Inconsistent GPH Reading on G3X

I have been flying the RV-7A, with IO-360, 2 pMags, and G3X system since 2017, with no issues.

Recently, the gph reading on the G3X Fuel Calculator tab on the Engine page became inconsistently high at times. When I would expect 7.0 gph, I may get that for some of the flight; then the gph would jump to approximately double that. No amount of throttle/mixture adjustment would return the reading to an accurate one. However, later in the flight, it may do it on its own (inconsistent, as I said).

That might be alarming (When it happened the first time, I really envisioned fuel draining out the bottom of the plane!), until I realized that the G3X reading of the flop tubes in the tanks (at the top of the Fuel Calculator tab), which I had to calibrate way back when, were reading accurately all the time. I verified this the last couple of flights by comparing the fuel I added at the end of the flight with the flop tube reading. So, it is only the bottom fuel readings on the Fuel Calculator page that are inconsistent.

I can only think of 2 things that might be the source. One would be the fuel sensor that is installed in the manifold on my firewall. The other would be the red cube. I am leaning (and hoping) that the fuel sensor is the issue. Before I plunk down any money, however, I wanted to pass it be the expertise on this site to see if they have experience with this and can validate my troubleshooting, or tell me other possible sources that I may be missing.

Sure sounds like the red cube to me. There are dozens of posts on VAF of people having issues with the red cube and needing to replace it. Thankfully it's easy to replace, and not that expensive.

My 10 year old Red Cube started giving inconsistent readings on my AFS 5400 EFIS. I read that they can go TU, often around the 10 year mark, which is how old mine is. I read that they can be "rejuvenated" with some sort of flushing maneuver (this thread...post 11). I elected to just buy a new one...my A&P is installing while he's doing my annual CI,

Have you noticed an altitude dependency? As in fuel flow is right until above 5K’ or so?

I chased this problem on an RV-14A (and posted about it). The red cube was installed per Van’s FWF kit instructions. It would read perfect until passing through ~5K’ then it would go to around 14GPH no matter what the throttle or mixture setting. We first replace the cube, no change.

The fix was to follow the red cube install instruction and not mount it on the engine. Problem solved.

I suspect a combination of hose length, engine vibration and such is the issue.

Carl

i had a similar problem with my two-year old RV-9A. Indicated fuel flow would drop to zero or some small number, even though I had normal fuel pressure and the engine was running fine. It would start reading correctly for a few seconds then drop to zero again. I sent it back to EI and they determined it was dead. These are delicate devices. A new one fixed the problem.