How Low is Low?
Boy am I glas to see this thread! I've been pulling my hair out. A month ago on a flight with my wife in our RV9A (O-320, carburated), the Low Fuel Pressure alarm went off on my EIS 4000. It was showing 1.7 PSI, when it usually shows between 3.5 and 5. I immediately switched on the electric pump and talked to our local tower, indicating I thought I'd lost my mechanical pump. I headed back to the airport, staying over roads as I went. I tried turning off the electric booster pump several times and each time the pressure would drop immediately and keep dropping until I got too nervous and turned it back on. I switched tanks and tied again. I don't remember the response but eventually I was able to switch off the electric pump and the fuel pressure stayed up in the 4 psi range.
Hanger talk between mechanics and non-mechanics attributed it to everything from a few ice dropplets in the fuel system restricting flow via the mechanical pump only, to not having the fuel selector valve at the proper "demark".
The plane has flow fine in 2 or 3 flights between then and yesterday when it happened again. This time I also immediately switched the electric booster pump on and noted that it didn't make any difference which tank the gas was being fed from. I tried several different times to turn the pump off but each time the pressure dropped to below 2.5 psi before I turned it back on.
Reading this thread has been very enlightening. First of all, I didn't realize I could let the pressure drop ot almost or even zero and not have the engine stop. Next time it happens, I might be a little more adventurous before I turn the booster back on.
One thing that might be different with my experience was that the loss of fuel pressure occured after a very very gradual climb or even what you'd probably call level flight and both times I lost pressure, it must have been 3, 4 or 5 minutes (maybe it wasn't that long but it seemed like an eternity) before the fuel pressure was up in the normal range again using the engine driven pump only. Also, when the plane was back in the hanger, I used the electric pump only to check fuel pressure readings from the transducer. They seemed very credible. I took a plug out at the pressure tranducer mount and the fuel pressue immedately showed Zero. re-installing the plug, I could cycle the pump and achieve pressues between zero and 5 psi depending on the amount of cycling and the pressures seemed to corrollate with the noise the pump was making as it labored at the higher pressures.
I'm somewhat relieved and will probably hold off on ordering a new mechanical fuel pump, or fuel pressure transducer, or gerry-rigging a second fuel pressure gauge into the cockpit...... for now anyhow.
Thanks guys and any further input would be appreciated.