Nick (ASH-SPEC) wrote: FUEL PRESSURE

In order to check the fuel rail pressure, you need a pressure sensor put inline with the fuel line to the fuel rail. The pressure sensor defined above will allow us to do this, but we have to connect it in. Since the fuel system always holds some degree of pressure, even when the car is not running, we must ensure that we don’t unhook a line a pour raw fuel into the atmosphere, or into a hot engine bay. We start first by cutting off the fuel to the filter as such.



Since the rest of the system is also under pressure, we must clamp off the other side where we are going to disconnect the fuel line from and tie in the pressure gauge. We also use a rag here because even in this short piece of hose, there remains enough pressure to spew a catastrophic amount of fuel out and start a fire. Ideally, one would do this while the engine is cold. Relieving as much pressure as possible by removing the gas cap also helps.
Once done, place your rag as follows and loosen the hose clamp and remove the hose from the filter.



In the VG30, as well as most other fuel injected vehicles, have an operating fuel pressure of ~3bar. 1bar = 14.5psi so 3bar is effectively ~43.5psi. The aspect that complicates this simplicity is the fact that the manifold is not always at 0psi. In fact, it is rarely at 0psi. Since the tip of the injectors is inside of the manifold, this means that the vacuum or pressure that the injector tip 'feels' also affects the fuel delivery. Since the ECU controls the duration of time that the injector is held open, it assumes that the fuel pressure is always the same, there must always be a 43.5psi differential between the fuel rail pressure and the manifold pressure. This is to ensure that no matter what vacuum/pressure the manifold is under, an 'x' millisecond pulse of the injector will always deliver the same amount of fuel. You can see what I mean here:



Ok, now that you have everything connected, you are ready to test your fuel pressure. You can see in this picture that there is about 10psi of pressure on the gauge. The engine is not running here and hasn't been started since the install of the gauge, but you can now see why I pinched off the hoses. 10psi of fuel pressure will puke enough fuel to start a Sonny's BBQ in your driveway so BE SAFE!

Now, in this picture you can see that the fuel pressure is appx. 33.5psi. Remember the pressure differential I was talking about? Well, at idle, the manifold is at about -10psi of pressure. In order to maintain linear fuel delivery, there must always be approximately a 43.5psi pressure differential; so 35 + 10 = 45psi. We are good here.



This next picture is a demonstration of how the fuel pressure regulator works. The hose I am holding in my hand is what connects the fuel pressure regulator to the manifold. The fuel pressure regulator is the device that maintains this 'pressure differential' such that the fuel delivery is linear per pulse-width of the injector. Since I have unplugged the FPR(fuel pressure regulator), the FPR 'thinks' the manifold is at 0psi. You can see here that the fuel pressure has now risen to ~44psi, as it should.



I want to point out that since the fuel pressure control systems are a 'passive' system and not 'active', there will always be slight variations in fuel pressure from what you see here. However, there should not be anything greater than about a 5psi difference in these tests. This is primarily what makes the difference between one Z and the next - some fuel systems simply work a little better/worse than the next Z, but the actual effect on the system as a whole is marginal as long as there aren't large variances.

I have setup the fuel pressure gauge as well as a manifold pressure gauge to demonstrate how the fuel pressure regulation system works in finer detail.



You can see here that the fuel pressure is slightly higher at 0psi of manifold pressure than it was in the original test. This is due to the fact that the ECU ALSO varies the fuel pump voltage (which affects its output). You can see here that at 45MPH with the manifold at 0psi, the fuel pressure is at 55psi. This is actually a little on the high side as we should be seeing a fuel rail pressure of ~44psi at 0psi of manifold pressure, but this is due to the fact that I am still using a non-turbo fuel pump controller in my car (my car was converted from non-turbo). However, this is not bad, if anything, it is simply safer. In this condition you should see at least 44psi at the fuel rail. If you see less than 44-45psi, you have a problem and it must be fixed.

In the following picture you can see that the manifold is at 5psi of pressure and we ALSO see that the fuel pressure has risen up from 55psi to 60psi. This is the fuel pressure regulator at work. Since the manifold is at 5psi more pressure, it also raises the fuel rail pressure so that the fuel delivery per injector pulse-width is consistent. This is important as the ECU is assuming that no matter what pressure the manifold is at, 'x' millisecond of injector pulse-width will ALWAYS deliver the same amount of fuel. This is very important when tuning a car too.



The next test is running the engine at 7000RPM, which is at 95% of its operating range. You can see that the manifold pressure is at ~15psi and the fuel rail pressure is at ~65psi. This is 10psi more at the fuel rail than when the manifold was at 5psi of pressure. 55+10 = 65psi of fuel rail pressure. This is consistent with what we should see.



IMPORTANT!! : The engine is running at 7000RPM here in the above photo. This is when the fuel system is at 95% of its expected delivery rate. You have to consider that as the engine RPM increases, so does the fuel rate. When I converted my non-turbo to turbo, I used the non-turbo fuel pump. It worked great at ~14psi. However, when I raised the boost to 16psi on the non-turbo pump, as the RPM increased to around 5500RPM, I began noticing the fuel pressure falling off all the way down to 45psi! This is VERY BAD! The reason this occurred is because the non-turbo fuel pump was unable to keep up with the demand of fuel at higher RPM. It maintained ~65psi until around 5000RPM and then sharply fell off at 5500RPM down to 45psi at the fuel rail. This is a catastrophic failure waiting to happen because when the fuel pressure falls, so does the fuel delivery. This is not a problem with the fuel pressure regulator; this was simply the non-turbo pump falling short of what was needed. I corrected this problem by putting a twin-turbo fuel pump into my car.

THIS IS THE PHENOMENON THAT YOU DO NOT WANT TO SEE!
You want to ensure that the fuel pressure is maintained ALL THE WAY THROUGH THE RPM RANGE that your engine operates within. If it does not maintain this pressure, the fuel delivery will fall and this will cause a lean condition. Lean conditions lead to detonation, broken pistons, burned valves, and catastrophic engine failure.

This test concludes the 'fuel delivery' aspect. If your fuel system does not maintain proper pressure, you simply need a bigger pump.

-Ash Powers
ASH-SPEC Performance & Tuning