I wanted to test out my fuel system components on my 98 Mazda 626 GF 2L, so I hooked up my fuel pressure gauge between the fuel filter and the fuel rail. I've used this gauge on this car before, and I don't have any particular reason to think that the gauge has suddenly gone bad. The car was running reasonably well before this ( just rough idle under load of varying intensity ). I am manually energizing the fuel pump with the engine off.
After I hooked up the gauge, I had to let the car sit for about ten days due to weather, so the fuel lines were probably empty for a while. I didn't get the hose all clamped down perfectly at first, and had some fuel spurting out at first. After I got it all clamped down I let the pump run about thirty seconds. I could hear what sounded to me like the fuel running rapidly through the fuel rail, and I verified the pump was drawing about 4.3 amps, which is the same it draws at idle. However, I couldn't see any pressure on the gauge, where as the last time I measured at idle a few months ago I got 35psi (close to the 36 max) and 44psi when I disconnected the vacuum hose from the regulator. Both in spec.
So here are my ideas. The fuel pump regulator is vacuum operated, and doesn't operate without vacuum applied. I disconnected the vacuum hose and could not observe any fuel leaking from there. So it would seem to me that pressure should be building up quite rapidly. Specifically to about 64 - 92 psi according to the WSM. According to Giles' book, fuel pumps usually take about 1 amp for every 10psi of pressure, so if the gauge was bad and pressure was actually building up I would expect to see at least 6 amps being drawn by the pump.
So it seems to me like maybe the FP regulator is stuck open for some reason, and the fuel is just flowing straight back to the tank through the return line, thus preventing any pressure buildup at all ( gauge reads zero ). However, I can't think of any reason why this would just happen to occur all of a sudden and am wondering if maybe I'm overlooking some other possibility? If this sounds like the most likely possibility, is there some way to check this and/or repair it short of swapping in a known good regulator? Maybe the gauge went bad and the sound I'm hearing at the rail is just a normal amount of flow that occurs even when the regulator is not actuated by vacuum?
Ideas are appreciated. Thanks!