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G3XT - Wandering Fuel Flow Readings

Mountain Time

Active Member
Over the last few flights I've noticed the fuel flow on my G3X Touch having frequent periods where it wanders all over the place during cruise - for example, where I should be at a steady 8.8 GPH or so, it will trend down to the low 7 or high 6 range, then trend back up into the 9s, etc. This happens quickly - you can see it quickly flicker through the range, up and down, in just 10-15 seconds (ballpark) or less. There's no apparent rhyme or reason. And the other engine data are normal and the engine runs as though there is nothing abnormal happening.

Other times it will settle in and stay in the expected spot, maybe varying .1 or .2 GPH depending on whether the engine loads up a bit as the autopilot works to maintain altitude in a slight downdraft , as an example. That's the normal behavior I'm used to seeing.

The sender is a FloScan 210 from when I built the plane with the RMI uMonitor and has 700 hours on it, about 150 of that with the G3XT. And, the interesting thing is that the total fuel used still comes out on the money (or dang close) when its time to fill up, as it always has. So that doesn't seem to be affected.

Any ideas from the collective mind? I haven't dug into anything yet but wanted to see if there's something specific I should be looking for.

Thanks in advance -

John
 
I'm not an expert on the FloScan but I'd suggest checking the easy things first, like wiring. An intermittent fault like that is often caused by a bad ground or loose connection.

The G3X's fuel totalizing function is sampling the FloScan's output pretty quickly, probably 1/second. An erroneous flow indication over a few seconds isn't going to make a noticeable difference in the total fuel used. 0.5 gph error divided by 3600 seconds/hour is a pretty small number! :)

HTH

Dave
 
Thanks Dave, that mirrors my initial thinking - I don't think there is much to the FloScan physically, a paddlewheel and pulse counter/generator inside the unit, and the wires to same. So likely either a connection issue somewhere, or something going flaky inside the FloScan itself, which would mean toss it out and replace.

John
 
My 0360 has always done that on my G3XT. I attribute it to the carb float opening and closing. Very rarely it is steady.
 
If the reading is always erring on the low side, I would suspect the paddle bearings in the floscan are failing. As the bearing creates drag, the moving fuel is read as lower than actual, due to having to overcome drag. On the Red cube, these are jewelled bearings with very low friction and expect floscan is the same.

Only use red cubes, but 700 hours seems to be a pretty good run for them. I have yet to see one last longer than 750 hours.

Larry
 
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Carburetor Cleaner

I have a Floscan 201B connected to the G3X Touch system in my plane.

Every 3 to 4 yrs it starts to read erratically and generally lower than it should and sometimes the vane even stops turning and it reads zero.

I remove the fuel lines on both sides of the flow sensor, and as gently as possible, spray carburetor cleaner (no compressed air!) in from both sides. Re-attach the fuel lines, and everything is back to normal.

I have had to do this a couple of times in over 10 years of using it.

Steve
 
My 0360 has always done that on my G3XT. I attribute it to the carb float opening and closing. Very rarely it is steady.

Yeah same with my O-360/Floscan/G3X combo, however I typically saw +/-0.3 gph variation. In this case where the variation has become much larger, but the fuel totalizer is still accurate, I wonder if the carb float hinge or needle has developed excessive friction or become 'sticky' and isn't operating as smoothly as it used to. Can't say I've seen that happen but it's a mechanical device with moving parts so it seems plausible. If that turned out to be the cause, your flowmeter may be fine and you need a carb overhaul instead.
 
Yeah same with my O-360/Floscan/G3X combo, however I typically saw +/-0.3 gph variation. In this case where the variation has become much larger, but the fuel totalizer is still accurate, I wonder if the carb float hinge or needle has developed excessive friction or become 'sticky' and isn't operating as smoothly as it used to. Can't say I've seen that happen but it's a mechanical device with moving parts so it seems plausible. If that turned out to be the cause, your flowmeter may be fine and you need a carb overhaul instead.

If that were the case, the fuel height in the bowl would be changing and changes in fuel height create changes in fuel delivery volume at a constant air flow. The OP would be seeing noticeable swings in engine performance if that were happening.

Larry
 
In response to Tom's comment...... It certainly does, but this time with a Floscan. There seems to be a common denominator evolving with these errant fuel flow readings.
 
I have an O-360 in my -7A and first used the Floscan 201 unit for 835 hrs. and never had a steady fuel flow indication on both EIS-4000 and then the G3X. It varied +/- .5 to 1.0 gph up and down. The Floscan hadn't failed, but I thought I'd install a "better" unit so I replaced it with my first red cube which failed after 550 hrs. The indications from the red cube on the G3X varied about the same amount as the Floscan unit.

