My fuel flow occasionally decides to read higher than it really is, then returns to normal some minutes/hours later.

Typical situation: I am flying in the cruise, lean of peak, and at some moment [for no apparent reason] the fuel flow increases by maybe 15%. I know this isn't reality, because the engine power output doesn't change - which it would do when running LOP and increasing fuel flow; there is no fuel leak visible (no dye stains visible on the lines or in the belly), and finally to clinch it - if I turn on the fuel boost pump (which is downstream from the transducer) then the flow reading goes back to what is was before the anomaly occurred, and remains normal for as long as the boost pump remains on. If I switch the pump off it starts telling lies again. The problem started after maybe 150hrs and has been recurring occasionally for 65hrs now.

Does anyone know what causes this kind of exaggerated reading?

I have developed a few theories, but I won't bias your thinking with them.
IO-540, Bendix injection, Dynon Avionics D100 and D120 computers, the fuel transducer is a common model, the EI FT-60 Red Cube, which is installed after the gascolator and fuel filter, but before the two fuel pumps, in a straight run of fuel line, roughly horisontal but running slightly uphill. I realise I could move the transducer to get a better reading after the fuel servo, in the vertical line. Of course, that is a fairly large job, and remembering I had no problems with my install for 150hrs prior, that is a last resort. I have not removed the transducer to perform an internal inspection, yet, as it seems to be working normally 95% of the time.

Does EGT, CHT change also during the FF spike?

I would suggest some form of contaminant in the FF transducer.

Maybe.

MTV

I had the same thing happen, once so far, also using the Red Cube. But with a GRT EIS 2000. I also did not see any other indications of greatly increased fuel use. It hasn't done it since, about 40 hours, so I'm not too concerned.

8GCBC wrote:Does EGT, CHT change also during the FF spike?

No change whatsoever. Nothing in the engine physically changes or moves a millimeter. It is a phantom increase.

But it is annoying, screws with the range, time remaining, and fuel level computations in the Dynon - which are nice to have, but not essential of course.

Is it exposed to any possible disruption of magnetisim, heat, vibration?

I had a oil temp guage on my Aztec #2 engine that went zero when I transmitted of VHF #1. Really freaked out my co-pilot!

8GCBC wrote:Is it exposed to any possible disruption of magnetisim, heat, vibration?

I had a oil temp guage on my Aztec #2 engine that went zero when I transmitted of VHF #1. Really freaked out my co-pilot!

Yes, that is possible, it is near the exhaust flow, and will also experience a little vibration. Although I have not noticed any trends which interlink increasing heat of vibration to the problem.

I wondered about air bubbles transitioning through the fuel system when a tank un-ports (I do long flights, and often times a tank will run low) and catching at the transducer. Seems to be an air bubble would lower the resistance to the paddle wheel spinning, compared to resistance the fuel provides.

I had also wondered about the electrical side of things. Perhaps interference, corrosion, or a bad contact - there are terminals down at the transducer. Also a mix of oil, brake fluid, fumes, etc, can collect / condense down by the transducer (droplets) which could be corrosive.

Have you called or emailed EI tech support?

I'd remove the transducer, carefully clean it and re install.

Or get in touch with EI.

MTV

I am sympathetic to your issue, however I have no insight. We also have a red cube FF transducer. The first one never made it 30 hours. The failure was it quit indicating all together. We've got about 30 hours on the replacement, so far so good. It was replaced no questions asked. I am starting to wonder if there might be a bad run of them?

mtv wrote:Have you called or emailed EI tech support?

I'd remove the transducer, carefully clean it and re install.

Or get in touch with EI.

MTV

I haven't emailed them yet.

I am thinking it might come to that removing it. The Tx takes some getting to, so I am trying to find easier solutions first

I would guess you have a leak between the engine driven pump and the boost pump. Since its before the engine driven pump the fuel is not under pressure and its sucking air into the system. We had this happen once and found the leak by capping fuel line going into the engine driven pump and then turning on the boost pump and the pinhole in the line was easy to find.