Revisiting the old issue
Dredging up an old thread...
I am finishing up an RV-9A project. The original builder installed the transponder antenna just inboard of the right fuel tank vent, and the red cube on a firewall-mounted bracket just above the antenna. Separation is less than 12 inches. With everything turned on, engine not running, the Skyview indicates 0.3 to 0.4 gph fuel flow rate. Pulling the transponder fuse drops it to zero. Clearly the same issue reported previously. Moving the transponder antenna would be a big project, moving the red cube only slightly less so. I have plenty of 3 conductor shielded wire so I plan to try that first. Question for the group: at which end of the cable should the shield connect to ground? Red cube or EMS?
Finally, the FF of 0.2-0.3 GPH error when parked, engine OFF, Comm's ON (i.e., Xpndr ON) ... has risen to the top of my list of TODO's.
The panel is ALL Dynon SkyView and AFP fuel injection FM-150 (filter, pump, servo) with Dynon "Red Cube" mounted between the engine mount (RV-6A). Elec power control is Vertical Power (VP-X Sport), integrated on the SkyView EFIS/ENG display). The Xpndr can be cycled ON/OFF with a softkey, while monitoring the FF. When OFF, 0.0 gph, ON, 0.3 gph, Andair fuel selector OFF, and there is no purge or return line. OFF is truly OFF. Clearly the Xpndr is a root cause of the FF reading in the RedCube, when zero fuel is flowing. The xpndr antenna (stub) is mounted adjacent to the right side fuel vent fitting, just behind the FW line.
After reading this and other threads the nexus appeared to be evident between the Xpndr and erroneous FF. The easiest fix is (will be) to relocate the Xpndr antenna, so I tested this first. Unscrewed the antenna, loosened the RG-400 coax and stretched it back to the wing root near the right inboard end of the flap. Mounted the antenna on a scrap of 0.025 alum and clamped it to the flap edge for grounding. Result 0.0 gph with everything ON. Then to be sure of the radius of the solution moved the antenna forward to the wing spar area (top side) and then to the LE of the wing root. Somewhere in the spare to LE the FF began to show erroneous flow again.
Conclusion - The xpndr antenna is causing erroneous Red Cube fuel flow, and was located too close to the Red Cube transducer. At least 36-40" (radius) away the antenna ceases to interfere, and beyond 48" it should be much less likely to be a factor. Now I have to extend the coax and route that thru the aircraft, drill a hole in the fuselage and hope it is perfect (ergo, doesn't cause RFI with my Comm antenna under the co-pilot seat pan).
The other option 'might' be to use shielded wire all the way to the Red Cube but that is a more difficult post-construction retrofit. Word of warning - do it that way during construction (add this to the gotcha-list.)
Last thought - the 0.1-0.3 gph fuel flow is not indicating actual fuel movement, and it would be easy to disregard this error but the EFIS doesn't 'know' it's not consuming fuel. Long 'buttonology/hangar-flying' sessions or other Master ON activities record this and result in consumption forecast errors. [Yes, the xpndr COULD be powered OFF to avoid this.] BUT - the real question is whether the 0.3 seen in the hangar is distorting the inflight consumption, too, and adding 0.3 gph to my entire cross-country trip? Why leave it that way and wonder?