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Fuel pressure indications on Garmin G3X

Fltnut

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I'm new to the forum and new to the RV12, so please bear with me.
The former owner had changed both fuel pumps for a "fluctuating pressure" problem. I don't call it fluctuating, it's more like crashing. It is very intermittent during cruise @ 7500 or 8500ft. Pressure goes to 0-2 then to 7.2, maybe 2 or 3 times, then back to normal. It may do that once or twice on an hour flight. It may not happen at all during the next 5 flights.
In my feeble mind it is an indication problem since there are no other indications of a problem. Has anyone else seen this type of problem on the G3X?

Ron
 
I don't have the legacy RV-12, so I can't address any specifics.. but there has been a few service bulletins regarding fuel pressure, which you may already be familiar with...

See SD-00017, SL-19-09-23 and N-18-07-12

If you're not sensing any change in performance/RPM..and since you say that the pumps have already been changed in an attempt to fix the problem.. then perhaps its just a sensor problem? According to the RV12 G3X electrical diagram, you have a Kavlico 101690-000 sensor.. which only measures up to 15psi...

Does the G3X need to know which sensor you have? I assume the Van's RV12 G3X default settings file will load that in automatically. (my rationale here is that the g3x needs to know that 0-12v maps to in terms of psi)

You can also try verifying that you have good electrical connections between the 3 wires at the fuel pressure sensor and the 3 pins at the GEA24 (apparently pins 1 2 and 4 of the largest D-sub connector).

You can also try just swapping out the sensor.
 
I shouldn’t tempt fate by saying this, but my original VDO fuel pressure sensor has been rock solid for 9 years and 1100 hrs. On the other hand I had to replace the oil pressure sensor. Then about 100 hrs later it went squirrely. Of course I bought another only to find out it was the lead connection. Checking the electrical contacts is definitely a good place to start.
 
Thanks for your responses. I flew this morning and the pressure was all over the place after flying for about 30 mins.
I'm going to order a sending unit and go through the wires when I change it.
 
Make sure the fuel line attached to the sensor is not touching the cooling hose to the radiator. This can cause the symptoms you are seeing.
 
FUEL PRESSURE HOSE* FLUSH (* Left Side of Fuel Return Assembly)
1. Place rags beneath the Fuel Pressure Sender.
2. Loosen VA-216 Fuel Return Assembly hose coupling while supporting elbow with
second wrench.
3. Place container beneath hose and remove hose from fitting.
4. With end of hose inside container, flush hose by turning on fuel boost pump for
two seconds.
Installation
1. Reinstall hose to elbow.

RV12 MM Page 13-9

(Service Sked 18-5 says Fuel Pressure Flush should be done during Annual or 100 hrs)

My top ideas for you to check:

1. Purple gasket removal on Kavlico sensor
2. Electrical connections
3. Actual fuel pressure problems to include flush of hose above, gascolator screen, screen engine fuel pump for Rotax S/B replacement, engine driven pump drain line clog
 
Make sure the fuel line attached to the sensor is not touching the cooling hose to the radiator. This can cause the symptoms you are seeing.
That's interesting because that's exactly where a zip ty is holding the line. That just got cut.
 
FUEL PRESSURE HOSE* FLUSH (* Left Side of Fuel Return Assembly)
1. Place rags beneath the Fuel Pressure Sender.
2. Loosen VA-216 Fuel Return Assembly hose coupling while supporting elbow with
second wrench.
3. Place container beneath hose and remove hose from fitting.
4. With end of hose inside container, flush hose by turning on fuel boost pump for
two seconds.
Installation
1. Reinstall hose to elbow.

RV12 MM Page 13-9

(Service Sked 18-5 says Fuel Pressure Flush should be done during Annual or 100 hrs)

My top ideas for you to check:

1. Purple gasket removal on Kavlico sensor
2. Electrical connections
3. Actual fuel pressure problems to include flush of hose above, gascolator screen, screen engine fuel pump for Rotax S/B replacement, engine driven pump drain line clog
Thanks for the trouble shooting tips.
The previous owner said that he had done the "flush". I've already done sort of a flush. I just cracked the line and let the fuel pump run while I controlled the leak in an attempt to bleed any air out of that line.
 
