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Another fuel pressure question.

Abo Bob

By Abo Bob,
in Engines & Forced Induction

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Abo Bob Grand Master

Abo Bob

Posted
Posted

I have a ROM tune in my R34 and a Walbro fuel pump. On the dyno the mixtures were ridiculously rich (off the scale) and the tuner (not present at the time) suggested fuel pressure.

We measured the fuel pressure and it was 3.3 bar at idle.

Changed back to the stock fuel pump (measured at 2.7 bar I think) and it ran fine.

Now obviously I need a bigger fuel pump so I figure I need an adjustable fuel regulator. Nobody makes a fuel regulator specifically for the GTT so I thought I'd put a second, adjustable one in series with the standard one.

The one this guy is selling looks about right I guess???

So the questions are:

Is this alright to put them in series?

Mostly people adjust the pressure up to push the injectors but I am adjusting down, is this ok?

Do I insert a T piece into the vacum line going to the stock regulator to feed the Sard one?

Does that sard one need this?

Anything else I should know?

Thanks.

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Sydneykid Grand Master

Sydneykid

Posted
Posted
I have a ROM tune in my R34 and a Walbro fuel pump.  On the dyno the mixtures were ridiculously rich (off the scale) and the tuner (not present at the time) suggested fuel pressure.

 

We measured the fuel pressure and it was 3.3 bar at idle.

 

Changed back to the stock fuel pump (measured at 2.7 bar I think) and it ran fine.

 

Now obviously I need a bigger fuel pump so I figure I need an adjustable fuel regulator.  Nobody makes a fuel regulator specifically for the GTT so I thought I'd put a second, adjustable one in series with the standard one.

 

The one this guy is selling looks about right I guess???

 

So the questions are:

 

Is this alright to put them in series?

 

Mostly people adjust the pressure up to push the injectors but I am adjusting down, is this ok?

 

Do I insert a T piece into the vacum line going to the stock regulator to feed the Sard one?  

 

Does that sard one need this?

 

Anything else I should know?

 

Thanks.

Changing the pump shouldn't make any difference to the fuel pressure, the fuel pressure regulator determines the fuel pressure, not the pump. You have something else wrong, I would strongly suggest that you or the mechanic have a good go at trying to find it. My suggestions for things to check ;

1. the vacuum line to the FPR

2. the fuel return line from the FPR

3. the fuel return lines under the car

4. the fuel return into the tank

Adding a second FPR in series with the standard one is good for raising the fuel pressure above that determined by the standard FPR. But you can't do that to lower the fuel pressure since the standard FPR is first in line.

We use the Nismo replacement style FPR, Nenun have them for ~$150.

Hope that helps:cheers:

Abo Bob Grand Master

Abo Bob

Posted
  • Author
Posted

The compatability table says that it doesn't fit the R34 GTT and I have been told elsewhere that Nismo don't make one for the GTT.

We put the pump in and out a couple of times with this being the only change and the result was the same. 3.3 bar with Walbro pump, 2.7 bar with stock pump.

I thought if I put the second regulator between the filter and the stock regulator it would sort of pinch off the flow a little causing a pressure drop after it but before the stock regulator (or is that before the injectors?)

Is the stock regulator plumbed so that it comes after the injectors?

thanks again.

R31Nismoid Grand Master

R31Nismoid

Posted
Posted

you shouldnt need an aftermarket regulator at all.

the stock ones are fine, im smashed out 369 rwkw on the weekend still using my stock reg.

I would do as SK suggested, check for the cause of the problem that you obviously have

RB30-POWER Mentor

RB30-POWER

Posted
Posted

You can't and don't want to use them in series. (Especially not for your problem anyway)

The regulator goes after the fuel rail/injectors.

What it does is bleed any excess fuel to keep the pressure at the rail at the factory setting. 2.7bar under vacuum at idle.

(Engine Vacuum will pull the spring/diaphram up and allow more fuel to bypass, when 0 vacuum or boost pressure is applied it will push the spring blocking of the passage a bit and therfore raising pressure)

Most people say that the regulator will be fine even with big pumps, but im not entirely sure that is correct. If the fuel pump is pumping a shitload of fuel and the regulator can't release enough of the fuel to drop pressure the pressure will rise.

With a massive pump and stock fuel lines i beleive its possible for this to happen.

RB30-POWER Mentor

RB30-POWER

Posted
Posted

Have you actually measured the pressure before and after the fitting of the pump though?

If you have aftermarket managment and its tuned for slightly higher pressure it won't matter, but increase pressure on the stock ecu car and it will shoot more fuel through the injectors.

Abo Bob Grand Master

Abo Bob

Posted
  • Author
Posted

Well my pump is not that massive. I would think that stock fuel lines would be ok at this level too.

There might be something wrong in the mounting because the pump is a very tight fit in the normal mounting bracket. Perhaps that's doing something although to be honest I can't figure out how. Still, it's worth checking out. I'll just pull it out of the mounting bracket and lay it in the bottom of the tank to test it.

There's just nothing else I can think of that changed with the different pumps to increase the pressure.

Nismoid - could it be you had no problems but you were running very rich without knowing it?

To address this:

1. the vacuum line to the FPR

2. the fuel return line from the FPR

3. the fuel return lines under the car

4. the fuel return into the tank

All unchanged with fuel pump change.

The more I think about it the more mystified I become.

Beer Baron Grand Master

Beer Baron

Posted
Posted

one question on this problem, doesn't the ROM tune compensate for this problem and fix up the AFR through the tune?

even so, there is certainly an issue there that is causing the higher pressure.

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