I did some trouble shooting on my fuel system this afternoon. With the engine off but the electric fuel pump (Carter P4600HP) turned on, I read 3.5psi on a small gauge mounted in the fuel log on my Holley 850 double-pumper. This is less that the pump is spec'ed to put out (4.5 to 8.0psi) so something is wrong.
I checked the voltage at the pump while it was @ 3.5psi and my digital voltmeter read 12.62V; the battery voltage was 12.68V so the wiring is losing only 60 millivolts (#10AWG) so that isn't the problem.
Next, to check for pump cavitation, I removed the 100u filter between the pump inlet and the tank (actually a can of gas slightly higher than the pump and slightly below the filter) but there was no change in fuel pressure. There is a 10u filter between the pump outlet and the carb fuel log but that really should not cause a problem since there was plenty of gas flowing when I first encountered the massive leak. This is a mystery so far.
I did order a Marshall 0-15psi liquid-filled gauge this afternoon so I'll have something to cross-check my existing gauge with.
While looking on the internet, I was surprised by how many people complained that their liquid-filled pressure gauge was no good-- it changed its reading when it got hot. They ALL do that-- the gauge case has to be sealed or the liquid would leak out. As the liquid heats up it generates pressure inside the case and, since the gauge reads the difference between the input pressure and the internal case pressure, the needle reading changes. Good liquid-filled gauges have a small vent so that the case pressure can be relieved to atmospheric to re-calibrate the reading. Plain gauges are vented to the atmosphere already so they don't change with temperature.
Regards, Neil Tucson, AZ