This is a bright bunch, so I'll ask for help troubleshooting a problem I'm experiencing with my carbureted, O-320 powered RV-6. Last Saturday as I was taxiing out for a flight, the GRT EIS master warning light came on. The digital readout indicated low fuel pressure (in the range of 0 PSI). I was taxiing with the electric fuel pump off and usually see ~5 PSI in those circumstances. I turned the electric pump on and the pressure immediately rose to 5 PSI. Then, I turned off the boost pump and the pressure dropped to 0. Once again, I turned the boost pump on and the pressure came back up to 5 PSI. I turned the pump off again and the indicated pressure dropped back to 0 PSI. I taxied back to the hangar and the reading never changed. During this whole process, the engine never gave any indication of fuel starvation. Going through the potential causes, I identified these, and performed some tests trying to isolate the issue: -Gauge or sender problem. To try and isolate this problem, I plumbed a mechanical gauge in the system. Today, it and the electronic gauge confirmed pressure both with the boost pump on or the engine (and engine driven pump) operating under power from the starter (with plugs removed). In all cases, the pressure was 5-6 PSI. Both gauges agreed at 0 PSI with the engine stopped and the boost pump off. -Failing mechanical pump. The mechanical and electric gauges both indicated appropriate pressure when the engine was spinning today. -Vacuum leak in the fuel system allowing air to enter the system. I got out the inspection light and checked all fuel connections for evidence of weeping fuel after pressurizing the system for 5 minutes using the electric pump. No fuel leaks were found. -Obstruction in the fuel lines. (Trash, collapsing hose, etc.). I have not done any testing to identify these issues. The only remaining tests I can think of are to disassemble the fuel system looking for blockages, cracked flares, or collapsing hoses. Also, to crank the engine and conduct a run-up, looking for the low fuel pressure indication on the mechanical pump. If that indication occurs, continue with the run-up (boost pump OFF) to see if the engine suffers from fuel starvation. Given that fuel is flight critical, I'm very interested in clearly identifying the root cause and correcting it. I do not want to shotgun this problem with replacement parts that may or may not fix the problem (or the indication of a problem). What are the collective troubleshooting suggestions of the group?