R32 GTR Losing 10psi Fuel Pressure at WOT

[TXSquirrel]

Hi folks, I would like to get some inputs on this issue with my R32 GTR... 

Car was on the dyno, the fuel pressure would rise to ~60psi no problem, stayed at ~60psi for about half a second then dropped to ~50psi, and then stayed at ~50psi until let go of throttle.  Idle fuel pressure stable at 43psi.

Made 315kw previously on 93 octane, now 275kw...  Injector duty cycle at 108%, but AFR stable at ~11. 

A quick run down of parts: Nismo fuel pump, R35 injectors, R35 AFM, NZEFI resistor delete, brand new OEM FPR, Nistune, Garrett -9s. 

Is the Nismo pump running out...?  Or could it be the pesky fuel pump control module...?  What does the FPCM do anyway?

Edited December 19, 2018 by TXSquirrel

[KiwiRS4T]

How old is the fuel pump? Did you buy it new? I would be inclined to replace it with something better.

[iruvyouskyrine]

Bypass FPCM using a relay and direct power feed, and put a new fuel pump of your choice (walbro 460 my recommendation) and go from there.

[GTSBoy]

Defo the pump.

[r32-25t]

And get rid of the little valve in the fuel line that’s inside the tank

[TXSquirrel]

My Nismo pump was purchased new from RHDJapan 3 years ago.  Sounds like the consensus is that the Nismo pump is much more likely the culprit than the FPCM? Is there any way to test the FPCM?

So if I'm to bypass the FPCM for direct feed, what gauge wire is recommended?

And what's that little valve inside the tank for? I always wonder why it's there...

 

[Dose Pipe Sutututu]

1 hour ago, TXSquirrel said:

My Nismo pump was purchased new from RHDJapan 3 years ago.  Sounds like the consensus is that the Nismo pump is much more likely the culprit than the FPCM? Is there any way to test the FPCM?

So if I'm to bypass the FPCM for direct feed, what gauge wire is recommended?

And what's that little valve inside the tank for? I always wonder why it's there...

 

FPCM is most likely exacerbating the issue, also that valve. Nissan put those OEM valves in to bleed once it exceeds a particular pressure. 

I beleive it was mainly a safety feature in the even of clogged fuel filters, etc. however a big pain the arse when you start applying boost.

I would just simply delete the pressure relief valve and ground off the fuel pump to the body, i.e. bypassing the FPCM.

[r32-25t]

The valve bleeds off fuel excess fuel pressure

[TXSquirrel]

I finally got around to diagnose this issue in detail, and I am not sure what to make of what I discover.

The FPCM is working fine, doing its thing by increasing voltage as engine load increases.  The problem is with 12v source for the pump.  I first measured the voltage at pump relay, and I was only getting 10.5v ignition on and 11.7v at idle, about 2v below battery voltage.  Then I measured at the fuse, same issue.  I tried the fuses next to pump, which are various lights, no problem at all, 12.5v with engine off and close to 14v with engine on.

So what happened between power source and the fuse harness?  How was it wired, straight off battery?  Look at the electrical diagram it's not too clear to me.  Looks like the EGT sensor is tee'd off the same source...?

[iruvyouskyrine]

Not surprising at all, 30 year old car means 30 year old wiring, and nissans wiring was nothing great to begin with. You are just getting a voltage drop over the fuel pump power feed wire.

I will quote myself here because that was and still is the correct thing to do.

On 12/20/2018 at 7:23 AM, iruvyouskyrine said:

Bypass FPCM using a relay and direct power feed, and put a new fuel pump of your choice (walbro 460 my recommendation) and go from there.

 

[Dose Pipe Sutututu]

Bypass the FPCM, useless thing to begin with. A drop resistor with new pumps is only going to reduce it's life.

You want to control pumps via PWM, if not just wire it direct to a relay.

I'm assuming you're American, stubborn and like to do things the hard way. I recall all of us telling you to go with modern larger injectors before too. 

[TXSquirrel]

When my tuner first noted the fuel pressure issue, he already suggested bypassing the FPCM or installing an Aeromotive fuel pump controller.  He did that for all the cars with aftermarket pump.  I am just trying to keep my car as close to factory and use Nissan parts whenever I can, so it's just me with my way.  That's why I'm running R35 injectors, which are still more than what the -9s can put out anyway.

After looking at the diagram more, I'm guessing this 12v source actually runs through the ignition switch.  So if I can get a confirmation that will be great.  Nissan did that with headlight harness, not a real surprise if 30 years ago Nissan decided to run fuel pump 12v through ignition switch.

[iruvyouskyrine]

On 12/20/2018 at 7:23 AM, iruvyouskyrine said:

Bypass FPCM using a relay and direct power feed, and put a new fuel pump of your choice (walbro 460 my recommendation) and go from there.

 

[Ben C34]