Garmin EIS Fuel Pressure Wild Fluctuations

[PeytonM]

1989 J-Model equipped with G3X with EIS. All sensors are stable, except for Fuel Pressure; check out a recent flight below, noting time scale. Equipment was installed March 2022, 350+ hours ago, and this problem has been there since Day 1. FP sensor was replaced, then a snubber installed, and fuel line to the sensor was bled. I tried turning on Fuel Pump in flight. No change when each of these changes were tried.  I have literally hundreds of tracings where fluctuations occur at all phases of flight. Recently, FP showed zero on taxi to the runway, then suddenly came alive! I’ve also seen an RED “X” thru the FP gauge occasionally! Air in line is suspect, but some of the tracing shows Zero(!) fuel pressure for minutes! Cannot believe that is air, but Garmin seems to think so! 

I was fighting altitude oscillations until recently when G came out with alternate gain settings. Now that’s fixed, so one last gremlin!

Suggestions, please.  Also, I’d like to see what others are seeing with their Garmin EIS.

Thank you in advance for your help. Peyton

[Ragsf15e]

22 minutes ago, PeytonM said:

1989 J-Model equipped with G3X with EIS. All sensors are stable, except for Fuel Pressure; check out a recent flight below, noting time scale. Equipment was installed March 2022, 350+ hours ago, and this problem has been there since Day 1. FP sensor was replaced, then a snubber installed, and fuel line to the sensor was bled. I tried turning on Fuel Pump in flight. No change when each of these changes were tried.  I have literally hundreds of tracings where fluctuations occur at all phases of flight. Recently, FP showed zero on taxi to the runway, then suddenly came alive! I’ve also seen an RED “X” thru the FP gauge occasionally! Air in line is suspect, but some of the tracing shows Zero(!) fuel pressure for minutes! Cannot believe that is air, but Garmin seems to think so! 

I was fighting altitude oscillations until recently when G came out with alternate gain settings. Now that’s fixed, so one last gremlin!

Suggestions, please.  Also, I’d like to see what others are seeing with their Garmin EIS.

Thank you in advance for your help. Peyton

If you solve it, let us know. My jpi930 fp has been fluctuating for years.

[Rick Junkin]

Gents, take a look at this document. I changed out the connectors as it recommends and have not had a problem since. I initially had fluctuation/dropout with my OEM fuel pressure gauge and this was the fix. I since have installed the G3X EIS with the same transducer and wiring and it is solid.

BENZ AIRBORNE-KULITE TRANSDUCER.pdf

A quick troubleshooting fix, not recommended as a long term fix, is to open up the black cylindrical connector between the fuel pressure transducer and the ship wiring and clean it with contact cleaner. I used Deoxit D5 https://www.amazon.com/gp/product/B00006LVEU/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1 and it gave me a short term solution and verified the connector was the problem. I changed out the connectors at the next annual.

Cheers,
Rick

[Ragsf15e]

2 hours ago, Rick Junkin said:

Gents, take a look at this document. I changed out the connectors as it recommends and have not had a problem since. I initially had fluctuation/dropout with my OEM fuel pressure gauge and this was the fix. I since have installed the G3X EIS with the same transducer and wiring and it is solid.

BENZ AIRBORNE-KULITE TRANSDUCER.pdf 982.35 kB · 0 downloads

A quick troubleshooting fix, not recommended as a long term fix, is to open up the black cylindrical connector between the fuel pressure transducer and the ship wiring and clean it with contact cleaner. I used Deoxit D5 https://www.amazon.com/gp/product/B00006LVEU/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1 and it gave me a short term solution and verified the connector was the problem. I changed out the connectors at the next annual.

Cheers,
Rick

unfortunately I’ve got a jpi transducer and connector.  I will try cleaning it, but I don’t think it’s the same or at fault for my application.

[PT20J]

This seems really common in IO-360 Lycomings especially with G3X EIS. Lot's of comments on vansairforce about it. I never noticed it with the factory gauge but I definitely get a lot of fluctuations with the G3X. I'm guessing that the factory gauge was much more heavily damped. I had Savvy do a deep dive into their database and they found the same thing -- common in IO-360s with G3X. Less common with JPI monitors.

I too tried the snubber to no real effect. I'm pretty convinced that it's due to fuel vapor forming in the engine driven pump. If I aim a heat gun at the pump with the engine cold and not running I can watch the fuel pressure increase rapidly. Also, the pressure will rise off scale from residual heat after shutdown. All the other fuel lines within the engine compartment are fire sleeved which is a pretty good insulator. If I fill the line to the fuel pressure transducer and then fly around, it will be dry when I check it again indicating that the fuel has vaporized.

The fluctuations don't seem to affect engine operation. As long as the servo is getting fuel, the servo is pretty insensitive to fuel pressure because the regulator maintains a constant pressure across the main jet which has unregulated pressure on one side and metered pressure on the other. If the unregulated pressure goes up (or down) the metered pressure changes by the same amount so that the pressure across the jet (which sets the fuel flow) remains constant.

[jetdriven]

Ours is pretty steady.  It has a PS8E snubber on it. 

[PeytonM]

Referencing the FP graph above, you will notice I have readings of Zero fuel pressure for several minutes. 

[PT20J]

On 8/23/2024 at 7:47 AM, PeytonM said:

Referencing the FP graph above, you will notice I have readings of Zero fuel pressure for several minutes. 

There are small variations and very large sudden variations. Clearly, the engine won’t run on zero fuel pressure, so there is likely a bad connection. I would fix that. The smaller variations might remain if they are caused by vapor as is common. 

[Pinecone]

Hmm, check the grounds.  It is quite often the grounding.