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Garmin EIS Fuel Pressure


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With installation of the EIS as part of a G3X, I’ve noted my fuel pressure indications vary wildly, from 5 to 40 psi.  FP is erratic from the time of engine priming throughout the flight. The FP transducer has been bled twice, so air is an unlikely culprit. Engine runs perfectly. My old analog gauge showed FP in the middle of the normal range. 

In contrast, the fuel flow accurately reflects fuel consumption within 1% of actual without wild excursions.

I’d be interested in other’s experiences with Garmin EIS, esp FP.

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All I can add is when my fuel flow transducer starts to go bad it runs between normal and infinite GPH and everywhere in between.  Have you tried swapping a different transducer and see if that clears it up?  Has it ever worked correctly?

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On the original Sigma-Tek gauge in my '94J, the cruise fuel pressure was a steady 23 psi. With the G3X EIS the average is about 23 psi but it wanders randomly about +/- 4 psi. I also bled the line to the transducer with no effect. I T'd in an analog gauge and both gauges were steady, so I think that the extra plumbing might have had a dampening effect. I have ordered a snubber and will try putting that on the transducer to see what effect it has. If that doesn't work, I will probably try replacing the transducer. My engine runs fine and the fuel flow is steady and accurate.

The transducer that Garmin has been using is a Kavlico P4055-5020-3. They have recently been out of stock everywhere. Back in June, Garmin approved a new GPT transducer (I don't know the manufacturer -- possibly Honeywell) which requires a G3X software update to v9.01. However the sensor has the same 1/8" NPT pipe fitting and connector, so it is a direct replacement. According to Trek, there is no technical advantage to the GPT over the Kavlico. The reason for approving it was to have an additional source due to supply issues with the Kavlico.

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There’s a previous thread about some JPI fuel pressure sensors doing this as well.  Mine was not fixed with a snubber, however I created a small leak somewhere when I installed the snubber.  While taking out the snubber I made sure all the fittings were snug and sealed… the oscillation stopped.  I have no idea why.  I probably had a very small air leak somewhere, but who knows?  This was months ago and it’s still good.  It had fluctuated for several years.

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5 hours ago, PeytonM said:

With installation of the EIS as part of a G3X, I’ve noted my fuel pressure indications vary wildly, from 5 to 40 psi.  FP is erratic from the time of engine priming throughout the flight. The FP transducer has been bled twice, so air is an unlikely culprit. Engine runs perfectly. My old analog gauge showed FP in the middle of the normal range. 

In contrast, the fuel flow accurately reflects fuel consumption within 1% of actual without wild excursions.

I’d be interested in other’s experiences with Garmin EIS, esp FP.

Read through this one, theres a lot of good advice in there…

 

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When FP, if being measured correctly… will be constant with constant FF…

The geometry of all the parts that the fuel flows through varies a whole lot… it’s not a simple tube…

But, that geometry doesn’t change… so that same fuel pressure… keeps delivering the same FF….

If the FP is all over the place… and FF stays constant….  Something in the geometry would need to change… or there is something wrong with what the sensor is reading…

 

In the case of the O360…. There is a fuel float in the carb… the float changes the position of the valve that controls fuel flow….into the bowl….

So there is a reason to expect a change in geometry of the fuel flow in some cases…

 

As rags points out above… some FF and FP data are noisy and oscillate…. And a snubber is a mechanical method of smoothly averaging the data…

If the data varies randomly in large amounts… it’s time to add another sensor to prove what the fuelP is really doing…

If the two sensors are both showing the same wild variations….  You get a better feeling about what is really going on and where…

 

The FF sensor is pretty far upstream in some installations… and may show nice smooth data…

The FP is downstream… and a leak may be occurring between the two sensors…

 

When you have a leak in the fuel system…. Bubbles are often getting delivered to the fuel injectors and getting separated out before they enter the intake system…

People have put a clear line when testing fuel injectors to see if bubbles are flowing in…

One source of bubbles found…. A leaky fuel selector valve….

very little fuel would escape while not running… lots of air entering the system when running…


Maybe a loose wire, faulty sensor, or air bubbles in the line….

