I'm completely rewriting this question since I really wasn't clear before.

When discussing a fuel system with a return line and a vacuum operated fuel pressure regulator such as this:

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Where it says "Pressure" in the diagram, I believe that's where vacuum is applied. If I understand correctly, the spring keeps the valve shut, and vacuum applied to the diaphragm opens the valve by varying amounts depending on manifold vacuum.

Should fuel flow back through the return line, even when no vacuum is applied to the regulator, i.e. does excess fuel pressure also force open the valve?

  • I'm not sure what you are trying to achieve here but the practice of clamping a return line and manually firing up a fuel pump to the point where it stops making noise and starts to draw excess current is dangerous. Please be mindful that doing this risks burning out the fuel pump and could also in certain circumstances cause fire. What is the specific running fault with the vehicle in question? Commented Dec 22, 2016 at 10:07
  • @SteveMatthews I only clamped it for a few seconds. I'm curious if the fuel pressure regulator is supposed to allow fuel to flow through to the return line even when it is not being actuated by vacuum. Just trying to understand how the fp regulator works in it's various states of operation. As a side, I suspect some fuel system problem due to rough idle under load ( and having eliminated virtually every other possibility ), but that's a separate question. Commented Dec 22, 2016 at 10:15
  • A fuel pump will deliver fuel at a high and fairly constant and consistent pressure. The FPR is responsible for reducing the pressure to the pressure it needs to be at the back of the injectors and sending excess back to the tank so the pressure in the fuel rail is neither too high or too low. What do you mean by rough idle under load. An engine under load is not at idle. Commented Dec 22, 2016 at 10:20
  • @SteveMatthews Load as in electrical loads, Radiator Fans, defroster, stereo, headlights, ATX in gear with brake depressed, A/C - but the car is at rest and main throttle is closed, not pushing on the gas pedal - that's what I mean when I say "at idle". Commented Dec 22, 2016 at 11:30
  • I'd have thought that would point to an issue with the alternator control circuitry. Commented Dec 22, 2016 at 11:37

1 Answer 1


In most cars, the fuel system should hold the pressure for quite a while. You shouldn't see a significant drop in fuel pressure after engine/pump shut off. What you are probably experiencing here is leak back at the pump. There should be a check ball or some means to prevent fuel from flowing back through the pump after it's shut off. This ensures fuel remains in the lines when the vehicle sits for longer periods of time (days). What you are describing seems to indicate the pump may be on its way out.

The other stuff you mention about amperage and the sort seems to jibe with my thinking of how things should operate normally. I don't think the issue lies with the fuel regulator. Only excess pressure should flow past the regulator, though.


Adding to help clarify a couple of points.

The idea of the vacuum on the FPR is to account for engine load. As you step on the pedal, vacuum goes down. As vacuum goes down, the FPR pressure goes up. This allows for more fuel to flow through the injectors at the same pulse width. I'd equate it to how the enrichment circuit on a carburetor works. The idea is to give the engine a little extra shot of fuel so it doesn't stumble as the fuel management is trying to catch up the needs of the engine. The vacuum response won't last very long, so the higher fuel pressure won't last very long.

As for your amperage going up, that's an easy thing. As fuel pressure goes up, the pump has to work harder. When you clamped off the line, the pump is fighting against the pressure harder, so the amperage draw becomes larger. If you had zero back pressure, the pump would be able to do its job much easier, meaning the amperage draw would go down.

  • My fp regulator is vacuum operated, so when the engine is off I assume it should be "closed". When it's in this engine off, closed state, should it still allow some fuel to flow back to the tank through the return line? I would assume that there should be higher resistance to the flow of fuel with the engine off and regulator "closed" (64-92psi) and that this would result in higher amp draw than at idle (30-36psi) since the pump has to work harder ( expected values ). However, I'm getting 4.3 amps in both these states and only seeing a higher 5.85amps with the return line clamped. ???????? Commented Dec 22, 2016 at 7:07
  • That just doesn't make any sense to me and I was hoping someone might be able to explain what's going on. Commented Dec 22, 2016 at 7:08
  • @RobertS.Barnes - Please see edits. Commented Dec 22, 2016 at 13:39
  • I apologize for not being so clear, I completely rewrote the question. Commented Dec 28, 2016 at 14:29

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