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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!

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    Could you attach a Mityvac to the regulator and see if you can get it to move, perhaps it is stuck in the open (low pressure) position.
    – dlu
    Dec 20, 2016 at 18:34
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    pinch off the return line. does your gauge use a schraeder valve? try removing it if it does.
    – Ben
    Dec 20, 2016 at 19:16
  • @Ben Yeah, I was thinking that if I clamp off the return line and I see the pressure rise I'll know it's the regulator... See if I can get to that tomorrow.
    – Robert S. Barnes
    Dec 20, 2016 at 19:37
  • @dlu a Mityvac is a hand vacuum pump? maybe if I just hook the injectors back up and try starting the engine that'll knock it loose? Of course if I don't have fuel pressure then I probably won't be able to start the engine - chicken and egg problem I guess...
    – Robert S. Barnes
    Dec 20, 2016 at 19:51
  • Yes, Mityvac comes in handy a lot, I got one for troubleshooting turbo controls, but I keep finding uses for it.
    – dlu
    Dec 20, 2016 at 20:26

1 Answer 1

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So I went out today and ran the fuel pump for a few seconds with the return line clamped off and saw that the amperage draw went up to 5.85 amps and the sound of flowing fuel stopped. At first I just figured the fuel pressure regulator must be stuck open / bad and that's why I'm seeing a jump in amperage from the 4.3 amps I was seeing ( which I also saw at idle ) and why the sound of flowing fuel stopped. However, I thought maybe even a non-actuated regulator allows a little flow ( a separate question ) so I decided to take a screw driver and press down on the bit in the valve which allows fuel to flow out into the fuel pressure gauge:

enter image description here

Low and behold some fuel squirts out! So even with the engine off and the clamp removed there is some residual pressure in the fuel line. So I figured I should be seeing something other than zero on the fuel pressure gauge. So I start inspecting the gauge and what do I see at the connector?

enter image description here

I see nothing! As in I'm pretty sure there is supposed to be something on the gauge side connector which presses down that pin on the valve side to let fuel flow into the gauge.

So now I have to figure out what happened to that part, what that part is and how to replace it.

Edit Jan. 8th 2017

So after a little back and forth with Actron / OTC / Bosch they agreed to ship a replacement part ( the bushing and depressor ) and here's what they look like:

enter image description here

enter image description here

and installed:

enter image description here

Hopefully, it'll work now!

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    pull the schraeder valve out of your fuel line t it'll be a little messy just use a rag to catch anything. there should be something in the gauge end that pushes down on the valve.
    – Ben
    Dec 21, 2016 at 16:09
  • @Ben I'm not quite sure how I'd do that and I'm a bit worried I might break it. By the way, any idea what piece is missing?
    – Robert S. Barnes
    Dec 21, 2016 at 16:11
  • it's threaded in you need a tool it's the same style valve used in wheel valve stems you might be able to use needle nose pliers to get it out. but those tools are generally cheap.
    – Ben
    Dec 21, 2016 at 16:13
  • The missing part should, I think, look just like the part in a tire pressure gauge that does the same job. Maybe a bit like an inverted 'T' or a peace sign / MB logo with a stem coming up from the middle. I think a cone that sat in that plastic cap with a few holes in it to let pressure get through to the gauge would also work. Great bit of troubleshooting btw!
    – dlu
    Dec 21, 2016 at 16:14
  • Pair of fine needle nose pliers or some tweezers might do it. Not all of them come out.
    – dlu
    Dec 21, 2016 at 16:15

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