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Fuel flow varying/wondering during cruise

FireMedic_2009

Well Known Member
I have G3X with a full engine monitor on my RV3. My fuel flow varies/wonders by 0.6gph at cruise over a 5 sec interval. I know the fuel line has to be straight 6” before and after the fuel cube however it a little difficult with a narrow firewall so one of the lines may be 4”. And I believe the fuel cube should be at or below the height of the carb. So I’m wondering if this is normal for carbureted engines since the float starts and stops fuel flow constantly thus causing the fuel flow to wonder around

Thanks
Daren
 
Fluctuating fuel flow indications

Fuel flow readings on my Dynon fluctuate around 2-4%, at the same throttle setting (Rotax), it hasn't indicated any problems or been any cause for concern.
 
It’s a flo scan. When I had my RV6A the fuel flow was rock solid on the IO-320.

So is it normal for a carbureted, not fuel injection, to fluctuate???
 
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So I’m wondering if this is normal for carbureted engines since the float starts and stops fuel flow constantly thus causing the fuel flow to wonder around

Thanks
Daren

The float doesn't work that way. It does not really start and stop flow and will definitely NOT cause fuel flow to wander, especially at higher flow rates. Having the sensor upstream of the pump can cause issues like this, though not universally.

If the sensor is downstream of the pump, I would speculate that the issue is in the floscan unit. An erratic fuel pump could cause these symptoms. Have you tried turning on the boost pump and observe if the problems remains or disappears?

Height of the floscan shouldn't matter.

Larry
 
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The short answer is that this is not uncommon, in my experience - I have a FloScan on the carbed RV-8, located per Van’s plans (from 20+ years ago) down by the left foot, (downstream of the boost pump, upstream of the firewall), and see fluctuations of probably +/- 0.2 (or 0.3) in cruise. That’s with my GRT EFIS, so I don’t think the EFIS has much to do with it. The good news is that it wanders around the mean flow, so the effect on the totalizer quantity is effectively zero. That’s 2080 hours of experience.

The answer is to “why” is also short - “I don’t know”……

(Similar setup on our -6, but with a G3X Touch system, and we see the same thing….)

Paul
 
have a FloScan on the carbed RV-8, located per Van’s plans (from 20+ years ago) down by the left foot, (downstream of the boost pump, upstream of the firewall), and see fluctuations of probably +/- 0.2 (or 0.3) in cruise.
Paul

From my research, the mechanical fuel pumps do not produce pulsing on the pressure side, BUT DO produce pulsing on the suction side. Input check valve is open on the pump upstroke and closed on the down stroke, creating the starting and stopping of flow with the slightest amount of backflow as the check valve closes. I believe this why the flow sensors are recommended to be installed downstream of any pumps. Haven't looked at floscan, but EI definitely makes this clear for the red cube in their documentation. The constant starting and stopping of flow on the suction side makes the sensor go wonky. It is counting the vanes moving by and the starting / stopping makes this very difficult to smooth out, as the vane can reverse a bit when flow stops and can be counted twice if the vane stopped right at the sensor and reversed a bit. The sensor is not directional, it just counts vanes moving past and therefore, reversals are not accounted for. The red cube is optical, but don't know if the floscan is optical or something else. Optical is VERY binary and the slightest fraction of movement in reverse can result in double counting a vane IF the van stopped very close to the sensor.

I suspect in your case, the fluctuations stop when the boost pump is on (iow, sensor now on the pressure side).

Both of my planes have the cube downstream and flow is solid as a rock in cruise.

Larry
 
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From my research, the mechanical fuel pumps do not produce pulsing on the pressure side, BUT DO produce pulsing on the suction side. Input check valve is open on the pump upstroke and closed on the down stroke, creating the starting and stopping of flow with the slightest amount of backflow as the check valve closes. I believe this why the flow sensors are recommended to be installed downstream of any pumps. Haven't looked at floscan, but EI definitely makes this clear for the red cube in their documentation. The constant starting and stopping of flow on the suction side makes the sensor go wonky. It is counting the vanes moving by and the starting / stopping makes this very difficult to smooth out, as the vane can reverse a bit when flow stops and can be counted twice if the vane stopped right at the sensor and reversed a bit. The sensor is not directional, it just counts vanes moving past and therefore, reversals are not accounted for. The red cube is optical, but don't know if the floscan is optical or something else. Optical is VERY binary and the slightest fraction of movement in reverse can result in double counting a vane IF the van stopped very close to the sensor.

