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very sneaky fuel level gremlin

Bsquared

Well Known Member
Hopefully this will help someone else with the same issue find this gremlin faster.... it took me four weeks and multiple flights.

For the first 60 hours on my new RV-14A the correlation between my resistive fuel level sensors, the Red Cube totalizer and the actual fill totals from the pump was almost perfect. However, shortly after returning from the paint shop in July I noticed that both the L & R fuel level readings were a bit more noisy but not enough to worry about. Then about a month ago on a 1 hour cross country (over mountains of course) my right fuel tank level dropped by 10 gallons in 10 minutes and then slowly returned to close to normal. Ever since that flight the left tank readings were still noisy.....going up and down by 1 gallon or so but the right tank level was totally wonky.

It seemed to be acting like a ground loop so I started with the usual suspects. Checked the airframe grounds (good), checked the EMS ground pins and re-seated them in the dSub connector (no improvement), looked at the calibration points in the hardware cal tables and they had not changed. Next I went to the wing root and checked the grounds and still no smoking gun.

Next I replaced the star washers on the fuel level positive terminals and re-torqued them and all of the sudden things got better.....for one flight..... and then the gremlin returned. I removed the wing root fairings for probably the 4th time and was staring at the fuel level sensor in complete despair when something caught my eye. The phenolic isolator bushing on the level sensor terminal, which should be a mustard/brown color, instead looked grey-ish.

Final root cause.....when the paint shop wet sanded all of that nice shiny pure aluminum Alclad surface the run-off with all of that aluminum slurry dripped off the wing and some of it landed on the isolator bushings and dried, causing a low level intermittent short. a bit of electrical contact cleaner on a rag returned the bushing to the correct color and my levels returned to their near perfect original condition.
 
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Final root cause.....when the paint shop wet sanded all of that nice shiny pure aluminum Alclad surface the run-off with all of that aluminum slurry dripped off the wing and some of it landed on the isolator bushings and dried, causing a low level intermittent short. a bit of electrical contact cleaner on a rag returned the bushing to the correct color and my levels returned to their near perfect original condition.
That is a tricky one! Thanks for sharing it!
 
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