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Why 2 fuel pressure regulators?

Cubes

By Cubes,
in Engines & Forced Induction

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Cubes Grand Master

Cubes

Posted
Posted

Recently she has been missing quite a bit of mid range.

Initially I thought it was a new set of plugs I had recently installed then maybe a bad batch of fuel but no.

Near the firewall on the end of the fuel rail there is a fuel pressure regulator as there is also one at the other end near the radiator where I have the fuel pressure bleed kit installed.

There is a vacume hose that attaches to the fuel pressure regulator so that it can see boost etc.. The hose was totally nackered, big split etc etc.. So I replaced it. The fuel pressure reg gets it source from the same hose that plugs directly in front of the throttle body.

The mid range is now back with the same top end.

What does this first fuel pressure regulator actually do?

I thought only one fuel pressure regulator was required to regulate how much fuel is let back to the fuel tank.. Hence if you block off the return line then fuel pressure will skyrocket.

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Cubes Grand Master

Cubes

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Here's a pic that was taken a little futher back that shows the AAC Valve (Idle Control Solanoid) and the vacume hose that runs to the fuel rail (fuel pressure reg)

I should take a pic of my injectors as I have never seen them on another RB20DET.. Apparently they are C18DET Injectors.. Tilbrooks said they are not stock or standard injectors.

Sydneykid Grand Master

Sydneykid

Posted
Posted

Hmm, interesting................

The reason why people have 2 FPR's is to allow sufficient capacity for the fuel to flow back to the tank. This is used when really big fuel pumps (usually 2) are employed, together with 1/2" fuel lines. One FPR is simply not big enough to return the excess fuel from this big a supply system.

The second, and more common, is to allow even fuel pressure and flow to all 6 cylinders. You feed fuel to the centre of the rail and have an FPR at each end.

Another is to allow for failure of one FPR, although I have never been able to see the logic of this one. If an FPR fails, it normally reduces fuel pressure, so having a backup would make no difference, you would still have low fuel pressure

Does any of this apply in your case?

Cubes Grand Master

Cubes

Posted
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Would any one have any links for pics of the RB20DET that possibly show fuel rails etc etc..

I've been googling but have came up with nothing.

I've just had a thought. Why is it the FPR towards the front of the engine reads from the Plenium Chamber but the rear FPR reads from in front of the throttle body?!?!?! :P

B-Man Grand Master

B-Man

Posted
Posted

Forgive my ignorance if I have got this arseways,

I have 2 FPRs on my car - the stock one and an after market 1:1.5 adjustable one - the people who put it on explained it to me this way:

The FPR regulates fuel from the exit end of the fuel rail - if you put a more restrictive FPR in line with and after the stock FPR, then you get the increased fuel pressure in the rail, and the stock one just stays there and does nothing.

The reason they didn't take it out was because it was a fiddly job and it didn't matter that it was still there ( I took this to mean that they couldn't be fcuked)

This may not even be close to why you have two Joel, but I thought I'd add my story anyways as it may be pertinent in another case.

Again, if I have got my facts wrong, please advise, cause I am just going on what I been told.

Cheers,

DAMQIK GTS-T Community Regular

DAMQIK GTS-T

Posted
Posted

I have a R33 and it and all others that I have seen have two FPR's

one on the inlet of the fuel rail and one on the outlet-return to tank,

I belive they do this to stop fuel presure hammering the lines and injector rail.

So maybe its factory or fitted to prevent this problem.

Regards

Damqik

nissky Community Regular

nissky

Posted
Posted

yeh i just checked the rear one allows fuel into the rail from the fuel filter, and the front one returns fuel to the tank.....

seeing as the rear would see slighty higher boost (due to boost signal coming from in front of throttle body) and always flow more i guess the front one will actually do the fuel pressure regulating??

(i think from the way its plumbed, rear one increases flow as boost increases, and front one decreases return flow as boost increases)

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