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Fuel Pressure Issue

Old 03-28-2013, 11:55 AM
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Fuel Pressure Issue

I read through several different forums and have yet to find an issue that is quite like mine. I have a 77 280z I put a fuel pressure gauge on. The pump initially builds pressure to 50psi then once the engine fires and begins to idle the fuel pressure immediately begins to fall until it hits 10psi and stalls (reeving the engine has no affect on symptoms). Once the engine stalls the pressure Slowly starts to build back to 50 psi. The fuel pump is being jumped straight to the battery so I know its not an electrical problem (at least not a fuel pump power supply problem)

I know fuel pressure at idle should be 36psi so I am leaning towards a FPR problem. What are everyone's thoughts?
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Old 03-28-2013, 12:05 PM
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Welcome to ZDriver!

Does the fuel pressure drop or hold when the engine is off (say for 5+ minutes) ?

If it drops, it's the fuel pressure regulator. Verify by a second test of the fuel pressure with engine off, and clamp a vice grip on the return line after the fuel pressure reg. Watch fuel pressure.
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Old 03-28-2013, 12:13 PM
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I don't know about 5 minutes but with the engine and the pump off it will still hold pressure I blocked off the return line and pressure hit 70psi and was still climbing before I turned the pump off

also without the engine running but with the pump running it maintains 50psi (with fuel return operating normally)
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Old 03-28-2013, 12:34 PM
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That's normal, and isn't testing the fpr... the pump SHOULD be able to flow that much. Pinching off the return line while the pump is still running only shows that the system before the fpr is holding pressure.

Do a 5+ minute [engine / power off] leak down test.
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Old 03-28-2013, 12:37 PM
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Alright I will, I won't be able to do it till next week sometime but I will and then I will post the results here, Thanks
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Old 03-28-2013, 02:02 PM
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Alright I was able to get the test done with the engine off and pump off and the return line operating normally after 5 minutes it leaked down from 50 to 42psi
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Old 03-28-2013, 03:31 PM
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You are checking static pressure. You have a flow problem, probably returning to much fuel to the tank or feed from the tank to othe pump is resticted. Could be fuel filter or fpr.
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Old 03-31-2013, 10:54 PM
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I was having the same issues with my 78' Z. I posted a few forums here and the thing was that there were a bunch of bad parts on the car. The fpr was definitely a culprit as well as the filter,injectors and afm. Try testing these before anything I spent a whole bunch of money trouble shooting and it were all these variables to do with my irregulated fuel pressure. Good luck
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