Help Troubleshooting Fluctuating Fuel Pressure

[jaylw314]

On 3/25/2022 at 2:57 PM, PT20J said:

I doubt it is the transducer as they all should be designed for fast response so that the smoothing can be tailored to the application in whatever processes the signal. Therefore, I would expect the damping to be in the gauge itself. Mechanical gauges have some natural damping due to the mass and spring mechanism. But who knows what JPI or Garmin do? It would most likely be done with some sort of software smoothing algorithm. 

Skip

And, yes, it’s annoying. 

There is DEFINITELY some signal damping in some electronic instruments.  I have both a primary EI FP-5L and a JPI 730 hooked up to the same Shadin fuel flow transducer, and mine spikes +/- 0.3 gph about 4-6 times a minute on the JPI, but only 1-2 times a minute on the FP-5L (it's irregular, not a fixed frequency).  That means the FP-5L clearly dampens the input, and while the JPI may or may not, I suspect it does, just less so.

Edited April 28, 2022 by jaylw314

[EricJ]

If the addition of the volume in the lines with the precision gauge in place is what's fixing it, is it possible to put a -4 or -5 line to the transducer (anything with more internal volume) and see if that makes a difference?

[T. Peterson]

On 4/27/2022 at 6:20 PM, EricJ said:

Is it?   I was interpreting that the hose has two ends, the source at the servo and the transducer/instrument at the other, and the placement of the snubber was either on one end or the other, i.e., at the instrument/sensor/transducer as usual, or at the source end.   Your circuit shown is the normal low-pass filter configuration, but it would require tapping the sensor directly at the snubber and what happens to the other end of the hose?   Cap it off?

If the idea is to tap the sensor at the middle between the R and the C, then swapping them converts it from a low-pass filter (as you've shown), to a high-pass filter, which wouldn't be very desirable.

So my understanding of the circuit model was that the mass of the fluid (the capacitor analog) would swap in series with the snubber (the resistor analog).   In that case I can't see a difference in the RC model since the order of an R and a C in series (or any two linear elements or processes) doesn't matter.  

I don’t have a clue about what you said, but it was sure said impressively!! Ya got my vote!

[carusoam]

Eric gets extra credit for his electrical knowledge…. 

Best regards,

-a-

[Ragsf15e]

On 3/25/2022 at 2:33 PM, PT20J said:

Well, the air in the line theory didn’t pan out. I removed the line from the transducer and it was dry. So, I ran the boost pump with it attached but the fitting loose so that it would fill the line with fuel. No change in the fluctuations. 

I noticed quite a bit of flow when I primed the line. So, I need to check that the fitting at the servo is restricted. I’m pretty sure we transferred all the old fittings when we swapped the engine, but I need to check it. 

Skip

Ok folks, new theory coming your way!  You guys are smarter than me on this though, so you won’t hurt my feelings if you poo poo this…. Supposedly the fuel lines can leak air and not fuel, which is why they can suck in bubbles (prior to the pump) but not show a fuel leak.  We’ve established that the line to the transducer will always have air in it because that drains out between starts.  The air in the pressure transducer line may or may not act as a “snubber”.  What if you have a small air leak at the transducer connection?  When you start the engine, fuel goes into the line but the bubble between the fuel and the transducer is pushed out through the leak. Now your fuel is right against your transducer and you see oscillations from the pump. If the line is tight, the air buffers out the oscillations. So far I am four flights in after removing cleaning and tightening that connection and I have no more fuel pressure fluctuations. 
 

@PT20J is there any chance there’s a small scratch or other possible tiny air leak on your fuel line to transducer connection? You just had a new one with your new panel?  Perhaps it’s letting out the air bubble each time you run the engine.

[PT20J]

46 minutes ago, Ragsf15e said:

@PT20J is there any chance there’s a small scratch or other possible tiny air leak on your fuel line to transducer connection? You just had a new one with your new panel?  Perhaps it’s letting out the air bubble each time you run the engine.

I don't think so. I examined the fittings pretty carefully and they looked sound. Also, my system will hold pressure for a long time. After shutdown with a hot engine, the pressure will actually rise as the fuel in the lines boils off. A day later it is still holding several pounds of pressure. Assuming the internals of the Avstar is similar to the Precision Airmotive servo, the ICO valve is just a flat plate covering an orifice and they generally leak some depending on how well the mating parts are finished during manufacture. 

