Could some one please link me a VFAQ or some article about this? I have been searching some, but most of the stuff I have been finding barely even mentions it. I am looking for some indepth detail about it.

Thanks guys!

If your talking about an actual over run occurring with a 255 fuel pump then I think I can help. Over run is when the injectors spray more fuel into your engine than what is regulated. I've only seen it happen at an idle. Your stock FPR can have more fuel bypassed because of the upgraded fuel pump's pressure. You need an aftermarket FPR. I think this thread can help you out.

That's an interesting twist of the issue.

Here some of what I've posted in the past. There's a ton more if you search.

FPR overrun is caused when the fuel pump provides more fuel than the FPR can dump out it's return port to the tank and looses it's ability to control (regulate) the fuel pressure.

Our fuel pressure regulators must adjust the fuel pressure during operation so that the fuel rail pressure is always:

Fuel Pressure = Base Fuel Pressure + Manifold Pressure

We set the regulator with the reference vacuum line disconnected so Manifold Pressure = 0 psi. Under vacuum the Manifold Pressure is negative, 18 in hg is about -8 psi. So the fuel pressure at idle with the reference vacuum line connected on a 1G should be 36.3 - 8 = 28.3 psi and the pressure with 20 psi boost should be 36.3 + 20 = 56.3 psi

Fuel pressure overrun is dynamic and not predictable by the existing hardware, software or piggybacks. Changes in engine load cause the regulator to work on not. The magnitude of the pressure change depends on how much the return port is being overrun.

WOT performance isn't where fuel pressure overrun shows up since the engine has consumed the surplus fuel that was causing the regulator to overrun.

Anytime you back off the throttle or have a significant decrease in engine load the fuel pressure spikes and since it's fuel consumption and not directly RPM based you can't tune it out with an SAFC. If you remove the extra fuel the car will be lean at those rpms when the regulator is functioning correctly.

The only way to fix this problem is to get a fuel pressure regulator that does NOT overrun with the fuel pump your using.

A little different attempt at explaining from 2005:

Our fuel pumps are what's called a constant displacement pump. The pump doesn't make pressure it makes volume. The FPR restricts the flow creating pressure in the fuel lines and rail. The diaphragm and spring hold the return port closed until the fuel pressure forces the diaphragm to compress the spring and open the port releasing some flow back to the tank and dropping the pressure until the port closes or the pump replaces the fuel. So far we have a constant base pressure based on the pump delivering enough fuel to keep the port partially open. If he pump delivers more fuel than the return port can flow fully open then the fuel pressure rises. This is FPR overrun.

The reference nipple on the FPR is used to increase or decrease the effective spring pressure by using manifold pressure to push or pull on the diaphragm. Our regulators are set up so that a change in 1 psi at the reference port equals 1 psi change in fuel pressure. Other regulators like the ones used for turboing NT cars have different rates. We use a 1:1 rate so that the injectors always deliver the same amount of fuel per unit of time at 20 in hg vacuum as they do at 25 psi boost. If we didn't track manifold pressure the vacuum would suck fuel from the injectors at idle and slow it to a trickle at full boost.

Just to add, I have not seen overrun on any 1g around me with a 255 and stocker FPR.

All of mine do . That includes m/t, a/t, and even n/t. The hand of my fuel pressure gauge (tapped at the rail feed) was fluttering at 60psi for all of them. They were different cars with stock fuel lines. I don't think it is just dumb luck that it happened to all of mine over the years and all of my brothers. Though I wish it were. That would beg another remedy other than buying an AFPR for me .

Small boost leaks or pre turbo intake leaks will mask the 'rich at low load' issue common to an overrun FPR. Load is directly related to airflow (give or take a few instances). So you CAN go in at the load cells and manipulate the low load target fuel. Or go in to the regular link format and alter the low maf calibration sliders. Seldom are you at high boost and low enough airflow that you'll be in those cells. But its just a band-aid to the real problem. And, it won't manage ALL the instances that overrun can affect your setup.

Also, I would think old fuel filters beginning to clog and gummed up fuel feed lines will mask the issue as well.

JayRolla said:

Just to add, I have not seen overrun on any 1g around me with a 255 and stocker FPR.

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Where I've seen plenty of cars over the years where the owners swore they didn't have FPRO until I showed them they did.

Mitsubishi knows it's a problem, that's why the 3000GT and EVO have dual speed fuel pumps. They can run them on low when they don't need the extra fuel and high when they do. This keeps the regulator from being overrun and the fuel pressure regulated.

steve said:

Mitsubishi knows it's a problem, that's why the 3000GT and EVO have dual speed fuel pumps. They can run them on low when they don't need the extra fuel and high when they do. This keeps the regulator from being overrun and the fuel pressure regulated.

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That's interesting. I wonder what made them go this route as apposed to an upgraded regulator that was made to handle (bypass) a greater volume of fuel.

99gst_racer said:

That's interesting. I wonder what made them go this route as apposed to an upgraded regulator that was made to handle (bypass) a greater volume of fuel.

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Definitely seems like this route would be a much cheaper and reliable solution for Mitsu.

habitatguy187 said:

Definitely seems like this route would be a much cheaper and reliable solution for Mitsu.

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I guess it depends on what it costs to develop, test, and qualify a new fuel pressure regulator compared to the extra cost of the control system.

i'd imagine it's cheaper for them to have a dual speed system for the fuel pumps than to get denso to completely redesign their regulator. companies do nothing but cut costs, so it's obviously cheaper for them to control the pump than to get a different fpr.

ramsack said:

i'd imagine it's cheaper for them to have a dual speed system for the fuel pumps than to get denso to completely redesign their regulator. companies do nothing but cut costs, so it's obviously cheaper for them to control the pump than to get a different fpr.

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Agreed.

