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My fuel pressure regulator needs replacing, however the original part from the manufacturer is outrageously expensive. While looking for less financially painful alternatives I've seen much cheaper universal units or similar units made for other cars (shape and fuel pipe diameters are the same), however the maintained pressure is a little different. The stock one is rated for an odd 3.09 bar, while the closest ones available are usually 3 bar or 3.5 bar. Is it absolutely critical to get exactly 3.09 bar, or the car will be fine with 3 bar? Maybe it is preferred to go for more, if not exactly 3.09? Doesn't the stock unit have some kind of an error anyways (+- 0.1 bar, for example).

The car is a 1999 Volvo V40 1.9 Turbo.

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  • So your question is about the impact of changing regulated fuel pressure on engine operation? Am I right in assuming that you don't want to address things like part durability/longevity in this question?
    – Zaid
    Commented Jul 28, 2015 at 9:34
  • Performance, economy and durability all interest me.
    – I have no idea what I'm doing
    Commented Jul 28, 2015 at 11:12
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    Unfortunately we can only address performance. There isn't enough information to objectively address things like economy and durability. Even if there was, these topics tend to be rather subjective, making them a poor fit for the site
    – Zaid
    Commented Jul 28, 2015 at 11:16
  • Understandable. I noticed some people suggest going for the 3 bar, as it's closer to the stock pressure, and other people suggest going for 3.5 bar, as going rich is preferable to going lean. Which one is the way to go?
    – I have no idea what I'm doing
    Commented Jul 28, 2015 at 11:26
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    Given the numbers you're dealing with, the rich/lean concern is highly improbable. I'd say stick with 3 bar.
    – Zaid
    Commented Jul 28, 2015 at 11:32

5 Answers 5

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You should be fine¹

The beauty of fuel-injection management systems is that they can compensate for slight deviations in operation via feedback.

The amount of compensation is commonly referred to as "fuel trim" (because the fuel injector pulsewidth is controlled ("trimmed") by the fuel-injection management). It is not uncommon for vehicles to accommodate upto ± 25% fuel trim before giving up the ghost and throwing an error code/CEL.

Getting a little geeky

Warning: Engineering calculations follow. Here be dragons.

A fuel injector could be approximated as pipe flow, so

sqrt( P1 / P2 ) α V1 / V2

In other words, doubling fuel pressure increases fuel velocity by 44% (√2 = 1.44)

If a 3 bar regulator is used, the relative change in fuel velocity is

√ (3.0 / 3.09) = √0.97 = 0.985

so the fuel trims should change by +1.5%. This is well within the compensation limits of any modern-day fuel injection management system.

You could even use a 3.5 bar fuel pressure regulator (6% fuel trim impact), though the closer you are to the pressure which the vehicle was designed for, the better.

¹ - This is assuming the rest of the vehicle is fine in terms of air-mass/MAP/lambda sensors, fuel pump, absence of vacuum/air manifold leaks, etc.

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  • is it 2.5% or 25%? the trim level?
    – Shobin P
    Commented Jul 28, 2015 at 10:17
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    @Anarach : Fuel trims up to 25% can be considered "normal" on some vehicles, though the CEL limit is something decided by the manufacturer.
    – Zaid
    Commented Jul 28, 2015 at 10:19
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    @Paulster2 : Your calculations look fine, but remember that the lambda sensors will pick up the difference and tell the ECU to dial back the injector pulsewidth
    – Zaid
    Commented Jul 28, 2015 at 10:23
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    The Lambda sensors can only do so much. It would be better to go with more pressure than with less, though. Not only would it be better to run a little rich (v. lean), an injector pulse width (IPW) can only be broadened so much before it is maxed out (in the case of lean). It is much easier for the ECU to pull back on the IPW. Ultimately, for a stock vehicle, the stock fuel pressure is going to be best. Also, you considering 3 bar v 3.09 bar is much less of an issue, considering I misread the OP thinking 3.5 was what was being suggested.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 28, 2015 at 10:32
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    @Paulster2 : Fair point. Better to fail rich than lean.
    – Zaid
    Commented Jul 28, 2015 at 10:35
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I'm afraid I can't say what effect changing the pressure regulator on your specific vehicle will have but I do know that many Golf VR6 owners (3 bar pressure regulator) add a slightly modified Corrado unit (4 bar) which sharpens up the throttle response.

