stands on the subject, but other companies (Advanced Flight and Dynon) that use those lower-pressure fuel pressure sensors recommend replacing with the newer Kavlico 150 psi sensors for both oil and fuel. At one point, they even used the "F" word...(fire).

https://www.vansaircraft.com/wp-content/uploads/2020/05/SB-00006.pdf
https://www.garmin.com/en-US/aviati...g3x-g3x-touch-equipped-experimental-aircraft/
https://dynonavionics.com/bulletins/support_bulletin_050620E.php
https://www.advancedflightsystems.com/service-bulletin-050620a.php
 
Hi Freemasm,

The aircraft is a standard Van's RV14 build and using Garmin avionics G3X etc. I think I have uploaded the relevant build section (49-14) and the fuel arrangement is on page 3 or 18.

I recall that the fuel line from the engine fuel pump to the sensor (VA-102) did have a restrictor.

Hope this helps.

John
 

Attachments

  • 49_14.pdf
    7.9 MB · Views: 365
I had the same issue 3 months ago when I entered phase 1 with my 14A. At idle without the boost pump on my fuel psi was between 40 and 45 and sometimes higher. Once I went above idle and more fuel was flowing the pressure dropped to 30. When I turned the boost pump on when starting take off it would spike the pressure up again until the engine accelerated. You can search my post on this forum. The high pressure without the boost pump on has improved with time and now I only occasionally see more than 40 psi at low idle speeds. Lycoming tech service told me the fuel pumps sometimes need to break in. That seems to be the case. I corrected the boost pump over pressure by adjusting the relief valve pressure on the pump. Andair can give you the process. FYI, when I went to adjust the grub screw on the relief valve I found the Jam nut finger tight. It came that way from the factory.
 
Sorry, I'm out of my element

This non-return type of FI system appears to be the legacy style. Sorry but I can't speak to the specifics of it but I have designed a lot of fluid systems for aerospace and power generation. Here's some thoughts. It appears to utilize a centrifugal type boost pump and a diaphragm type engine driven mechanical fuel pump (assumptions on my part). With very rare exception, pumps are delta pressure devices. Some fluid system background info:

The boost pump will have an associated pressure versus flow curve. Low flow = higher pressure and vice versa. The diaphragm pump is a positive displacement (PD) type that provides constant flow for a given speed. Theoretically, it can provide infinite pressure as determined/required by the downstream system and the available power from the pump driver (engine cam shaft here). There are of course mechanical limits. Since there's no fuel return to the tank (which would use a back pressure regulator), a PD type pump must utilize a different pressure relief (think partial flow diversion) to limit forward flow; here probably a relief valve internal to the PD pump that sends "excess flow back to the pump inlet.

Why all the rambling?

One pump has a non-linear pressure vs. flow curve. The other pump controlled by a relief valve that will be hard to maintain a constant pressure over a wide range of flow (much easier on something that that doesn't have to be small and light enough to fly). So, two non-linear performing devices, in series (when both one), that cover a wide range of flow demand, etc. I would not expect a constant pressure.

If you could give us some fuel system pressure versus RPM data while including pump operation state for each, we could probably settle some of these suspicions. All that said if fuel pressures stay in the green and behaves as expected (reference above), I don't see a problem.

I'm following this and want to learn more. Your original suspicion appeared to be instrumentation related. I'm not there with you, yet. If the low flow pressure exceeds the instrument range, obviously, the related instrument could get damaged.

Sorry I couldn't help but I'm staying tuned.
 
fuel Pressure High

Hi,

I've isolated the Fuel Pressure and the RPM from the G3X flight Log spreadsheet and converted it to a pdf for uploading here.

Each row is a second in the flight.

Not sure when the booster pump was ON, however, it was on when landing and I presume when taking off.

Regards John
 

Attachments

  • Fuel Pressure & RPM.pdf
    912.5 KB · Views: 226
This non-return type of FI system appears to be the legacy style. Sorry but I can't speak to the specifics of it but I have designed a lot of fluid systems for aerospace and power generation. Here's some thoughts. It appears to utilize a centrifugal type boost pump and a diaphragm type engine driven mechanical fuel pump (assumptions on my part). With very rare exception, pumps are delta pressure devices. Some fluid system background info:

The boost pump will have an associated pressure versus flow curve. Low flow = higher pressure and vice versa. The diaphragm pump is a positive displacement (PD) type that provides constant flow for a given speed. Theoretically, it can provide infinite pressure as determined/required by the downstream system and the available power from the pump driver (engine cam shaft here). There are of course mechanical limits. Since there's no fuel return to the tank (which would use a back pressure regulator), a PD type pump must utilize a different pressure relief (think partial flow diversion) to limit forward flow; here probably a relief valve internal to the PD pump that sends "excess flow back to the pump inlet.

