Flight Deck Problems Fuel Flow

[Emaroo]

Hi
I have had a flight deck installed in a Jabiru 230 for 8 months. Have been very pleased with the fuel flow meter and I have fine tuned it to be very accurate. But in the last couple of weeks on some flights I have been getting high readings . I usually use around 20lts/hr 2,800 rpm but occasionally for a whole flight it will remain at 24-28 lts/hr then the next flight it will be back to the normal 20lts. I have checked for leaks but can not see anything that would suggest there is a problem. Do you have any idears.

Cheers Rick ( South Australia)

 

[dynonsupport]

One way to check whether the fuel flow sensor is lying or not is to compare the fuel used across a tank with the amount of fuel that you're actually filling up with. Beyond that, it's tough to troubleshoot those guys. High readings don't clearly point to a failed sensor directly - it implies that the impeller is spinning faster than usual. A more expected failure would probably be an impeller that is has gotten stuck or "gummed" up, though we've seen very little incidence of failure of these sensors, period. If you have a filter slightly upstream of the sensor, check it for debris as well, as something changing the flow characteristics in the line could change the perceived flow at the sensor.

 

[jaba-who]

The fuel flow sensor is very sensitive to position. I presume you have checked the position of the sensor since it became unstable?
Originally mine was not completely horizontal and gave high readings (48 - 54 l/hr) especially in the climb but it then dropped to high 28 - 30s in cruise. Though when measured by filling the tank back up I was using 18 l/hr.
I went through a lot of re-positioning to try to make it sit truly horizontal in flight. When I did this it gave pretty accurate flow rates until recently when it has started going high again (but not as bad as before) with no apparent change in position of the sensor.

I have to go through it all again and see if I can nut out what's going on but sounds similar to your problem.

As an aside - your true fuel flow rates sound a little low. Should be 22 to 24 l/hr for the 3300 engine. Mine was only 18 litres and my temps were fine and I was saving fuel to - I thought it was great. But I spoke to Jabiru - because it was low and thought I'd better check it out. Their advice was to change the needle in the main jet in the carby (and the float valve seat) because it was too lean - would get leaner at altitude and give hot spots and risk valves damaging prematurely.

So for what it's worth you might like to think about changing the carby jets.
If you solve the fuel flow readings let me know.

 

[JR]

I too have a 6 cyl Jab which has this fuel flow problem, seems to be high especially after take off, mostly settles down in the cruise although sometimes stays high throughout the flight
The readings are STABLE, ie if you alter rpm it drops and raises quickly and settles on a number, so seems to be reading OK
Its just this central number seems to vary
Instructions on the sensor call for slight incline so air doesnt become trapped
Maybe level is the way to go, but theres not much coice in the 12 inches or so of fuel line there is to play with
JR

 

[jaba-who]

I went through a lot of testing and emailling people including the makers of the sensor.
I originally also read the instructions to mean the sensor should be on an incline. After talking to people I came to the conclusion I had read it wrong. The information I got in the end was the ideal was - straight section (at least six inches) of fuel hose both before and after sensor, the hose upstream of the sensor should have a slight incline upward the sensor should be horizontal with the wires coming out - upward perpendicularly.

Now in a Jab you can not achieve all these at the same time - there is just no room (unless you add a great length of hose and reroute it completely).

I asked them (I can't recall whether it was Dynon or the makers of the sensors) if you can't do them all which ones should take precendence over the others. The answer was that primary thing should be the sensor position - horizontal. Then after that they were not really sure. They said I would have to just experiment.

What I have now is - The sensor horizontal ( zip tied to the engine mount frame and fuel pressure sensor). The hose upstream has an incline but has an S bend - at best the last 3 inches are sort of straight.
The hose downstream is again a short straight bit that is sort of horizontal then bent and slopes downward to the mechanical pump.

So for what it's worth I went the way of sacrificing the long straight sections. It seemed to work for maybe 6 months. Now I seem to have gone inaccurate again.
Maybe its time for another positioning experiment.

 

[dynonsupport]

I'd guess inaccurate after months of being OK means there's an air bubble trapped inside. If you look at the sender inside, it's a spiral raceway. If you install it with the wires pointing up, any air spirals right out. If the wires are any other position, air can get trapped inside, and this just messes everything up. Even if you have the wires pointed up, you may want to try bleeding the sensor.

Straight sections before/after are important if you want the exact K factor that was printed on the tag that came with the sensor. The Dynon EMS lets you adjust K factor so this isn't much of an issue.