Fuel flow reading zero at low power settings. Why?

[alpinelakespilot20]

D180 with floscan transducer

While flying today I noticed my fuel flow reading zero a few times, sometimes for sustained periods.  When I got back and downloaded my data, I noticed that I was getting those zero readings generally only at low power settings and when my boost pump was off; at higher power settings, or when I turned the boost pump on, the fuel flow seemed relatively normal.  At all times during the flight the engine ran normally with no stumbling whatsoever.

I'm not sure this picture is readable, but in the top chart the green is manifold pressure and the blue line is fuel flow.  The bottom chart's red line is altitude. 

You  might be able to tell that I did three touch and gos to start, then climbed to 8500' and cruised for a while, then descended to land.  If you look at my first 2 touch and go's, for example, the fuel flow seems relatively normal while the power is in and I'm climbing to pattern atittude.  However, once I pull some of the power at pattern altitude my fuel flow drops immediately to 0.0 until I take off again.  The only exception is my third touch and go when I used the boost pump.  There you can see that some fuel flow is indicated.  Likewise, after cruising for a while, once I pulled power to begin my descent, fuel flow pretty quickly goes to 0.0, only picking up again when I put my boost pump on for landing. 

Any ideas what might cause this? I have a floscan that I'm guessing is either faulty or has something stuck in the vanes.  My floscan is between my fuel selector and my boostpump, so I'm wondering if it takes the extra suction from the boost pump to get the vanes turning at the lower power settings.  Would that make sense?  I don't think it is electrical, b/c on the Dynon if you lose an electrical connection, you usually get a 99.9 reading rather than a 0.0 reading.  Kind of stumped here.

Any help appreciated!

[Just to clarify, I was intentionally not using my boost pump the first couple takeoffs just to compare full power fuel flows with pump off versus pump on.  Indicated f.f. changes with my transducer located where it is.  It is one of the disadvantages of this location.  It makes it a bit trickier to calibrate or use the fuel totalizer if you do a lot of pattern work where the pump is on most of the time.]

 

[Dynon]

The fuel flow sensor works a bit differently than some others. It pulses, so if there was a connection problem, the system wouldn't see pulses, and that would be the same as zero. So it could be that. Or, if there was something causing friction in the impeller, low flows might cause it to not move at all, perhaps. You might expect that to cause lower than expected flows when it's moving, though. We're not experts in the internals and all possible failure or performance degradation issues of the sensor, though.

 

[alpinelakespilot20]

Thanks for your reply. If checking electrical connections doesn't do the trick, will my D180 work with either another floscan or a red cube as a replacement? I've heard that the red cubes are a bit less sensitive to plumbing configurations than the floscan so I thought I might give one a try.
Thanks again.

 

[dynonsupport]

Yes, the D180 can use either the Floscan or the EI "red cube".

 

[alpinelakespilot20]

Just an update. Confirmed that it was a transducer issue. I pulled the transducer, tapped on it lightly, sprayed some carb cleaner through it, lightly blowed through it, and reinstalled it. Just put it through 4.1 hobbs hours of flying yesterday and everything works again as it should.