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There are a few different active fuel pressure tests listed in the 97 Mazda 626 GF Work Shop Manual ( European ). The first two are an idle pressure test ( 30 - 36 psi ) and a fuel pressure regulator test ( disconnect vacuum from the FPR 37 - 45 psi ). These first two tests are listed as "Pressure Regulator" tests and it says to replace the FPR if not as specified.

Then there is a third test, the "Fuel Pump Maximum Pressure" test, in which the engine is off and the fuel pump is manually energized giving an expected value of 64 - 92 psi. For this it says to replace the fuel pump if not as specified.

One thing I noticed about the maximum pressure test, is that the WSM only ever calls for it when the idle pressure is below it's expected minimum value. It doesn't seem to be a stand alone test.

Why would the pressure with the engine off be so much higher than with the engine idling and the FPR disconnected from vacuum? I understand that the injectors are working, so that causes a certain amount of pressure decrease, but does that completely account for a theoretical maximum drop from 92 psi down to 37 psi? Or is something else going on?

Is it possible that the first two tests could be good and the third test fail and it still be a problem with the FPR? On the other hand, if the first two tests are OK, but the maximum pressure test fails, does that necessarily indicate a problem with the pump?

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There are two values for the pressure since the maximum pressure is achieved at low or zero flow when the engine is not running and the other when the engine is consuming fuel. It is possible that the pump can deliver maximum pressure at zero flow, but not be capable of supplying fuel at the rate that the engine is using it at the required pressure - 30 to 36psi etc.

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    In the specific case I'm looking at, I can hear quite a bit of fuel flowing through the fuel rail back to the return line when I do the maximum pressure test with the engine off, which confuses me since the FPR on this car is purely vacuum actuated. – Robert S. Barnes Jan 8 '17 at 21:09
  • The fuel is going back through the return line to limit the maximum pressure... Are you sure that the FPR does not have an internal pressure relief valve that limits the maximum pressure? This would by-pass the operation as controlled by the vacuum. – Solar Mike Jan 9 '17 at 8:48
  • That's why I asked this question a while back: mechanics.stackexchange.com/questions/40054/… – Robert S. Barnes Jan 9 '17 at 10:39
  • So, look at the diagram - specifically at the pressure spring : when the fuel pressure exceeds the force supplied by the spring then it will open, the vacuum applied will cause the valve to open earlier as the force (fuel pressure) opening the spring is augmented by the vacuum applied... – Solar Mike Jan 9 '17 at 11:36
  • So as long as the fuel pressure is correct both at idle and with the FPR disconnected, then the FPR is probably fine, but if the the MAX pressure with the engine off is below the minimum required then the FP may not be supplying sufficient volume at idle, even though the pressure at idle is OK? – Robert S. Barnes Feb 28 '17 at 8:21

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