Just an update. I separated the fuel pressure line from the cooling hose and for the last 3 hrs. of flying time the fuel pres. has been stable. I'll believe it's fixed after about 10 hrs.
 
Update

Any recent updates on low or erratic fuel pressure issues? I've tried a variety of suggestions from multiple threads, poured over hours of flight log data to identify a pattern, but no joy. I'm approaching 150 hours and still disturbingly random. No change in fuel flow or rpm just that pesky alarm?
 
I can't tell you that I haven't seen a fluctuation but it's very rare. Before moving the fuel line the pressure would jump around on every flight, once the coolant warmed up.
Any changes that do happen are very small and of no concern (to me anyway) now. There are no alarms now.
 
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Scott--as we tell everyone that has an issue like you are describing, check the actual fuel pressure with a know good mechanical gauge tee'd into the FP hose. That way you can see if its a 'real' pressure fluxation, or instrumentation.

But from the sound ot things, with no other erratic engine operation, no surges, no rpm or map, cht, egt fluxuations, it points to a sender/wireing/ground/power, interface problem.

Tom
 
I have the same issue with my SLING 2, with 912 ULS. It started a year after I had it and frankly, it scared the **** pout of me the first time i noticed it. Other than the drop in pressure on the G3X screen, normally I see 3.8 and as high as 4.2 or so, it dropped as low as 1.7, 1.9, but usually around 2.0 for a few seconds, the 912 ran strong without issue.

I came to find this is a well know issue and there was an SB from The Airplane Factory (Sling Aircraft) and ROTAX that addresses the issue…but wait, there’s more…my SLING came with the additional IN-LINE filters, meant to regulate pressures better.

I know its not the answer or solution we all want, but I’ve been told again and again…they know about it and absent any rough running or stumbling there is no cause for alarm. Really?

Ive learned to live with it. The scary part for me, is it normally happens when I toggle off the fuel pump once I reach my desired altitude and sometimes on climb out. I’m running 91 no ethanol unleaded 95% of the time and 100LL when I need to flop off on a X-country. Test with both and no difference, so it ain’t the fuel.
 
Last edited:
John---again the big thing is to determine if its a 'real' pressure drop due to the pump, or minor fuel leak somewhere, or if its instrumentation. From what you say, this is a constant thing no matter if the boost pump is on or not.

Personally speaking, the instrumentation issue is one thing, but obviously real pressure drop is a bunch more serious. We always recommend that a known good mechanical gauge be installed along with the electrical sender, so you visually see the 2 different types of systems read the same pressure. Obviously the mechanical gauge is a pain, but its NOT dependent on any electrical system component to read the fuel pressure. This way, you can monitor the pressure and when the electrical version starts to fluxate, you can do a verification really quick. If the mechanical gauge agrees with the electrical version, then you have a realtime issue. IF the mechanical gauge reads fairly normal pressure and the electrical shows the fluxuation, then that validates your theory of instrumentation.

I would caution however, dont just blame the sender, the wiring or connections for the issue. You might have to dig deeper to find the culprit.

Tom
 
Fuel pressure fluctuations

On my RV-12 (912ULS) I was experiencing rapid fuel pressure swings from normal to the high end causing an alarm message. As the pressure indications fell the alarm would cease. A Rotax dealer indicated most likely a sensor as no surging of the engine was noted.

A friend who has built several 12’s said most likely an obstruction in the return line and suggested I remove the fuel line at the sensor on the firewall and the return line to the tank on the firewall and blow back the line catching anything at the sensor. I couldn’t get to the return line going to the tank so I VERY CAREFULLY put some pressure into the tank which effectively blew the return line back to the sensor. I did not see any debris but this did solve my high pressure fluctuations.

I hope this helps someone with the same issue. I have 275 hrs on my 12.

Alan
 
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