Help Troubleshooting Fluctuating Fuel Pressure

[Mac80]

On 7/1/2023 at 3:49 PM, Davidg said:

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! 

My temperature gage does this until it gets mid range.

[Ragsf15e]

18 hours ago, alexthe5th said:

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.

I started this, and thought I had mine fixed (unknown why though).  It was only about 3 months before fluctuations started again.  I’m currently ignoring it until one of us figures out a better fix.

[alexthe5th]

18 hours ago, PT20J said:

@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
 

I don’t recall it ever happening on the original instrumentation, although I hadn’t owned the plane very long before the EIS upgrade. I looked at some old videos I have of the panel in flight and hadn’t seen it occurring.

No plumbing changes that I’m aware of, although my A&P installed a snubber after we first saw this occurring. It seemed to stabilize after that, but maybe a month or two later the problem came right back. I’m wondering if the transducer or snubber is loosening slightly and allowing air in.

The installed transducer is the Garmin 75 PSIG brass transducer, P/N 011-04202-20. 

The problem persists on the ground after landing (but the amplitude of the fluctuations are lower at low RPMs). I’m not sure what happens if I shut down and restart the hot engine again, I can give that a try sometime.

[PT20J]

5 minutes ago, alexthe5th said:

I don’t recall it ever happening on the original instrumentation, although I hadn’t owned the plane very long before the EIS upgrade. I looked at some old videos I have of the panel in flight and hadn’t seen it occurring.

No plumbing changes that I’m aware of, although my A&P installed a snubber after we first saw this occurring. It seemed to stabilize after that, but maybe a month or two later the problem came right back. I’m wondering if the transducer or snubber is loosening slightly and allowing air in.

The installed transducer is the Garmin 75 PSIG brass transducer, P/N 011-04202-20. 

The problem persists on the ground after landing (but the amplitude of the fluctuations are lower at low RPMs). I’m not sure what happens if I shut down and restart the hot engine again, I can give that a try sometime.

Thanks. This is the same as my installation in my 1994 M20J. I never noticed the fluctuations in the OEM installation and I was was paying a lot of attention because the original Kulite transducer failed and I had to replace it with an Omega. 

My installation is the same as yours except that I have a G3X instead of the GI 275. What seems to be happening is that heat in the engine compartment boils the fuel in the lines causing vapor which seems to cause the fluctuations. I tried filling the line to the transducer completely with fuel and then checking it again after a flight and it was dry. I have checked carefully for leaks and there are none (I've had enough to recognize the blue stains left from a minor fuel leak). I did a test on the ground where I could reproduce the problem after which I teed in an analog gauge and repeated the test. With the extra gauge and it's plumbing, the fluctuations did not occur in either the analog or digital (G3X) gauge.

I also note that the fuel pressure rises rapidly off scale after a hot shutdown as the fuel in the lines boils into vapor. I have an AvStar fuel servo and I believe that it has a tighter ICO valve than the Precision RSA which is designed with some leakage so this may be more prevalent with the AvStar.

I added a snubber just before the transducer and it seemed to reduce but not eliminate the fluctuations.

My working hypothesis is that this fluctuation exists in M20Js due to vapor in the fuel lines and does not affect the operation of the servo and occurs at a frequency outside the bandwidth of the original instrumentation. It just becomes evident with the modern digital instrumentation.

Skip

 

[Ragsf15e]

19 minutes ago, PT20J said:

Thanks. This is the same as my installation in my 1994 M20J. I never noticed the fluctuations in the OEM installation and I was was paying a lot of attention because the original Kulite transducer failed and I had to replace it with an Omega. 

My installation is the same as yours except that I have a G3X instead of the GI 275. What seems to be happening is that heat in the engine compartment boils the fuel in the lines causing vapor which seems to cause the fluctuations. I tried filling the line to the transducer completely with fuel and then checking it again after a flight and it was dry. I have checked carefully for leaks and there are none (I've had enough to recognize the blue stains left from a minor fuel leak). I did a test on the ground where I could reproduce the problem after which I teed in an analog gauge and repeated the test. With the extra gauge and it's plumbing, the fluctuations did not occur in either the analog or digital (G3X) gauge.

