Strange fuel pump issue

[chaseincats]

Hi guys,

So I'm having a strange issue here.  I recently changed my fuel injectors and fuel filter (for the first time in 4 years) and the car started up on the first crank but gradually started taking longer and longer to start with each drive.

When I unhooked my old fuel filter to change it, the gas coming out of the bottom was black for some reason (the gas coming out of the filter was clear though, so the filter was working).  I went to my local parts store and rented a fuel pressure gauge then spliced it in between the fuel filter & fuel rail.  When I primed the pump by turning the key to "on", the gauge read 10psi.  I cleared the gauge and primed the pump with the key again, this time it read 5psi.  I cranked the car, and after a bit, the gauge hit 36 psi and the car started immediately.  I checked the pressure a couple years ago when I had a different issue and I remember the gauge reading 36 immediately with the key set to "on."

Note: the later 280s prime the pump with the key in the "on" position, not just when the car is cranking like the earlier cars.

I would have thought it was the fuel pump, but the car doesn't stumble when revved high or on inclines and once it gets to 36PSI, it stays there.  The tank looks clean when I look in the inspection hole (the sending unit is accessible via the hatch on a '78).

Could the seals in the pump be coming apart (do pumps have these)?  The black gas that came out of the bottom of the filter was liquid and didn't have any goop or chunks in it indicating a clogged rail.

 

The only strange thing that comes to mind is the pump used to hum for a good 5-10 seconds when I put the key in "on" but it no longer does that - it clicks on then off immediately when putting the key in "on."  Not sure if that helps diagnose this but it's something I noticed.

 

Any ideas?

-chase

[Zed Head]

34 minutes ago, chaseincats said:

Note: the later 280s prime the pump with the key in the "on" position, not just when the car is cranking like the earlier cars.

Note: this is incorrect, if you're talking about 280Z's.  👽  The ZX's used a priming time at Run, but no 280Z's did.

34 minutes ago, chaseincats said:

When I unhooked my old fuel filter to change it, the gas coming out of the bottom was black for some reason

Seems like your hoses might be falling apart.  You might check those.

Could also be your pump check valve or FPR leaking down.  The fact that you measured 10 and 5 is a sign.  Usually they'll hold 20 or better even over a few days, even weeks.

You can run your pump by removing the yellow solenoid wire and turning the key to Start.    

[chaseincats]

34 minutes ago, Zed Head said:

Note: this is incorrect, if you're talking about 280Z's.  👽  The ZX's used a priming time at Run, but no 280Z's did.   

It depends if you have the 5 or 7 pin AFR connector.  The later 280z AFRs had the fuel pump hardware removed from the AFR's jurisdiction which is how the pump can be primed in the "on" position.

The problem isn't (for the moment) that the pressure isn't holding in the rail overnight, it's that I'm not getting 36psi to the rail until a fair bit of cranking.  Good point with the hoses.

Edited May 4, 2021 by chaseincats

[Zed Head]

Only if you remove the oil pump switch plug on 1978 280Z.  It's a flaw.  A 280Z fuel pump will only run with the key at Run if the operator intentionally changes something or there is an error in the system.  It is well described in each individual 280Z Factory Service Manual in the Engine Fuel chapters. 

Except for the flaw in the 1978 fuel pump control system, which is described on one of the AtlanticZ.CA pages.

[chaseincats]

3 minutes ago, Zed Head said:

Only if you remove the oil pump switch plug on 1978 280Z.  It's a flaw.  A 280Z fuel pump will only run with the key at Run if the operator intentionally changes something or there is an error in the system.  It is well described in each individual 280Z Factory Service Manual in the Engine Fuel chapters. 

Except for the flaw in the 1978 fuel pump control system, which is described on one of the AtlanticZ.CA pages.

My car and my friends' 78 work like that and our oil pump switches are plugged in 🤷‍♂️

[siteunseen]

Try the gauge before the filter. I'm sure the FPR vacuum hose is in good shape?

You may have a collapsing hose somewhere.

Edited May 4, 2021 by siteunseen

[chaseincats]

1 minute ago, siteunseen said:

Try the gauge before the filter. I'm sure the FPR vacuum hose is in good shape?

You may have a collapsing hose somewhere.

I'll try that today and report back.  I actually tested it with the FPR vacuum hose unplugged (and capped) and got the same result.  I also checked the FPR for vacuum leaks but it's solid.

[Zed Head]

9 minutes ago, chaseincats said:

My car and my friends' 78 work like that and our oil pump switches are plugged in 🤷‍♂️

Two non-rights do not make a non-wrong.

[Zed Head]

1 hour ago, chaseincats said:

When I primed the pump by turning the key to "on", the gauge read 10psi.  I cleared the gauge and primed the pump with the key again, this time it read 5psi.  I cranked the car, and after a bit, the gauge hit 36 psi

Explain the above in terms of the below.  Obviously your pump is not working the way you think it is.  Good luck..

11 minutes ago, chaseincats said:

My car and my friends' 78 work like that and our oil pump switches are plugged in 🤷‍♂️

 

[chaseincats]

1 minute ago, Zed Head said:

Explain the above in terms of the below.  Obviously your pump is not working the way you think it is.  Good luck..

 

The pump (when I last used a gauge between the filter and rail a few years back) read 36 when the pump was primed and during operation.  Today, it reads 10 during priming and 36 during operation which seems to be the issue.

[Zed Head]

Whatever was malfunctioning back then fixed itself, possibly.  Switches do stick.  Even a bad connection could cause it.

Why not assume that the system works as Nissan intended it to and take the appropriate actions?

Edited May 4, 2021 by Zed Head
+d

[chaseincats]

7 hours ago, Zed Head said:

Whatever was malfunctioning back then fixed itself, possibly.  Switches do stick.  Even a bad connection could cause it.

Why not assume that the system works as Nissan intended it to and take the appropriate actions?

So I went down just now with a fuel pressure gauge and unplugged the oil pressure switch connector and the pump indeed did come on.  Does that mean my fuel pressure control relay was stuck open all these years?