[Ragsf15e]

5 minutes ago, PT20J said:

I don't think so. I examined the fittings pretty carefully and they looked sound. Also, my system will hold pressure for a long time. After shutdown with a hot engine, the pressure will actually rise as the fuel in the lines boils off. A day later it is still holding several pounds of pressure. Assuming the internals of the Avstar is similar to the Precision Airmotive servo, the ICO valve is just a flat plate covering an orifice and they generally leak some depending on how well the mating parts are finished during manufacture. 

mine held shutdown pressure before and after this change as well.  I would think if any bubble at that connection had already been pushed out, and it was holding ~30psi (or increasing like yours), then this hypothetical air leak could still exist if it didn’t let fuel through?

[N201MKTurbo]

My fuel pressure gauge started vibrating in 2017 after I installed my new avionics. The only connection is I removed the gauge during the panel work. I have replaced or overhauled every component between the fuel tank and the gauge and nothing has stopped the vibration. The engine runs fine, but I wonder how long the gauge can take it. I replaced all the hoses, all the hard lines. The fuel pump has been replaced with a rebuilt pump. The servo was overhauled when I did the engine. The engine was overhauled. I rebuilt the selector valve. OK, it is the same Weldon boost pump. I replaced the gauge with two other gauges. Nothing has had any effect on the problem. I have blown out all the fuel in the line and it gets better a little. I have filled the line with fuel and it doesn't change. If I leave the mixture in ICO, it will have the same pressure a week later. Its driving me crazy!

Edited May 9, 2022 by N201MKTurbo

[Ragsf15e]

4 minutes ago, N201MKTurbo said:

My fuel pressure gauge started vibrating in 2017 after I installed my new avionics. The only connection is I removed the gauge during the panel work. I have replaced or overhauled every component between the fuel tank and the gauge and nothing has stopped the vibration. The engine runs fine, but I wonder how long the gauge can take it. I replaced all the hoses, all the hard lines. The fuel pump has been replaced with a rebuilt pump. The servo was overhauled when I did the engine. The engine was overhauled. I rebuilt the selector valve. OK, it is the same Weldon boost pump. I replaced the gauge with two other gauges. Nothing has had any effect on the problem. I have blown out all the fuel in the line and it gets better a little. I have filled the line with fuel and it doesn't change. If I leave the mixture in ICO, it will have the same pressure a week later. Its driving me crazy!

Well at least we’re not alone!  (But mine is still steady after an unknown cure…)

[N201MKTurbo]

3 minutes ago, Ragsf15e said:

Well at least we’re not alone!  (But mine is still steady after an unknown cure…)

I wish you could figure out what it was.

I might hire a witch doctor to dance around it.

Edited May 9, 2022 by N201MKTurbo

[Davidg]

Hello, I’m having a similar problem. My fuel pressure can stay steady for 3 or 4 flights  and beginning to fluctuate from 14 to 28/29 psi without any warning. One or two times it even drops near 0 psi. The engine still run smoothly. I've shown videos of the instrument to my mechanist. I was anable to do it again during the tests he ran. He finally told me that as long as it was random, with no consequence on the engine and other parameters, it feels to him that it was only a trouble with the instrument. He also said that the electric pump is here to help in case he would have been wrong. Do the others mooney owners here have found a solution? Did someone here experienced a failure of the fuel pump after having those fuel pressure fluctuation?
 

[Davidg]

Sorry, I forgot! M20J 

[Davidg]

Here is what it makes… despite those heavy fluctuations, engine always runs smoothly

[Davidg]

Yes, it is completely random! It can happen at cruise speed, or with low power during descent. It appears regularly during  3 or 4 flights then disappear for one or two flights. The only time I’ve never had this problem is during climb at full power…

[skykrawler]

Voltage goes through the gauge then on to the transducer.  The second terminal on the transducer is connected to ground.

Verify the connections at the transducer tight and that the wire going to ground has a good ground.

You can test the gauge (without the engine running) by grounding the the input of the transducer which should then display full scale pressure.

This is where I would start.

[Davidg]

Ok, I’ll try. Thanks!

