Sticking Moeller fuel gauge

I have a 2 year old SLSA and I've got just under 240 hours.

Fueling history: I flew plane home from Vans on 100LL using Decalin. For a few months I continued this until I began bringing 93 E0. I used this until about 3 months ago when the supply of non ethanol began to dry up. Currently been using Shell E10 for past few months.

Currently EMS reads fairly accurately as reflected by fuel loaded against fuel used and the Moeller gauge has worked well. Longest plane has sat without being flown is has been a couple of weeks.
I plan on 5gph but actual use age has been around 4.5 to 4.8 varying with type of flight. When selecting fuel on SkyView I typically see the sender showing a little lower than the fuel computer but based on fuel loaded after flit the numbers Re accurate.
Recently as I added fuel the Moeller gauge didn't move and it appears stuck at the 11 to 11.5 point on the gauge. I rocked the plane gently and the gauge rose to 16 gallon pong and began to move in the air once I'd burned fuel down to 16. Next flight gauge stayed at 16 even though I knew it was below until it suddenly unstuck and seemed to read normally.
Yesterday gauge was stuck at 11 despite me filling upto full. Jiggling plane and gently tapping glass gauge cover did nothing. Later I tapped it and it went to 16.Confident I had full fuel I did a1.45 flight and gauge stuck at 16.
After a gentle landing I noticed gauge was reading 11 galls which was about accurate for time flown.
I unscrewed sight gauge and gently tapped top of Moeller float but saw nothing out of the ordinary.

Okay...does anyone have any input as to how I should proceed? It's going for annual next week but trying to get. Head start. I'll call Vans on Tuesday and I read a previous thread here on Moeller gauges but it didn't really answer my issue.
Not sure exactly how gauge works and whether the glass gauge is the problem which seems an easier fix than replacing whole gauge mechanism. When you remove the gauge itself I expected to see metal touching gauge but instead there's a smooth metal diaphragm so not sure how it works.

I'd appreciate any input . Thanks!

Update today I came by hangar to check in gauge. Yesterday it was stuck at 11.4 and I added 4.5 and it should have moved to 15 but it stayed at 11. Today it's at 15 but when I added fuel it didn't move or wobble when I gently rocked plane.

You can remove the "glass" gauge by removing the 2 retaining screws realizing no seal violation. The gauge is activated by magnetic movement. Highly unlikely it is the gauge, but, an easy thing to try.

Thanks Larry, you're right.

I removed my gauge today and realised its a sealed unit. Yesterday I left the hangar with my gauge indicating 11 even after adding 4.5 gallons so it should have read 15. When I arrived today it had decided to move to 15.
I installed my RV-12 builder hangar neighbor's gauge and it went to 15. Reinstalled mine and it went to 15.
However, I added another 2 gallons to see if it would move and it didn't.

So I'm assuming the issue is with the float mechanism in the tank itself, just not sure what.

I don't put much faith in fuel gages. After all the FARs only require that they read empty when the tank is empty. I use a dip tube made from nylon instrument tube that I Calibrated during my electric gage calibration. Usually the dip tube and D-180 readings are within in few tenths of a gallon.

Top Cat said:

Thanks Larry, you're right.

I removed my gauge today and realised its a sealed unit. Yesterday I left the hangar with my gauge indicating 11 even after adding 4.5 gallons so it should have read 15. When I arrived today it had decided to move to 15.
I installed my RV-12 builder hangar neighbor's gauge and it went to 15. Reinstalled mine and it went to 15.
However, I added another 2 gallons to see if it would move and it didn't.

So I'm assuming the issue is with the float mechanism in the tank itself, just not sure what.

Click to expand...

This gauge looks similar in design to the ones used in propane tanks. And if so, it has a stamped (cheap) gear at the bottom of the shaft that has been known to get stuck..... If you remove the unit, it is very easy to understand how it works. If you want to check the indicator pointer of the gauge, just circle around it with a magnet. The needle should track smoothly as you move the magnet. If it does, then the problem is with the gear.

Thanks Gasman!

The gauge is a sealed unit moved by a magnet. I've swapped it out with another plane and got the same result although the mechanism may be moving at its own pace. I''l give it further observation before opening up the tank.

The float and stem should lift out of the top of the tank. It must be empty. The counter balance will be next to the shaft as you lift the unit, meaning that the float MUST be all the way down for the unit to be removed.

Thanks! Its an SLSA and I'm hoping that it'll 'maybe' sort itself out.