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High Fuel Pressure

rofocale

By rofocale,
in General Maintenance

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rofocale Contributor

rofocale

Posted
Posted

Hi folks,

This is on a 1995 R33 GTR V-spec with a stock FPR and injectors and 2 year old fuel filter. With an unknown fuel pump...initially... and the car is definitely running on the rich side with a powerfc using the base map and the idle routine is complete. It drives okay...but definitely rich (on mixture and pocket!)

Some time ago I bought a mechanical fuel pressure gauge to plug into the fuel line just after the fuel filter before heading off to the fuel rail and injectors. I measured the pressure according to many articles I've read on this and other skyline forums and my results have been:

at idle: ~43psi

at idle with vacuum line to FPR disconnected: ~50psi

These figures I believe are high for a gtr...i've seen posts which basically say it should be 2.5 bar(or 36psi) at idle and 3.0 bar(or 43 psi) with vacuum line disconnected. The R34 gtr service manual and R32 service manual also corroborates these figures(I haven't seen specific references in any Nissan documentation for the R33 GTR)

Now my old fuel pump was never holding line pressure with the power switched off and it was very likely just the stock fuel pump which would make it pretty old so I recently had the opportunity to buy a Nismo GTR fuel pump and I have installed that and also replaced all the pressurized fuel lines(ie the ones from the fuel tank to the metal runners behind the right rear wheel and the ones in the engine bay) with some new high pressure fuel tubing and for good measure I also replaced the fuel filter. So after replacing all the stuff and measuring the fuel pressures I get exactly the same figures...

My thoughts are:

Is this fuel pressure within some level of tolerance and effectively normal? (I doubt this but it might be possible) Hence I need not go any further except get the car tuned.

This fuel pressure is high and hence raising the amount of fuel going into the engine - more than what the powerfc thinks is going in.

If this fuel pressure is too high and should be addressed, I suspect that the only piece left in the equation is the FPR which is just a stock item. By chance I happen to have a nismo adjustable FPR I got some time ago cheap and never had a need to use. Is it worth me changing over to the nismo FPR and setting the idle fuel pressure to 2.5 bar (or 43 psi)? Or would this potentially mask the real problem whatever it may be and not be a good idea (yet?).

Are there any other possible causes for this high fuel pressure? (Other than the mechanical pressure gauge is reading incorrectly... I shall investigate this just in case...)

Any thoughts would be appreciated.

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scotty nm35 Grand Master

scotty nm35

Posted
Posted

Of course you get the same figures, it is the reg that controls the pressure, not the pump.

You must have a very high idle, with a low idle there is much more vacuum which will drop the pressure even more. If you are still worried, swap the reg. More than likely the gauge is out by a bit though.

89CAL Grand Master

89CAL

Posted
Posted

Of course you get the same figures, it is the reg that controls the pressure, not the pump.

You must have a very high idle, with a low idle there is much more vacuum which will drop the pressure even more. If you are still worried, swap the reg. More than likely the gauge is out by a bit though.

This

Pressure is restriction to flow, the Pump creates the flow, the regulator restricts the flow on the exit of the fuel rail.

It is possible your gauge is out as above or the FPR is a bit dodgy. But it isn't really that big a deal. I know its not the cheapest option but you can just go and get it retuned (It looks like you have a Power FC) and the car will run fine.

Having said that, if you drop the base pressure, you'll have to get a re-tune anyway as the fuel pressure under load will be a tad lower and thats a bit more dangerous then it being a bit rich

I'd just take it for a retune, I dont think your Dangerously above the standard base pressure but if your that worried about it, replace the FPR (Maybe borrow a stock FPR if you know someone) and see if that fixes it and get a re-tune then

  • 2 weeks later...
_x_FiReStOrM_x_ Community Regular

_x_FiReStOrM_x_

Posted
Posted (edited)

Possible causes: slight blockage in return line, either gunk in hardline or internal swelling of a rubber hose. Blockage or fault in fuel pressure reg itself. Aftermarket fuel pump delivering a high volume of fuel, which the regulator can not return quick enough.

For a start i'd try get a hold of a good quality or known accurate fuel pressure gauge and then go from there. You can also do a simple fuel flow test to determine the actual flow rate of the pump if it's unknown.

Edited by _x_FiReStOrM_x_

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