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Fuel Flow Quit Working

JackinMichigan

Well Known Member
On last flight (20 hrs total) noticed the fuel flow reading quit. I have the "red cube" flow sensor, whatever it's official name. I assume everybody has the same.

Asking for your opinion. Is it most likely an electrical problem or could it be in the mechanical function of the sensor (or how reliable are they?).

My first test will be to disconnect a fuel line in the engine compartment and with the electric pump flow (rapidly) about 5 gallons thru the sensor to see if I can unstick it. Maybe use air to blow reverse air thru it.. worrying what harm that could do.

Any opinions guys??
 
Before you dive into disconnecting fuel lines, I?d take a look at the wiring from the cube all the way to your avionics, particularly any splices along the way, to see if a connection has come loose.
 
Maybe use air to blow reverse air thru it.. worrying what harm that could do.

Be careful... these things have tiny little bearings, and compressed air will spin them much, much faster than they're designed to turn in normal service. Probably better to blow manually, and gently. I'm talking less than birthday-candle strength.
 
Yeah, what Matt said! Whatever you do don’t blow air into it. You be almost guaranteed to damage the cube if you do.
 
I agree

First, thanks for the warning about blowing air thru it. I won't.

I guess Todd's opinion is that it is most likely an electrical problem. That is my first inclination too. However, lots of work to get to the wiring connectors down in the tunnel. The fuel flow test is a lot easier to try first.
 
I have a Floscan, not Red Cube, but they make a simple tester for the FS. Perhaps they have a tester or have a way to check electrically?
 
First, thanks for the warning about blowing air thru it. I won't.

I guess Todd's opinion is that it is most likely an electrical problem. That is my first inclination too. However, lots of work to get to the wiring connectors down in the tunnel. The fuel flow test is a lot easier to try first.

It?s not that I think it?s more likely, just that in my mind it?s the easiest first troubleshooting step. But I guess that depends on your specific installation? mine?s slung under the engine between the mechanical fuel pump and the throttle body and is wrapped in fire sleeve so it?s not easily accessible. There?s no right or wrong, just be methodical and only test one variable at a time.
 
Got it Working

Did the fuel flow test and it's working again.

Don't know if I unstuck the sensor or the electrical gremlins blessed me just to keep me confused.

for your info, with an open fuel line the electric pump puts out 54 gph..
 
Fuel flow changes with pump on.

My fuel flow increases by 2 gph when I turn on the fuel pump. This is a carbureted O-540. Any ideas?

Mark
 
My fuel flow increases by 2 gph when I turn on the fuel pump. This is a carbureted O-540. Any ideas?

Mark


Pulsation in the flow causing the Hall effect sensor to register double when the impeller reverses briefly?
 
Heat protection is needed on the red cube.

If it is in the engine compartment and not wrapped with some heat protection they will eventually fail. Tend to work fine when cool, but will fail in flight.

Vic
 
Not brief, stays high until the pump is turned off.

No, I mean the impeller reverses briefly during flow pulses, causing the sensor to count two impulses rather than one when the impeller blade passes it, once in the forward direction, and then briefly in the reverse direction. The sensor can't tell the flow direction, the flow is simply calculated proportionally to the number of blade passes.
 
If it is in the engine compartment and not wrapped with some heat protection they will eventually fail. Tend to work fine when cool, but will fail in flight.

Vic

I have always wondered why one would put the sensor in the engine compartment as opposed to the tunnel or similar place. Is there a benefit to doing this that I'm missing?
 
I have always wondered why one would put the sensor in the engine compartment as opposed to the tunnel or similar place. Is there a benefit to doing this that I'm missing?

A number of folks early on reported variations in reported fuel flow at the stock location depending if the boost pump was on or off. As a result, it became common to mount it FWF and down stream of the mech fuel pump.
 
My experience - -

I have the Flow-Scan. I called the factory, and a great guy said to : " Take it out and put Carb cleaner in it and shake it for a minute, then wash it out with gas or another fluid " It worked. I had it happen again within the last 2 months. Did the same thing again, and it works again. I think a tiny piece of something gets into the very sensitive bearings and slows it down, or stops it.
 
A number of folks early on reported variations in reported fuel flow at the stock location depending if the boost pump was on or off. As a result, it became common to mount it FWF and down stream of the mech fuel pump.

Sounds like it would be exposed to a lot of heat, might just live with the high reading for the brief time I run the boost pump.
 
Sounds like it would be exposed to a lot of heat, might just live with the high reading for the brief time I run the boost pump.

That’s what I do. Per plans location; it shows 1 gal/hr higher with the boost pump on during takeoff. This means the fuel totalizer function is slightly conservative, e.g., if it shows 10 gal remaining, there’s actually slightly more.
 
Better accuracy

Sorry, late on the response. But, as it's been pointed out, it does give better accuracy when mounted after the engine-driven pump, which of course places it in the hot engine compartment.
However, wrapping the sensor with some heat protective barrier, seems to work quite well.

Vic
 
I literally just mounted my red cube yesterday. The instructions say to put it after the last fuel pump, which except for efii, thats the mech pump. I talked to a few people and decided to put it after the gascolator and boost pump which in my case are both inside the cabin, so the cube is inside the cabin as well. I blew past some of the info but pretty sure it said its jeweled bearings, which like swiss watches that means industrial rubies and pin-point axes (plural of axis)....
 