I suspect in your case, the fluctuations stop when the boost pump is on (iow, sensor now on the pressure side).

Both of my planes have the cube downstream and flow is solid as a rock in cruise.

Larry

Good theory, and I agree with most of it, but alas - the fluctuations do not stop with the boost pump on or off…but the flow does become greater by about 1.5. Fortunately, we don’t fly with the boost pump on for very long, so again - it makes little difference to the totalizer fuel quantity.
 
How the fuel flow sender is mounted makes a difference. The rules I stick to:
- Do not mount on the engine. This goes against how Van’s tells you to mount it on the RV-14 - which I had first hand experience with huge fuel flow measurement issues. After moving the sender off the engine the problem was resolved.
- Mount it down stream of the mechanical fuel pump. On the first RV-10 I mounted it in the tunnel like Van’s says to do (between the boost and mechanical fuel pumps). It was accurate unless the boost pump was on - then it shot up 1-2 GPH.
- Mount it so the hose run from the sender to either the carb or fuel manager is up hill. Need not be much, but up hill.
- No 90 degree fittings - other than the nice smooth tube kind.

Of late I mount the sender on the engine mount using Adel clamps.

Carl
 
When I initially bought the plane in April of last year the flo scan was mounted right after the booster pump on the floorboard and produced big variations in fuel flow when the boost pump was On. So I’m mounted it on the firewall attaching it to the engine mount with adel clamps and is pretty accurate on total fuel used. It is mounted after the mechanical fuel pump and used braided hose to connect it between the mechanical fuel pump and the carburetor. Coming off the mechanical fuel pump I swept the hose towards the right side of the firewall and attached the flow scan about 4 inches after the bend coming back towards the left and then made a sweeping bend coming off the flo scan to the carburetor . The flo scan’s height might be a slightly above the carburetor to keep some space between the exhaust pipes. The fuel flow meanders whether the boost pump is on or off by 0.5gph or so. Didn’t know if flo scans are finicky about not having 6 inches of straight hose before and after the unit.

I just didn’t know if this is typical on carbureted engines since my fuel injected engine on my 6A, which used a red cube, was pretty rock solid.

Thanks again!
 
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I was having similar fluctuations and noticed it even with the engine off and mixture idle cutoff.

Turns out I had my transponder / ADS-B antenna mounted forward near the firewall. As soon as I turned off the transponder it stopped. Ended up moving the antenna back behind the aft bulkhead and that solved it.
 
I was having similar fluctuations and noticed it even with the engine off and mixture idle cutoff.

Turns out I had my transponder / ADS-B antenna mounted forward near the firewall. As soon as I turned off the transponder it stopped. Ended up moving the antenna back behind the aft bulkhead and that solved it.

RFI will drive you to drink.
 
How the fuel flow sender is mounted makes a difference. The rules I stick to:
- Do not mount on the engine. This goes against how Van’s tells you to mount it on the RV-14 - which I had first hand experience with huge fuel flow measurement issues. After moving the sender off the engine the problem was resolved.
- Mount it down stream of the mechanical fuel pump. On the first RV-10 I mounted it in the tunnel like Van’s says to do (between the boost and mechanical fuel pumps). It was accurate unless the boost pump was on - then it shot up 1-2 GPH.
- Mount it so the hose run from the sender to either the carb or fuel manager is up hill. Need not be much, but up hill.
- No 90 degree fittings - other than the nice smooth tube kind.

Of late I mount the sender on the engine mount using Adel clamps.

Carl

As many have reported, there are a lot of varying factor and one's experience differs from another.
My last RV14 had the Red cube mounted per VANS instruction and it was rock solid. My new 14 had it mounted to the case and it was pretty solid but I had to change the k-factor for more accuracy. I have since changed the mount to the case with some rubber in between as a means to reduce any vibration and the result has not been as steady as before but I need more hours to say for sure.
 
My setup is identical to Paul Dye's (mine has a carb) and my indications are identical, right down to the higher flow with the boost pump on. My totalizer is always within .1-.2 gallon at fillup. I installed the Red Cube in 2012 along with a legacy G3X system. Back then the instructions didn't mention installation downstream of the engine driven pump, either. I'm satisfied with what I have.
 
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Of course they are, you are running fuel injection...

It was also downstream of the pumps when it was a carb'ed as well and was also rock solid. The recommendation is that the cubes are downstream from the pumps. No recommendation that it is downstream of the FI servo, though I and others often do this also.

Larry
 
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