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noelawinslow

High fuel pressure, bleeds off quickly

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Hey all, picked up an NA auto z31, trying to track down a hard start and misfire condition.

Checked fuel pressure just after the filter and it kept shooting down to 0 after I shut it off, so I crimped off the return line, primed the system, and crimped off the supply line. Still bleeds out.

 

Replaced FPR.

Fuel pressure stable at 60 psi at idle. Much too high. Holds pressure long enough to watch the needle drop now though.

 

Pulled the evap check valve. One way it slowly releases air and the other way it lets out plenty, so apparently that's good.

 

I hooked up an air compressor to the return line, wondered if that might jack up the pressure, blasted air through it at 5psi at first, no issue. Moved up to 60 psi and there's no blockage. Might as well clean the supply side out too but now I'm kinda out of options unless I want to check the Evap system further.

 

I didn't think evap could cause problems like that.

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I thought the system was supposed to retain pressure with the engine off. I cleared the lines and started the car up again, 60 psi still and it barely idles. I'll run a return line to a gas can and see how that works for me.

 

Is it safe to run compressed air through the return line in the tank with the fuel cap off?

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There's a check valve in the pump.  They fail sometimes.  Crimp the supply line before it leaks down and see what happens.  

 

You didn't say anything about what the pressure was before you replaced the FPR but the fact that fuel dribbles from the FPR instead of flowing could mean that the FPR is blocked or defective.  If you knew what it was before it would give a pretty clue about blockage versus FPR.  Some aftermarket FPR's can be connected backwards and will give full fuel pump pressure in the rail if that's done.  I'm not familiar with the Z31 fuel system in general beyond the basics, besides the fact there are 6 years worth of them.  There may be differences.  Give some details on yours.

 

Think in terms of pressure instead of flow direction.  Pressure works backward too.

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You might also measure pressure with the FPR not installed.  Just to be sure there's not something blocking the ports.  The FPR is just a pressure relief valve.  If you don't have some monster fuel pump pushing so much fuel it can't bleed off pressure, it should work if the flow paths are clear.

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