Does anyone know what engine procedures "Pressure release in rail" conducts in VCDS?

[abamfo]

Does anyone know what engine procedures "Pressure release in rail" conducts on the engine in VCDS-Engine-Settings?

It tells you to hold the brake and accelerator down and it conducts some form of test / procedure on the engine. Would greatly like to know what it is if anyone knows! Cheers in advance

[SwankP3RF3ct10n]

Most likely releasing all pressure in the fuel rail by not engaging the low pressure fuel pump in the tank and probably opening the injectors. This is just an educated guess though (as to what it would do to release the pressure), but whenever you see the terms pressure and rail in relation to an engine, it's talking about the fuel rail/fuel system.

[abamfo]

Most likely releasing all pressure in the fuel rail by not engaging the low pressure fuel pump in the tank and probably opening the injectors. This is just an educated guess though (as to what it would do to release the pressure), but whenever you see the terms pressure and rail in relation to an engine, it's talking about the fuel rail/fuel system.

Thanks! Outside of the fuel injectors, what role do you think the PCV plays in releasing pressure in the fuel rails, if any?

[jmaddr]

Ask Ross-Tech. They are very helpful and should know what this does.


Sent from my iPhone using Tapatalk

[SwankP3RF3ct10n]

Thanks! Outside of the fuel injectors, what role do you think the PCV plays in releasing pressure in the fuel rails, if any?

The PCV plays no role in releasing fuel rail pressure. The PCV is simply/usually a one way valve which is part of the emission control system and is usually in the part of the system that takes crankcase vapors and plumbs those back into the intake tract of the motor. Audi's PCVs tend to be a bit more intricate than a simple PCV valve, but the principal is the same. The PCV system on this engine is below the supercharger and plumbs directly into the charger itself in the same housing as the twin screw impellers.

[abamfo]

The PCV plays no role in releasing fuel rail pressure. The PCV is simply/usually a one way valve which is part of the emission control system and is usually in the part of the system that takes crankcase vapors and plumbs those back into the intake tract of the motor. Audi's PCVs tend to be a bit more intricate than a simple PCV valve, but the principal is the same. The PCV system on this engine is below the supercharger and plumbs directly into the charger itself in the same housing as the twin screw impellers.

Thanks for your response! In the end I asked Ross Tech but they had no reali idea what it did.

For me, it seems that whatever the part which releases pressure in the fuel rail (for example when decelerating) isn't performing 100%. Outside of the fuel injectors, is it the HPFP which could be letting me down?

I'm by no means an expert but surely there is a pressure release function for the fuel rails?

[SwankP3RF3ct10n]

Thanks for your response! In the end I asked Ross Tech but they had no reali idea what it did.

For me, it seems that whatever the part which releases pressure in the fuel rail (for example when decelerating) isn't performing 100%. Outside of the fuel injectors, is it the HPFP which could be letting me down?

I'm by no means an expert but surely there is a pressure release function for the fuel rails?

Dude, what are you talking about? Because what I understood from your OP is that you were just wondering what the terminology meant... not that you actually had a problem. If you have a problem, state that and ask about that... but that's not what I understood. The fuel system doesn't lose pressure while you're decelerating or while the engine is engine-braking (i.e. injectors fully closed and engine is still running due to mechanical lock/drive of the engine). The LPFP is electronically controlled and pumps fuel into the fuel system from the tank to the engine bay where its pressure in raised by the HPFP which is mechanically driven by a CAM lobe. The only time pressure is ever released in the system and usually only partially and if anything just on the low pressure side is if the ignition is completely Off and hence the LPFP has not current running it. Audi's fuel system is returnless, so I believe what happens is that LPFP has a range of volume that it can send to the HPFP. Since the HPFP has one level of pressure increase static to the same pumping as engine RPM, it stands to reason that Audi could be controlling how much fuel they need via telling the LPFP to back off or ramp up on amount... or they could just be controlling it all via shorter injector pulses if they need less fuel in the engine which would be calculated in relation to the fuel rail pressure, but eventually too much would build up and you couldn't back off enough. That's why I say that since they can't control much with a mechanical pump's volume on the HPFP, it would be done on the LPFP side of things, so in that case, lower pressure from that one can only be ramped up so much by the mechanical one at any given/respective engine RPM giving less volume of fuel to the injectors. Higher pressure from the LPFP or more volume, would give much more fuel once ramped up by the HPFP at any given/respective engine RPM. As long as you now that more pressure at 1ms of injector pulse width would be more fuel than less pressure at the same 1ms of injector pulse width, you can make sense of what I'm saying.