... snip ...

Sorry I couldn't help but I'm staying tuned.

I believe the engine driven pump develops its pressure due to a spring pushing the diaphragm back after the cam displaces it; this puts an upper limit - around rated pressure - these pumps can provide. Over time this decreases as the spring loses some of its elasticity, but in my experience this decrease is minimal.

My FI setup uses an Airflow Performance boost pump and associated relief valve in series with the engine driven pump. In operation the engine driven pump maintains about 27 PSI but the boost pump will sometimes spike the pressure to over 30 PSI before the relief valve kicks in and drops it back to about 28-29, especially near engine idle RPM.

I had a sensor go bad once and show me a lot higher pressure than that, but the pressure indications were erratic and clearly a sign of the sensor malfunctioning.
 
FWIW I fought a "high" fuel pressure problem for months when I first my engine monitoring system installation. Turns out I had programmed the wrong sensor definition in the EFIS module. I programmed the correct one and it has been working perfect for 7yrs. YMMV

:cool:
 
Here goes nothing

I believe the engine driven pump develops its pressure due to a spring pushing the diaphragm back after the cam displaces it; this puts an upper limit - around rated pressure - these pumps can provide. Over time this decreases as the spring loses some of its elasticity, but in my experience this decrease is minimal.

My FI setup uses an Airflow Performance boost pump and associated relief valve in series with the engine driven pump. In operation the engine driven pump maintains about 27 PSI but the boost pump will sometimes spike the pressure to over 30 PSI before the relief valve kicks in and drops it back to about 28-29, especially near engine idle RPM.

I had a sensor go bad once and show me a lot higher pressure than that, but the pressure indications were erratic and clearly a sign of the sensor malfunctioning.

Thanks for that info and it makes some sense. If the bellows force and resultant system pressure is provided by an internal spring and engine only provides the required "reset", the engine driven pump would have a "variable displacement" characteristic. Very simple design elegance.

As for the data, some quick observations and guesses. Pump status is not known but guess-able. Obviously no conclusions, but:

- One would guess the engine was started with the boost pump off.
- The engine fuel pump appears to be behaving quite nicely. This assumes/it appears that it alone (no boost pump) was on through idle, taxi, and run-up. The pressure was very constant through a decent range of fuel flows.
- You kept the boost pump on through climb-out and cruise. The associated high fuel flow kept the system mostly in range.
- You descended and landed with the boost pump on. Low flow kept the system pressure mostly off-scale high (no FP data).
- You kept the boost pump on through taxi and shutdown.

All that said:

- The data appears valid. No issue with sensors.
- What is your fuel system pressure operating limits? Green arcs and redline?
- Measure FP pressure with only the boost pump = ON
- Can the boost pump relief be field adjusted? If so, a small setting move could be all that is needed

As mentioned, I'm good with fluid systems but no first hand experience with yours. Find someone with that expertise and keep us informed. Best of luck. Sorry I couldn't be of more assistance.
 
Reply

In my research today

The Lycoming 390 has a max fuel pressure of 45psi inlet to fuel injectors.

I will check the pressure with only the booster on when I visit the airfield (could be tomorrow).

The output pressure of the booster pump can be adjusted.

I appreciate you assistance.

Regards John
 
Been doing some digging

Let me state again that I don’t have experience with traditional or legacy type mechanical FI systems but have been doing some studying. The boost pump is probably a vane type (positive displacement) type pump. I gleaned this from some old Dukes and Airflow Performance manuals.

Why does this matter?
There is a Positive Displacement Pump in Series with a Variable Displacement Type pump. The VD type pump (engine driven fuel pump) should produce a rather steady pressure over a wide range of flow. As I previously stated, from the data you posted, this pump appears to be performing as expected with a fairly constant pressure of ~27-28 psi over a wide range of flow. This assumes boost pump = off during what appears to be start-up through taxi and run-up (your posted flight data).

The boost pump should have a full flow bypass/relief valve; meaning, pressure should change very little from zero system flow (engine demand) through full flow plus some margin. Remember that actual boost pump flow is pretty constant as it bypasses excess fuel flow back to it’s own inlet.

Some findings.
The engine pump appears healthy and performing as expected at the pressure setting per one OEM’s website; 26 - 32 psig setting. Per a couple of manufacturer’s releases i’ve read, the boost pump relief valve setting should be set a few psi greater than the main fuel pump actual supplied pressure. This is to ensure an instrument response (secondary info for pilot) that the boost pump is actually operating when energized. For you, a setting of 30-31 psig based on your operational data.