I'm pretty sure I have FPR Over run, but would like someone to attempt to describe the symptoms.

it will be different on every car. you need to have a fuel pressure gauge to know for sure.

If you have any doubts about your system, you should definitely upgrade to an AFPR. You might as well upgrade it either way if your modding your car. If your going for big power gains and are gonna be tuning your gonna want it anyway.

BlackMount said:

I'm pretty sure I have FPR Over run, but would like someone to attempt to describe the symptoms.

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Simple, the fuel pressure regulator doesn't read the correct pressure at idle.
On a manual 1G with the vacuum line disconnected and plugged it should be 36 to 38 psi and with the line connected it should drop to about 27 psi depending on idle vacuum. If your higher that this your overrunning the regulator, have a bad one, or your gauge is hosed.

What about on a 2G NT? i hooked my car up just to see what the fuel pressure is. (No, i did not hook up a new gauge in my car) Just a fuel pressure gage to the fuel rail, and mine read about 55psi. is that good for my car? Or should i look into getting a new FPR?

nothingfiction said:

What about on a 2G NT? i hooked my car up just to see what the fuel pressure is. (No, i did not hook up a new gauge in my car) Just a fuel pressure gage to the fuel rail, and mine read about 55psi. is that good for my car? Or should i look into getting a new FPR?

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Did you look up what your base fuel pressure should be?
If your measuring the pressure per the factory spec for a 2G NT and the fuel pressure is over that you have a problem, don't you?

The USDM 2G NT is unique in that the FPR is back by the tank and there isn't a return line from the fuel rail. I don't know enough about the rest of the details.

good info. I have a afpr to install into my 90 gsx and this has provided good info for me to set the afpr to and what psi to aim for also based on how the engine runs at the time.

However as it was explained i guess i dont understand 100%.

After installing the afpr. I need to set it at first with out the vacume reference line attached? if so then i aim for that 36 to 38psi range. This is changed by chaning the psi on the afpr with the line not attached and plugged. Then when the line is reattached i should see about 27psi? If so then I've done a pretty good job setting it up. Of course some fine tuning based on the wideband and how the engine runs at idle will confirm it's good?

Just making sure before i install this i know what i'm doing and what to look and aim for. Thanks.

according to the hp upgrade page...

10 Air/Fuel Controller
Allows tune-ability of the air/fuel mixture to avoid detonation and major engine damage.

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Would this be an adjustable fuel pressure regulator? Or is that two different things?
i have been looking online about fpr's and most of everything i manage to find just talk about guys with fmu and with turbo (even in the 420a section). Not much talk about them on the non turbos.

With the fpr there is some discrepancy there. the location is relevant to the year i'm told. some years they did it back near the fuel pump. some years it is on the fuel rail. Correct me if i am wrong. that is just what i have heard and read on this site.

the reason i ask about fpr's in general is because i have this low idle and i get something that i could only think would be fuel cut. Given i am not 100% sure what fuel cut is, but from what i read about it it kind of seems like what i am going through. i can hit the cruise at any given speed and be fine... then while driving it feels like it loses power (not electrical power)... or bogg down i guess. then i notice my shortband o2 go all the way lean. Which tells me it isn't getting fuel. Then it will kick back in and my shortband will be normal until it does it again. Via why i'm thinking fuel cut. i took my car to a shop and they said i probably have a bad fuel pressure regulator (given why i am looking it up and reading what i can about it).

As for the base fuel pressure... only thing i have found is about guys with fuel management units running 12:1's. Not really much info on here that i have found about it.

JRSTANGE said:

After installing the afpr. I need to set it at first with out the vacuum reference line attached? if so then i aim for that 36 to 38psi range. This is changed by changing the psi on the afpr with the line not attached and plugged.

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Without the vacuum line connected, you get the base fuel pressure. It should be 36.3 psi on a 1G MT but the factory FRP isn't adjustable and has a range of acceptable values.

JRSTANGE said:

Then when the line is reattached i should see about 27psi?

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Once the vacuum line is reconnected the fuel pressure regulator needs to track the reference port 1:1 so with a normal sea level idle vacuum of 18 in hg (-9psi) the fuel pressure should drop from 36.3 psi to 27.3 psi.

If your AFPR is working it will always keep the fuel pressure = base pressure + reference pressure. This keeps the pressure difference between the inlet of the fuel injectors (fuel rail) and the outlet (intake manifold) constant.

The injector flow rate is dependent on that difference in pressure, the base fuel pressure, and the ECU is programmed with the resulting flow rate which is used in all the fuel calculations to figure out how long to open the injectors for. If you change the base pressure, and you can to make the injector flow more or less, the ECU need to know the new flow rate. Changing the fuel pressure is one of the ways you can fine tune.

steve said:

Without the vacuum line connected, you get the base fuel pressure. It should be 36.3 psi on a 1G MT but the factory FRP isn't adjustable and has a range of acceptable values.



Once the vacuum line is reconnected the fuel pressure regulator needs to track the reference port 1:1 so with a normal sea level idle vacuum of 18 in hg (-9psi) the fuel pressure should drop from 36.3 psi to 27.3 psi.

If your AFPR is working it will always keep the fuel pressure = base pressure + reference pressure. This keeps the pressure difference between the inlet of the fuel injectors (fuel rail) and the outlet (intake manifold) constant.

The injector flow rate is dependent on that difference in pressure, the base fuel pressure, and the ECU is programmed with the resulting flow rate which is used in all the fuel calculations to figure out how long to open the injectors for. If you change the base pressure, and you can to make the injector flow more or less, the ECU need to know the new flow rate. Changing the fuel pressure is one of the ways you can fine tune.

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Awesome! Thanks for confonfirming. Makes perfect sense! Hopefully this post works from the BB.