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fuel injection pressure wierdness...


Dave WM

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280z 1975

I pulled the FI's and cleaned by running cleaner thru them while cycling.

Put it all back together, at 1st no issue, then hard starting, have to throttle it to get  it going. Once started no issue.

Spark, check while cranking, all good.

started disconnected ign to start fuel pressure, slow to come up then bleeds off fast. Has a new OE pump.

get out some clamps, clamp the return line, no effect same thing slow to pressure then drops off fast when start returned to run (will not hold pressure)

One more time this time get assistant to turn key to start while I prep to clamp the fuel in line (and the return line is still clamped).

while showing the asst how to turn to start for 1 second, I notice the pressure gauge does NOT drop even with out the second clamp.

hmmm so I go and disconnect the return line clamp, still no drop sitting at about 36 psi.

Attach the starter solenoid wire, cranks right up. Turn it off, pressure holding at 30 psi.

so for some reason I was losing pressure and for the life of me cant figure it out, seems intermittent.

I am seriously considering getting some rebuilt FI's. Oh I clamped off the cold start supply 1st no effect.

Note all this began AFTER I pulled the injectors for cleaning (I really just wanted to base line the spray patterns).

 

Edited by Dave WM
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Not uncommon for the FPR to have an iffy seat, that doesn't always seal.  It's just a metal tube pressed against a flat piece of metal.  And the check valve in the pump is just a ball and seat design.  Both are are all metal, no rubber, so any small piece of debris can hold them open.  The injector seats are the same, so a tiny piece of grit can cause problems.

Seems like you might just be bored and creating problems for yourself. :Zap: [Zing!] It happens  The spray patterns for the 280Z injectors are typically just a narrow stream.  No real pattern.  Primitive.

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EuroDat, your guide is pretty much everything I did but for the flow rate

Now that it is holding pressure again I will see if the problem comes back. I had the same issue when I 1st got the car, problematic starting. It resolved on its own back then as well.

Maybe there was a bit of junk in the new lines I installed.

I know while I had it on the test stand there was NO leakage from the injectors (they were mounted to the rail and the entire setup was pressurized. I don't recall any leakage at that time from the FPR vacuum line so its prob ok.

 

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Sounds like it could be something getting stuck in the check vavle at the pump and then clearing or something like that. Are you running a filter between the tank and pump? That might stop it from hapening again.

Loosing fuel pressure is not a big problem, as long as it not through the injectors, csv or leaking externally. Its just irritating and takes longer to start the engine. Some people wire a switch to activate the pump and manually prime it before starting.

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Yes have the fram G-3 prefilter, the fuel pump is a new OE Nissan.

Of course it started right up this morning, and the pressure is holding, not indefinitely but much better that when it would drop in seconds.

I think I need to drive the car more and work on it less....

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1 hour ago, Dave WM said:

I think I need to drive the car more and work on it less....

I'm guilty of the same.

Dave, I am having the same "bleed down" with my fuel rail pressure. Takes between 4-5 hrs to completely bleed down. I tried to diagnose with no luck. I would clamp off lines and still get the pressure lose. I haven't given up yet just postponed for now. I have a starting problem because of it when the car sits over night and its not cold enough the next morning for the cold start to work. But when it is cold enough she starts up nicely. Mine has been like this for a couple years now. I have been running an aftermarket fuel pump for over 10 years now. May or may not have anything to do with it. I just want to know why its not holding pressure any longer than it is. I have a used OE pump with a NEW check valve that are possibly none existent anymore. Figured I would install that next to see what happens. 

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sounds like the same story here. cold never really an issue (assuming cold enough for the cold start to fire, which in florida is not often, even changed it for a hotter one just so it would fire at 85f). The leak down is a real mystery, did the same thing, clamping various parts.

I am starting to think it maybe just the fuel lines themselves may account for some of the different times. If you think about it, assume the lines were all metal, ANY loss of fuel would result in a dramatic loss of pressure in a static (pump off) situation (in fact it would prob be zero the instant the pump was off). Just guessing of course. Maybe a comparison to a modern car will show the same inconsistency, seems like yours and mine a perhaps a lot of other Z's sure have the same issue, so maybe its not really an issue at all.

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