What’s next.
To me, everything points to the boost pump relief valve performance. The pump can obviously make the pressure. It is either set wrong, some debris is not allowing the relief valve to properly actuate, etc. Again, I would verify the system pressure with only the boost pump running as a start and troubleshoot from there. When all is behaving properly, the fuel system pressure should be fairly consistent across rpm (27-ish engine running now) when the boost pump is off and 30-ish when the boost pump is on

I would not fly this aircraft until the total FI system is behaving as expected. Extrapolating your flight data against one manufacturer’s published pump data, there are times it is theoretically drawing almost 8 amps.

Now my disclaimer. As mentioned, I’m new to this type of feed forward type FI system. My recommendations are based off of my fluid systems design experience and the info I’ve recently gleaned from some suppliers of said systems. Find the required expertise for the FI system/components installed for a safe return to flight.

Keep us informed.
 
Last edited:
This thing has gotten way more complicated than necessary.

The engine driven pump cannot generate pressure higher than its spring dictates. So, if the indicated pressure is significantly more than normal pump pressure (25 psi ballpark), it is an indication problem. Confirm by connecting an analog gauge.

If you're not sure how a Bendix-style FI system actually works, call Don or Kyle at Airflow Performance, or sign up for the AFP school.
 
It is a pretty consistent 27 psi until the boost pump is energized; then 48 to off scale high. The boost pump output makes the engine pump op benign as it should. My money is on the boost pump relief valve. I have infinite respect for Mr. DanH but will gladly bet him or anyone else a craft brew.

I’ve already won. Had a reason to geek up on a system unknown to me.

Looking forward to hearing the root cause once determined by the OP.
 
It is a pretty consistent 27 psi until the boost pump is energized; then 48 to off scale high. My money is on the boost pump relief valve.

It is possible. Assuming it is an AFP pump, past reports of stuck poppet valves typically involved o-ring swelling with an earlier rubber compound and auto fuel. The OP is reporting this to be a brand new installation, so all it would take is some dried test fluid residue after a long sit, or maybe just rubber unlubricated by fuel.

Again, the sure diagnosis is to connect a mechanical test gauge to determine actual pressure vs indicated pressure. Want to cheat? The Bubba method would be to whack the relief valve with a rubber/nylon/wood mallet while the pump is running, engine not running, mixture in idle cutoff...but Don and Kyle may not approve.
 
Hi,

Been at the airfield today. The booster pump was 49.2psi with engine stopped.

I wanted to check the psi with the engine running, however, she would not start. The small (tiny) battery was soon dead and no one at the field for assistance.

Anyway, I thought I would alter the setting on the booster pump. I was surprised to find only 2 threads of the grub screw holding the locknut and grub in the housing. I did screw out a little and gained a pump pressure of 43psi, but worried about the thread engagement.

I have just emailed ANDAIR for a technical analysis.

Regards John
 

Attachments

  • Fuel Pressure 010 (640x360).jpg
    Fuel Pressure 010 (640x360).jpg
    85 KB · Views: 122
Solution

Hi everyone,

Thank you for all the assistance. I am pleased to say the problem of high fuel pressure from the booster pump has been solved.

Andair (UK) supplied me with a new fuel pump. The existing pump (supplied in the Van's Kit) must have been calibrated incorrectly.

I must say that I have received terrific service from Andair both technical and customer service.

I really value the Van's Airforce forum. It is an essential route to solving problems.

Regards John
 
It is possible. Assuming it is an AFP pump, past reports of stuck poppet valves typically involved o-ring swelling with an earlier rubber compound and auto fuel. The OP is reporting this to be a brand new installation, so all it would take is some dried test fluid residue after a long sit, or maybe just rubber unlubricated by fuel.

Again, the sure diagnosis is to connect a mechanical test gauge to determine actual pressure vs indicated pressure. Want to cheat? The Bubba method would be to whack the relief valve with a rubber/nylon/wood mallet while the pump is running, engine not running, mixture in idle cutoff...but Don and Kyle may not approve.

I have heard a few reports from folks that when a new AFP boost pump was first run after sitting for years that it would run high until they hit it or did something proactive to loosen the stuck relief valve ball.

While I respect your experience and knowledge, I do not agree that a traditional fuel pump CANNOT fail with high pressure. I had a std AC pump on our boat engine (small block Chevy) fail by delivering 10-12 PSI (supposed to be 3-4 PSI) and consistently overflowed raw gas into the intake. I can't tell you the specific issue that causes it, only that it can happen.

Larry
 
Back
Top