[Davidg]

Updates of my fuel pressure problem. I’ve checked the ground, it seems to be ok. I’ve flown 4 hours this week end, trying to note when and how the fluctuations appears. So no trouble at all during climbing. It seems that the fluctuations increases withe height and flight time. It also seems to be in relation with the plane movement . On turbulent air, when the plane shake, the fuel pressure too. Engine always running smooth despite the fuel pressure fluctuations… and to finish, fluctuations are very present during descent…

[MikeOH]

What is the pressure reading when at full power? What is the pressure when you ground the transducer as skykrawler suggested?

[Davidg]

At full power it’s 27/28 steady. I did not tried to ground the transducer. 

[MikeOH]

39 minutes ago, Davidg said:

At full power it’s 27/28 steady. I did not tried to ground the transducer. 

My suspicion is that there may be a fault with the transducer unless it is at the upper limit of the TRANSDUCER's range.  By shorting the transducer to ground you can establish what full scale on the GAUGE is.  If they are the same number (i.e. 27/28) I would be even more suspicious of the transducer.

[Ragsf15e]

@Davidg
I was going to say, “they all do this” until i see you have a mechanical gage.  @PT20J and I have fought this on and off for a while, but we’ve sort of concluded that it’s something to do with the electrical transducer and eis (he has a g3x and i have a jpi930).  Mine has done this on and off for years.  Occasionally I try to fight it with a snubber (which caused a self inflicted leak), new lines, tightened connections, etc.  I’ve had it steady for about a year at most but it has always returned and I still haven’t nailed down what causes it.

However, you would be the first one Ive heard of with a mechanical gage that’s showing the same thing… 

[Davidg]

I’ve eventually found where is the problem. Not the transducer or the gauge, but the display! If I push hard just near the needle, it goes back to the right position… it’s a good news for me! I now know how to check if the fuel pressure is still ok! 

[Woodpile]

I'm hoping to contribute a bit to the topic. I just had a GI-275 EIS installed in my 1970 20F. Love the gauge; rebuilt fuel sensors and have much better info in cockpit. Two issues arose. The MP was bouncing all over the place. A different thread suggested a particular snubber; $14 later and nice stable MP. The second was with instantaneous fuel pressure dropping to 0 - continuous pressure was normal at 16-18 on the mech pump, and 18 with electric boost. Whenever the boost was off, the instant needle would periodically drop to 0 and trigger the alarm. This usually happens at max power and only with boost off. There are no abnormal indications in flow. I suspect this is only something that is visible due to the sensitivity of the transducer - never saw this on the analog. I just got off the phone with Garmin and they said the 275 display could not be adjusted (sensitivity reduced etc) - it only displays what the sending unit says. It is my thought that even a fraction of a second buffer would eliminate the indication. One mech suggested to replace fuel pump - other said don't waste time/$. After reading this thread, and the success with MP, I think a bit of extra fuel line or some sort of snubber is worth the effort.

[alexthe5th]

Joining the fuel pressure fluctuation gremlins club  - curious if anyone can help provide some ideas on how to diagnose this one. Since our GI 275 EIS (and Garmin 75 PSIG 011-04202-20 fuel pressure sensor) installation in our M20J (w/ the IO-390-A3A6 STC), we've been dealing with this sporadic issue of erratic fuel pressure indications - a few observations:

• The fluctuating fuel pressure indications don't occur every flight, rather they seem to occur on random flights or partially through a flight. We haven't figured out specifically what conditions are necessary for the fluctuations to start happening.
• The engine runs completely smooth regardless of the fluctuations. Fuel flow is rock solid.
• On flights when the fluctuations are happening, reducing the RPM from 2600 to 2500 makes the amplitude of the fluctuations significantly worse. Raising the RPM back to 2600 dampens the fluctuations again.

Below are a couple of data plots from two flights I recently took on the same day. The first flight had very minor fluctuations present until the end of the flight when I reduced power, and at that point the FP started fluctuating:

Then the second flight, the fluctuations continued to be present, and became severe when I reduced the RPM from 2600 to 2500:

It seems to occur randomly on flights for no particular rhyme or reason. Curious if anyone has any ideas here on next steps, or has come across anything similar. My A&P added a snubber to the fuel pressure line which seems to have had no effect.

[PT20J]

@alexthe5th 

Did you notice this on the original instrumentation before installing the GI 275 EIS?

Did you change any plumbing except the transducer?

What transducer was installed with the GI 275?

Can you duplicate this on the ground after a flight when the engine compartment is hot?

Skip