I also note that the fuel pressure rises rapidly off scale after a hot shutdown as the fuel in the lines boils into vapor. I have an AvStar fuel servo and I believe that it has a tighter ICO valve than the Precision RSA which is designed with some leakage so this may be more prevalent with the AvStar.

I added a snubber just before the transducer and it seemed to reduce but not eliminate the fluctuations.

My working hypothesis is that this fluctuation exists in M20Js due to vapor in the fuel lines and does not affect the operation of the servo and occurs at a frequency outside the bandwidth of the original instrumentation. It just becomes evident with the modern digital instrumentation.

Skip

 

I think as good a guess as any, although mine starts fluctuating within minutes of starting even on really cold days.  I’d be surprised if it was hot enough to vaporize.

[PT20J]

1 hour ago, Ragsf15e said:

I think as good a guess as any, although mine starts fluctuating within minutes of starting even on really cold days.  I’d be surprised if it was hot enough to vaporize.

It's probably already vapor from the residual heat after the last flight. Everytime I open up the line to the transducer it is dry.

[Ragsf15e]

22 minutes ago, PT20J said:

It's probably already vapor from the residual heat after the last flight. Everytime I open up the line to the transducer it is dry.

I guess I was thinking first flight of the day, cold, plane sat for two weeks.  It starts its fluctuating pretty quickly.
 

Do You just mean that dry bubble in the line that’s always there?  

[PT20J]

11 minutes ago, Ragsf15e said:

Do You just mean that dry bubble in the line that’s always there?  

I’ve come to believe that’s the case. 

[alexthe5th]

Interestingly though, looking at my data logs - whatever my mechanic did when he installed the snubber seemed to have fixed the problem for about a month. There were about 15 flights over that period, and if this issue of vapor in the lines was a fundamental design issue with the M20J, it probably would have manifested itself at some point during that month - but it didn't.

I wonder if it's just a simple issue of everything being fully tightened up, and then the transducer (or the snubber) loosening itself and allowing air in.

[Ragsf15e]

On 7/25/2023 at 4:23 PM, alexthe5th said:

Interestingly though, looking at my data logs - whatever my mechanic did when he installed the snubber seemed to have fixed the problem for about a month. There were about 15 flights over that period, and if this issue of vapor in the lines was a fundamental design issue with the M20J, it probably would have manifested itself at some point during that month - but it didn't.

I wonder if it's just a simple issue of everything being fully tightened up, and then the transducer (or the snubber) loosening itself and allowing air in.

I’ve had the same experience… “fixed” for a while (months at a time) and then not.  Sometimes after maintenance, sometimes randomly.  It’s definitely weird.

[haymak3r]

I haven't wanted to post in to this thread, since it is a bit old.. But, my gi-275 is showing the same thing. My pressure fluctuates from its norm of about 26-27psi up to 30-maybe 35? It spikes so quickly that I can't really see the max. No change in engine sound/FF. 

Here is a flight that I had with the issue. It seems to be doing it every flight at this point.

https://apps.savvyaviation.com/flights/shared/flight/7126879/b3505625-7cbe-4ac0-95b8-0ecae8eb3724

If a snubber fixes this, then I will go that route. It sounds like it only temporarily resolves the issue though. 

[PT20J]

27 minutes ago, haymak3r said:

I haven't wanted to post in to this thread, since it is a bit old.. But, my gi-275 is showing the same thing. My pressure fluctuates from its norm of about 26-27psi up to 30-maybe 35? It spikes so quickly that I can't really see the max. No change in engine sound/FF. 

Here is a flight that I had with the issue. It seems to be doing it every flight at this point.

https://apps.savvyaviation.com/flights/shared/flight/7126879/b3505625-7cbe-4ac0-95b8-0ecae8eb3724

If a snubber fixes this, then I will go that route. It sounds like it only temporarily resolves the issue though. 

If it didn't do this with the factory gauge, and it just started doing it after the  installation of the Garmin gauge, and the plumbing hasn’t changed except for the transducer, and the engine runs normally, I’d just ignore it. It has probably always done that except that the variations were outside the bandwidth of the old mechanical gauge. The variation frequency is also outside the servo’s response, so as long as the average pressure is in the green, the servo won’t care.