In each case, the totalizer was pretty accurate as I could predict how much fuel I'd take on a topoff within 1/2 gallon. My perception is the Floscan unit varied just slightly less. My current red cube has 250 hrs.on it and if/when it fails, I'll probably just reinstall the Floscan unit.
 
What we know of was with the FT60, not a FloScan. So I cant 'say' with a FloScan. But if you look at the similarity of those that have posted here, you'll see what we have seen, and gotten some confirmation of.

Tom
 
Looks like its time to read up on FT60 issues, and maybe spend some quality time with the multimeter this weekend (the FloScan is mounted on the firewall just ahead of the passenger rudder pedals - oh joy, some more upside down time under the panel... :rolleyes:)

Thanks for the suggestions and ideas, everyone.
 
If that were the case, the fuel height in the bowl would be changing and changes in fuel height create changes in fuel delivery volume at a constant air flow. The OP would be seeing noticeable swings in engine performance if that were happening.

I agree in principle that changing levels in the carb bowl would have the effect of slight changes in fuel volume vs. air flow (effectively changing the mixture at a given RPM & MAP), but I doubt the pilot would perceive noticeable swings in engine performance (we're talking about the fuel level varying fractions of an inch here, not like it's several feet :)). If you look at the design of an MA-4-5 and think about the physics of how it works throughout various aircraft attitudes, G loads, and power changes, the fuel level in the bowl can and does vary a fair amount without having significant effect on engine performance. The ability of the venturi to draw the proper ratio of fuel out of the nozzle at a given mass airflow just doesn't seem to be that sensitive to fuel level variations in the bowl.

If you're running a CS prop, the governor keeps the RPM steady and the MAP is steady at given throttle setting, making it unlikely that the pilot would see/hear/feel any slight changes in power from the engine due to small mixture variations associated with varying fuel levels in the carb bowl. And with a fixed pitch prop, the rpm tends to wander anyway (for unrelated reasons).

As a side note, the fuel flow indication through the Floscan or EI red cube is typically more steady on a fuel injected engine than on a carbureted engine, and this is because the fuel level in the carb bowl isn't perfectly steady even when the needle & float are working like new...
 
I agree in principle that changing levels in the carb bowl would have the effect of slight changes in fuel volume vs. air flow (effectively changing the mixture at a given RPM & MAP), but I doubt the pilot would perceive noticeable swings in engine performance (we're talking about the fuel level varying fractions of an inch here, not like it's several feet :)). If you look at the design of an MA-4-5 and think about the physics of how it works throughout various aircraft attitudes, G loads, and power changes, the fuel level in the bowl can and does vary a fair amount without having significant effect on engine performance. The ability of the venturi to draw the proper ratio of fuel out of the nozzle at a given mass airflow just doesn't seem to be that sensitive to fuel level variations in the bowl.

If you're running a CS prop, the governor keeps the RPM steady and the MAP is steady at given throttle setting, making it unlikely that the pilot would see/hear/feel any slight changes in power from the engine due to small mixture variations associated with varying fuel levels in the carb bowl. And with a fixed pitch prop, the rpm tends to wander anyway (for unrelated reasons).

As a side note, the fuel flow indication through the Floscan or EI red cube is typically more steady on a fuel injected engine than on a carbureted engine, and this is because the fuel level in the carb bowl isn't perfectly steady even when the needle & float are working like new...


Debate is healthy, but I disagree.

Specifications for float height are typically given in 32's or 64's of an inch. Small changes in fuel height have a profound impact on air fuel ratios in a carb. A carb only works properly IF it's fuel height is kept constant. Just look at the impact of a leaking float or fuel boiling in the carb bowl. If the fuel height were changing in flight, I have all confidence that this would be very noticeable. EGT's would be constantly swinging around, as EGT is very sensitive to AFR.

If you want to discuss the physics, Fuel metering is based upon the differential from pressure in the venturi (which is directly related to velocity, which is indirectly related to flow volume through a fixed size venturi) and pressure acting on the fuel, which is based upon a combination of the fuel height and ambient pressure on the top of the fuel (both pressure and gravity are in play here-This is why carb's don't work at 0 or negative G's), along with the diameter of the main jet.

Larry
 
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Debate is healthy, but I disagree.