Sounds like it would be exposed to a lot of heat, might just live with the high reading for the brief time I run the boost pump.

So to piggyback on what Vic said, mine has worked without issue for 231 hrs?time will tell. Mine is covered in a large firesleeve and I suspect that between that and the fuel flowing through it that the temps are tolerable. The accuracy is phenomenal?about a tenth of a gallon based upon fuel flow tests and real world comparison of data at fill ups.
 
Vic or Todd (or anyone who'd like to jump in),

Can you be more specific about the heat shield on the flow sensor? A picture sure would help.

Thanks.

Jeff
 
Sorry, no pics handy, but mine is just a large length of silicone firesleeve about 10-12? in length and has a big enough diameter to slip over the cube. Got it from Aircraft Spruce
 
Heat and transducer

After my first FWF red cube failed, I relocated it to a cooler spot and wrapped it with self adheasive aluminized material I purchased at the auto parts store in the exhaust wrap area. Just cut and applied it overlapping pieces until it was covered completely.
I like the reflective qualities over the fire sleeving material plus it is adhesive backed and thinner.
 
Another example

The instructions also state to mount it somewhere other than the engine. The engine vibration causes quicker fatigue on the internal parts. I got about 850 hours on mine until it quit working. Mine has had the firesleeve on it from the start.
 
I've been using Red Cube's with the MGL iEFIS system. I have 3 installed, 2 inside the cockpit and 1 behind the baffling (firewall side). I have never had a problem with the 2 inside but constantly have a problem with the one under the cowl. I've replaced the Red Cube 3 times and replaced the RDAC twice. Runs intermittent during fights. Sometimes worse than other times. I've done everything but add a heat shield around it since it's not on the hot side of the baffle, between the servo and spider... BUT... I think I'm going to give the heat shield a try, see if that makes a difference. I monitor the temps at the firewall, they run around 140F, doesn't seem that hot for a Red Cube...
 
I monitor the temps at the firewall, they run around 140F, doesn't seem that hot for a Red Cube...

Have you looked at the temperature, say, 10 minutes after shutdown? With only convective airflow, and no fuel flow, I wonder if the temperature peaks some time after shutdown?
 
Have you looked at the temperature, say, 10 minutes after shutdown? With only convective airflow, and no fuel flow, I wonder if the temperature peaks some time after shutdown?

Yes, when the heat propagates the whole area it will get over 160F on the firewall side. I haven't monitored the cylinder side but I'll guess it's closer to 250F + after shutdown. BTW, this is in my Lancair, not my Staggerwing.
 
It quit working again

I started this conversation when my fuel flow sensor quit. Fixed it (about reply #6) by flowing a lot of fuel thru it. But now it has quit working again. The trouble shooting begins anew. Will post whatever findings.
 
Grumman?s too

I tossed this out on the Grumman Gang and they are having the same debate about why Fuel flows increase with pump on. One friend has his transducer mounted in the cabin near the Fuel selector. That is far UPSTREAM of even the electric boost pump and sees 10% rise in fuel flow with the pump on...I am still puzzled.

Mark
 
Red Cube erratic operation

Hello,

My Red Cube seems also to go wrong for 2 or 3 flights. Sometimes it works during 10 minutes and after nothing during 15 minutes and so on.
I have 180 hours hobbs (2 years). Before that it worked fine and very accurate. My red cube is installed juste after the injection system behind the alternator (as the same like Jason Beaver).
Is there is somebody who have the same installation with no problem after several hundred hours ?
260$ for a Red Cube...I do not want to change it every 2 years...
https://hebergeur-images.com/fr/vi/11df640e1920a8615822d4945f30f71d[/
 
Hello,

My Red Cube seems also to go wrong for 2 or 3 flights. Sometimes it works during 10 minutes and after nothing during 15 minutes and so on.
I have 180 hours hobbs (2 years). Before that it worked fine and very accurate. My red cube is installed juste after the injection system behind the alternator (as the same like Jason Beaver).
Is there is somebody who have the same installation with no problem after several hundred hours ?
260$ for a Red Cube...I do not want to change it every 2 years...
https://hebergeur-images.com/fr/vi/11df640e1920a8615822d4945f30f71d[/

We?ve had ours mounted in that spot, hanging from the sump bolts, for 650hours on the RV-3 with no issues.
 
Mine lasted 4 years and 600 hours in that location mounted to the front of the oil sump with no insulation
 
Update on the failure

To update everyone. I posted originally that the fuel flow sensor quit (at 20 hrs). Next, I flowed a lot of fuel thru it into a gas can. Must have unstuck the rotor inside because it started working again. After about 1/2 hour of flight time, it quit again.

Current situation.. troubleshooting determined the problem was the red cube, not the wiring. After getting another red cube.. standing on my head for hours, the red cube was replaced and now works again.

FYI. The cube is mounted in the tunnel after the booster pump and filter just as Van's directed me to do. The tunnel is vented and does't get over 150 degrees. Don't know why these fail... heat, vibration, etc...
 
It quit AGAIN

The brand new red cube was installed, leak checked, tested by running fuel into a gas can. Today on engine start, it quit working, with essentially zero hours.

Very, very frustrated. With 20 hrs on this plane have replaced fuel tanks, fuel level sensors, alternator, auto pilot servos, etc.. all brand new parts.

Ready to pour gasoline on it and put a match to it.
 
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