So, with all that said, if you've got some sort of fueling issue where you're not getting enough fuel, then chances are good your LPFP in the tank is not up to snuff to keep up with the volume needed for the mechanical HPFP up in the engine side of things. Either you've got a leak somewhere in the system or that pump in the tank is just weak or not up to the task for some reason. But again, I don't think you said anything about having a problem, so I'm just guess as to what you might be getting at here at this point.

[S4'ed]

The only way that the pressure in the rail is reduced (with the engine running) is the normal operation/opening of the injectors. I suspect the VAG-COM "Pressure Release" function is to allow the bleed-off of fuel rail pressure if a Tech wants to replace a fuel injector, or the fuel rail high-pressure sensor, or the High Pressure Fuel Pump. This command probably just opens all the injectors (when the engine is not running).

[SwankP3RF3ct10n]

^^^ I agree. The high pressure side in the rail is between the injectors and the HPFP, so there's no way to bleed the pressure off short of just opening the injectors. Well, that or disconnecting the LPFP and trying to start or run the engine. Eventually it'll starve out and die from no fuel and hence no fuel pressure in the rail.

[abamfo]

Dude, what are you talking about? Because what I understood from your OP is that you were just wondering what the terminology meant... not that you actually had a problem. If you have a problem, state that and ask about that... but that's not what I understood. The fuel system doesn't lose pressure while you're decelerating or while the engine is engine-braking (i.e. injectors fully closed and engine is still running due to mechanical lock/drive of the engine). The LPFP is electronically controlled and pumps fuel into the fuel system from the tank to the engine bay where its pressure in raised by the HPFP which is mechanically driven by a CAM lobe. The only time pressure is ever released in the system and usually only partially and if anything just on the low pressure side is if the ignition is completely Off and hence the LPFP has not current running it. Audi's fuel system is returnless, so I believe what happens is that LPFP has a range of volume that it can send to the HPFP. Since the HPFP has one level of pressure increase static to the same pumping as engine RPM, it stands to reason that Audi could be controlling how much fuel they need via telling the LPFP to back off or ramp up on amount... or they could just be controlling it all via shorter injector pulses if they need less fuel in the engine which would be calculated in relation to the fuel rail pressure, but eventually too much would build up and you couldn't back off enough. That's why I say that since they can't control much with a mechanical pump's volume on the HPFP, it would be done on the LPFP side of things, so in that case, lower pressure from that one can only be ramped up so much by the mechanical one at any given/respective engine RPM giving less volume of fuel to the injectors. Higher pressure from the LPFP or more volume, would give much more fuel once ramped up by the HPFP at any given/respective engine RPM. As long as you now that more pressure at 1ms of injector pulse width would be more fuel than less pressure at the same 1ms of injector pulse width, you can make sense of what I'm saying.

So, with all that said, if you've got some sort of fueling issue where you're not getting enough fuel, then chances are good your LPFP in the tank is not up to snuff to keep up with the volume needed for the mechanical HPFP up in the engine side of things. Either you've got a leak somewhere in the system or that pump in the tank is just weak or not up to the task for some reason. But again, I don't think you said anything about having a problem, so I'm just guess as to what you might be getting at here at this point.

Thanks for your lengthy response and yes you are correct, in fact I do a few problems with my car. It has been at Audi since last Friday and they have't come back to me with a solution. The problems are covered in the link below but I didn't want to bother this thread with it, really just wanted to do some investigate work on that VCDS test

http://www.audizine.com/forum/showth...3#post11752233

I've read through all of your comments and it seems to all make sense to me now. For background, the reason why I was asking is because there seems to be at times strong variances between my actual and specified fuel rail pressure under different RPM loads, and where there happens to be larger variances there are misfires, albeit not perfectly correlated. This is why I am looking into the fuelling system to work out what might be causing the issues. The problem is that I have replaced the LPFP and although this did slight improved my fuel trims, it did not solve the misfiring issue. The reason I was asking about the "Pressure release in rail" test on VCDS was because while conducting it (which asks you to brake and accelerate in P like most of them do) it never finished and the car misfired like crazy and it felt like it was running very rich and idling rough. I definitely don't know enough about fuelling systems or cars for that matter and probably shouldn't have conducted it in the first place but it lead me to think what is this test doing and why didn't the car pass. I am now thinking that the fuel injectors might be clogged after reading the above comments.



Thanks for everyone's input, it's great that you help each other out!