Specifications for float height are typically given in 32's or 64's of an inch. Small changes in fuel height have a profound impact on air fuel ratios in a carb. A carb only works properly IF it's fuel height is kept constant. Just look at the impact of a leaking float or fuel boiling in the carb bowl. If the fuel height were changing in flight, I have all confidence that this would be very noticeable. EGT's would be constantly swinging around, as EGT is very sensitive to AFR.

If you want to discuss the physics, Fuel metering is based upon the differential from pressure in the venturi (which is directly related to velocity, which is indirectly related to flow volume through a fixed size venturi) and pressure acting on the fuel, which is based upon a combination of the fuel height and ambient pressure on the top of the fuel (both pressure and gravity are in play here-This is why carb's don't work at 0 or negative G's), along with the diameter of the main jet.

The engineers at Marvel Schebler were pretty smart guys, and the shape of the bowl and geometry of the nozzle are designed to minimize the effect of changes in attitude, but the fuel height with relation to the nozzle cannot always be a nice constant level (like the one shown in the diagram below) when you're in a sustained nose down steep descent or a steep climb...even including nearly vertical flight, like where you're going almost straight up decelerating for a wing over. While these changes in fuel level undoubtedly do affect the AFR, and you could probably see it in the data, the O-360 on my -8 ran fine in all of those flight conditions without any obvious changes in how it was operating.

Pulling G should have a similar effect as a lower fuel level in the carb bowl. If small changes in fuel height have such a profound effect on AFR, I would expect significant changes in performance when pulling G as well. In my -8 I never felt like the engine was going excessively lean when pulling G (and I often flew it aggressively lean to begin with, so it wouldn't have to get much leaner to notice).

Anyway, back to the OP's dilemma...if he has an SD card in the GDU he can log all the engine data. It might be worth graphing out the data to see if there's any observable variations in EGT that align with the same period as the fuel flow variations. If so, then it's likely that the flowmeter is telling the truth and he needs to go looking for why the fuel flow actually is varying between high 6's and low 9's gph. If EGT and other engine data all look perfectly smooth while those variations are occurring in fuel flow, it's probably time for a new flowmeter.
 

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SD Data

Good suggestion, Mark - I reviewed the engine data in the Garmin Pilot logbook, and you can see the periods where the flow is varying up and down and yet CHT and EGT don't change, and they remain the same when the fuel flow data settles down to its normal, steady value.

That would match my seat of the pants observations in flight, where engine performance seemed completely normal with no variation; with the fuel flow #s dipping into the 6-7 GPH range on the indicator, I'd be experiencing severe lean misfire if those #s were true.

So that helps - pretty clearly a sensor issue. Now for a little yoga to limber up for that dive under the panel......:eek:

JHW
 
The Plot Thickens...

Tom, I searched and reviewed the FT60 threads - very odd symptoms (albeit similar to mine) and it sure looks like you and others have been investigating pretty thoroughly, with no apparent answer and no rhyme or reason as to when issues arise and when they don't.

I guess I can take some comfort in not being the only one with this issue :)

I'd be very surprised if there were an electrical issue with a connector or wire, although its possible and I'll check for it soon. That, and keeping an eye on when it happens and trying to identify any common factors (power setting, etc.) is about all I can think of at the moment.

JHW
 
John, there has been quite a bit of investigating, and quite a bit of expense to do so. In one case of a well documented instance on a VAF members plane, the FT60 cube in this case was moved to 4 different FWF locations, meaning that 4 different plumbing packages were built, 3 complete rewiring done,and new calibrations. Same results for all 4 install locations, including a trip back to EI for 'inspection and testing" which it passed, then a new, totally separate FT60 a dn new wiring was installed. Same scenario.
WE heard basically the same story from a builder at OSHKOSH, that described almost exactly the same thing, except his numbers were different. He also had moved his FT60 around, new wiring, new calibrations, and even with a new unit, he had the same anomaly.
From what I'm seeing on this thread, this flow anomaly isnt as strange as we thought it was, and actually seems to be somewhat common, to a greater or lesser extent. Read through the posts and you'll begin to see a pattern. Pull your Savvy data and really look at all the parameters, EGT, CHT, RPM, MAP, FP, FF, and it will begin to paint a pretty good picture.
I dont know when all of this started, but the first I heard of it was 3 years ago. Prior to that nothing, so whatever is going on was created somewhere around 2018.
Thats about all I can say for sure, and I dont want to get into speculation. But various aircraft, various install locations--some per the kit manufacturer, some different, but those exhibiting the anomaly have a common denominator.

Tom
 
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