I would assume that a small increase in pressure available to the injectors wouldn't cause a problem because the ECU is checking lambda so will adjust injector time to compensate. I would suggest that a big increase may cause the injectors to leak and that a decrease may mean there may not be enough fuel available at full load.

If it were my car, I'd fit the 3.5 Bar unit but like I say, I'm not familiar enough with Volvo fuel systems to be able to say for sure.

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    I doubt there's anything Volvo-specific going on here (they tend to use the Bosch fuel-injection philosophy). I'd recommend staying as close as possible to the design pressure, so 3.0 bar in this case.
    – Zaid
    Commented Jul 28, 2015 at 10:17
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Getting the fuel pressure right is absolutely critical for how the engine operates. The reason why is it directly affects the amount of fuel the injectors can pass when open. You can affect fuel delivery in three ways:

  • Change the injector (increase size)
  • Change the pulse width (increase how long the injector stays open)
  • Change the fuel pressure (flow change is directly proportional to fuel pressure change)

Since you are increasing the pressure, the flow output would increase as well.

You can calculate the exact amount of fuel increase you'll see at the injector by using this equation:

New Flow = SQRT(New Pressure/Old Pressure) * Old Flow

I'm not going to run through the math here, but you can find a calculator at this page.

In your case, you are talking about increasing the pressure about 1/2 bar, or ~7psi. I'm going to use the case of increasing the pressure from 43.5 to 50.5 psi. If the injectors originally flowed 240 cc/min (22.84 lbs/hr) they would now be flowing 258.59 cc/min (24.6 lbs/hr). This is a large difference in the amount of fuel you'd be putting into the cylinder. More fuel than the fuel map and O2 sensors could overcome without a retune. You'd be running very rich.

One of the ways you could overcome this issue is by replacing your old regulator with an adjustable fuel pressure regulator. This would allow you to tune the fuel pressure to it's original pressure without too much of a fuss. There are many options out there for this, as is told in this Google search.

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    I believe certain fuel injection systems alter their pulse width in response to the value received by the O2 sensor and on such a system, provided that the system has scope to alter the pulse to the required level, the ECU would effectively compensate for the pressure difference.
    – Steve Matthews
    Commented Jul 28, 2015 at 10:35
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    I will caveat my answer by stating I misread the original post. If you are considering dropping to 3.0 bar regulator, it is not that big of an issue and the computer will pick this up and adjust accordingly. What @Zaid said in his post is spot on.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 28, 2015 at 10:35
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The answer is "depends".

In general, Direct Injection systems (both gas and diesel) are usually more sensitive to a deviation in fuel pressure from the norm.

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As others have stated, OBDII vehicles (1996 and newer) can adjust the injector pulse width (PWM) to maintain a stoichiometric (the "ideal" 14.7:1) air/fuel ratio. Some older systems also had this capability to a limited extent.

Having a slightly higher fuel pressure is good as the higher pressure means the computer adjusted amount of fuel will be delivered quicker and will atomize better.

The PWM of the injectors is known as Duty Cycle. Most systems are designed around 80-85% Injector Duty Cycle. Higher fuel pressure will result in a lower Duty Cycle for the same amount of fuel delivered.

Lower duty cycle will result in less wear and tear and heat produced by the injector, potentially leading to longer service life.

Another way to improve air/fuel mixture is to upgrade to newer injectors that have more holes in the nozzle. 12 Hole is common on modern vehicles, 4 hole injectors appeared some time in the 90s.

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  • Welcome, and nice answer. Keep in mind that not all cars know the fuel pressure via a sensor reading, they are "hardcoded" to the stock fuel regulator's pressure. So if you up the fuel pressure it will attempt to adjust the PWM accordingly but may not run right if the pressure change is drastic enough. Also, if there's an open loop mode (sometimes Wide-Open Throttle) that will also be off.
    – Nick
    Commented Sep 7, 2018 at 2:08

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