Start with determining if it is a sensor issue a real physical issue…

PP guesses only not a mechanic…

Best regards,

-a-

 

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  • 3 weeks later...
On 9/4/2022 at 8:45 PM, PT20J said:

On the original Sigma-Tek gauge in my '94J, the cruise fuel pressure was a steady 23 psi. With the G3X EIS the average is about 23 psi but it wanders randomly about +/- 4 psi. I also bled the line to the transducer with no effect. I T'd in an analog gauge and both gauges were steady, so I think that the extra plumbing might have had a dampening effect. I have ordered a snubber and will try putting that on the transducer to see what effect it has. If that doesn't work, I will probably try replacing the transducer. My engine runs fine and the fuel flow is steady and accurate.

The transducer that Garmin has been using is a Kavlico P4055-5020-3. They have recently been out of stock everywhere. Back in June, Garmin approved a new GPT transducer (I don't know the manufacturer -- possibly Honeywell) which requires a G3X software update to v9.01. However the sensor has the same 1/8" NPT pipe fitting and connector, so it is a direct replacement. According to Trek, there is no technical advantage to the GPT over the Kavlico. The reason for approving it was to have an additional source due to supply issues with the Kavlico.

Skip

Today I installed a Omega PS-8E snubber in the fuel pressure line where it enters the Garmin Kavlico transducer and it cured the wandering fuel pressure issue.

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5 minutes ago, PT20J said:

Today I installed a Omega PS-8E snubber in the fuel pressure line where it enters the Garmin Kavlico transducer and it cured the wandering fuel pressure issue.

Skip

Invite @Ragsf15e to see that… :)

And @PeytonM because he started this thread…

IIrc… rags has the memory for snubber issues…

Go MS!

Best regards,

-a-

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2 hours ago, PT20J said:

Today I installed a Omega PS-8E snubber in the fuel pressure line where it enters the Garmin Kavlico transducer and it cured the wandering fuel pressure issue.

Skip

You know skip, there’s only one thing you’ve got left to put this to bed… take off the snubber and see if it’s still cured.  I bet it’s a 50/50 chance that you fixed it by messing with the connections while installing the snubber.  Either way, enjoy it because it’s a great feeling to see it steady!

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49 minutes ago, Ragsf15e said:

You know skip, there’s only one thing you’ve got left to put this to bed… take off the snubber and see if it’s still cured.  I bet it’s a 50/50 chance that you fixed it by messing with the connections while installing the snubber.  Either way, enjoy it because it’s a great feeling to see it steady!

I don't think so. I've had this apart several times trying to see if it made a difference if I filled the line with fuel or not (it didn't make a difference). Then I had it all apart to tee in the analog gauge for testing. 

BTW, I had a lot of trouble after initial installation getting the transducer fittings not to leak fuel. The Kavlico has a brass 1/8" male NPT that screws into an aluminum adapter. The installer sealed the pipe threads with EZ-Turn, and it seeped. I took it apart and used a liquid Permatex Teflon thread sealant, and it seeped. I tried Permatex Aviation No. 3, and it seeped. I tried Teflon tape, and it seeped. A mechanic friend suggested Armite LP-250. Success! I've had this lash up apart several times and when putting it together with Armite, it never leaks. It's magic stuff. It's basically powdered lead in mineral oil. It won't gall, seize or leak. Good from -350F to +2987F. McMaster-Carr carries it in small tubes, but you have to call them as it doesn't show up in the online catalog for some reason.

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Fantastic insight Skip!

How easy is it to keep an eye on the new assembly?

In the event it decides to leak over time….

Of course with this magical material… its time frame may get measured in Centuries or eons….   :)

Thanks for sharing the details!

Best regards,

-a-

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On 9/23/2022 at 6:34 AM, PT20J said:

In my installation it’s easily visible through the oil door.

Yeah totally depends on the installation. I have one on oil pressure and it’s right under the oil door.  The fuel pressure sensor is down low on the firewall behind several scat tubes.  Impossible to see or tinker with unless you first provide a few choice curse words, some blood and at least 45 minutes.  I’m kind of glad my fuel pressure fluctuations were fixed without a snubber.

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