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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.

I have no idea what I'm doing
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5 Answers5

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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.

Community
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Zaid
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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.

Steve Matthews
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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.

Pᴀᴜʟsᴛᴇʀ2
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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.

kmarsh
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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.

WingMantis
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