Skip

[haymak3r]

Yep. It's a bit annoying though, as it shoots up over redline every so often, and that's causing an alert with the system. it didn't happen right away either. Maybe a few months after the install was finished.

[Ragsf15e]

55 minutes ago, haymak3r said:

Yep. It's a bit annoying though, as it shoots up over redline every so often, and that's causing an alert with the system. it didn't happen right away either. Maybe a few months after the install was finished.

Mine is very similar. Causes an alarm at 30psi, normally fluctuates between 20-30.  Annoying, but (unfortunately) I learned to ignore it. A snubber might be a good idea just so you don’t have to start ignoring engine gages.

[haymak3r]

Ya, well I have a checkride scheduled for the 12th... Hopefully the DPE understands what he is seeing. I don't think FP is a required IFR instrument, but still.

[PT20J]

I put an Omega Engineering PS-8E snubber ahead of the transducer. It reduced the peaks but did not completely eliminate the variations. 

[Woodpile]

I was very happy with the PS-8G to fix my wild MP swings

[Ragsf15e]

16 minutes ago, Woodpile said:

I was very happy with the PS-8G to fix my wild MP swings

Yes, I have Omega snubbers on my oil and manifold pressure, and they work great.  I tried one on the fuel pressure and I caused a small leak (and it didn’t seem to help much), so I took it off.  At least on my setup, putting a snubber in the line adds 3 connections because the snubbers are npt fittings and require an adapter on each end to mate with the flared fittings on the fuel line.

[PT20J]

A year or two ago, the Kavlico pressure sensors became scarce and Garmin approved an alternate GPT sensor. I believe the GPT is more expensive and I don't know who Garmin's vendor is. A possibility is to try the GPT sensor to see if it is less sensitive. However, I did ask Trek Lawler about it and he said that both sensors have the same specs.

[haymak3r]

16 hours ago, Woodpile said:

I was very happy with the PS-8G to fix my wild MP swings

I too had to get the 8G for my MP. Rock solid now. Hopefully the 8E solves this in my case. 

[Ragsf15e]

4 minutes ago, haymak3r said:

I too had to get the 8G for my MP. Rock solid now. Hopefully the 8E solves this in my case. 

Definitely post if you’re able to solve it.  I thought Omega made snubbers matched to viscosity (8g being for gas/air) and the next closest one was for oil.  100ll was between the two.  Which one are you using?  The one I tried was for “light oil” and it didn’t work, although I also caused a leak, so that could have been part of it.

[Woodpile]

The description on their website was light oil or water, IIR. That would probably put 100LL right in the middle 

[haymak3r]

2 hours ago, Ragsf15e said:

Definitely post if you’re able to solve it.  I thought Omega made snubbers matched to viscosity (8g being for gas/air) and the next closest one was for oil.  100ll was between the two.  Which one are you using?  The one I tried was for “light oil” and it didn’t work, although I also caused a leak, so that could have been part of it.

I have the 8g on my MAP sensor. I ordered the 8E yesterday to try with my FP sensor. Will definitely let the thread know what the result is once I get it installed.

[generalaviationguru]

On 7/24/2023 at 7:29 PM, alexthe5th said:

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.

 FYI our F (J engine - A1B6) developed this same behavior.  Upon investigation, a worn out, brittle, near-failure 25 year old fuel hose was the cause of the fluctuations.

Normal flow, engine performance, but pressure repeatedly bled into the red and had to be brought back up with boost pump. 

[Ragsf15e]

14 minutes ago, generalaviationguru said:

 FYI our F (J engine - A1B6) developed this same behavior.  Upon investigation, a worn out, brittle, near-failure 25 year old fuel hose was the cause of the fluctuations.

Normal flow, engine performance, but pressure repeatedly bled into the red and had to be brought back up with boost pump. 

Well that’s definitely different than what we’re discussing on this thread, at least my symptoms.  Mine consistently bounces between about 20-30psi, never settles low and the electric pump doesn’t change the behavior at all.  The electric pump works fine but it doesn’t settle the fluctuations or raise the psi of them.