EI FT-90 “gold cube” fuel flow transducer - intermittent sticking problem

I’ve used a red cube for 5-10 years without issues. The EI red and gold cube sensors are ‘everywhere’, don’t cost a lot and don’t seem to have many problems by my observation. Nothing is perfect but I’m curious on what basis they might be judged as not reliable – it’s a genuine question as these have been very widely used for a very long time.
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My red cube has a problem, after approx 70hrs use. The problem first showed around 20hrs, whereby at low fuel flow such as idle/taxi it read nothing, then in flight it under read and eventually read nothing at all.

Today I removed it and found the impeller was basically stuck. It would spin if you blew through it, but it was clearly stiff. Running fuel through it into a container using the fuel pump wouldn’t show a reading, and connecting an oscilloscope to the output showed nothing.

I read through the many threads on VAF about red cube failures. It seems common, often with the same failure mode as mine. One post said that normally if you blow through the sensor it will sound like a kazoo, unless the impeller is sticky. Sure enough mine made no noise. I also read that squirting wd40 through it can free up the impeller. Sure enough, this freed it up and afterwards it had the correct noise, and it also worked fine and read correctly and accurately.

This is odd, because the thing is constantly sitting in avgas , but maybe some kind of deposit had made the impeller bearing sticky. I note from the video that unlike the Floscan 201b which uses jewelled bearings, the Red Cube doesn’t, it just has the impeller shaft sitting in holes in the alloy case.

I don’t believe it’s due to temperature expansion problems because the alloy case will expand more than the steel impeller shaft.

I’ve read about premature failures too but similarly with other transducers. There are a huge number of red cubes in service so who knows.

From what one can find on EI red cubes specifically, they seem to die early or not die at all, which is different than one would expect if the problem were contamination. However if yours was stuck with contamination at 70 hrs TIS, based on my experience if you’re by chance running auto fuel I’d be looking closely in that direction.

If WD-40 didn’t work and it was just contamination, I’d just stick a new one in myself, they are cheap.

I’ve only run UL91 and 100LL – no car fuel, but at the moment it’s looking like a warranty replacement.

Good luck, hope the next one works.

Mine has not given any problems, and with roughly $500 total cost including the STC’d transducer and gauge I can’t complain.

Superior on what basis? I’ve honestly heard about an equal number of stories of EI and Floscan types failing, and if mine were ever to fail I’d prefer the replacement to be $250 delivered, directly interchangeable and available over night.

To each his own, I have no real comparative data either. My personal experience is knowing of a lot of EI Red Cubes that haven’t needed to be replaced. I’d rather go with something I know and I’d have no reason to spend more to replace a design which doesn’t in my experience fail very often. YMMV.

My datapoints are:

-Flowed 7500USG through a floscan xdcer on our prior aircraft over 8yrs. One failure needing replacement in the period.
-Flowed 8400USG through an EI red box over 6yrs on our current 210 aircraft without a problem.

FWIW I like the engineering of the the slimmer, lighter Floscan better.

Good data

I like cheap, effective and STC’d.