# Pressure Limiting Valve (High Pressure Fuel Rail)



## tautvydasv (Mar 13, 2007)

Hi guys, well I had mine changed like two months ago. Symptoms were, that the car was not able to built pressure in high fuel system. 

Well unfortunately, now again I have same symptoms + after logging I saw that I have problems with low fuel pressure aswell. For example driving at 70 miles per hour I have only 1,8 bar there... 

How about others? Do you often have limiting valves failing after upgrading to apr/awe/autotech fuel pumps? Well it is not an easy job to change it, so if it is going to be failling every month or second, then maybe it would be reasonable to upgrade to rs4 one?


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## CoNGLoMeRaTeGTi (Apr 4, 2007)

I've upgraded to the RS4 valve and logged 134 (+-1 bar). 

Car has ran like a champ since. 

Sounds like you may have a LPFP issue.


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## tautvydasv (Mar 13, 2007)

Now I have two problems 

On idling I have low pressure around 6 bar, however still high pressure system pressure is fluctating between 6-14 bar, so it is really too low. 

You changed to RS4, because you had a malfunctioning one, or just wanted to make an upgrade?

At the moment my car is at the dealer. I have autotech upgrade kit installed. So again if dealer is going to want to change my HPFP, I will need to reassemble it and this is so annoying. It is practically nothing that can fail there... There are two possibilities how it can be loosing pressure due to the high pressure pump failure. First is broken plunger, second is leaking seal around the plunger (there is a third one high pressure indicator, but then you would get an error). However in the first case, engine would make an additional sound and the car would not be able to drive. On the second event happening, you would have a lot of fuel beeing flushed into the engine, therefore you should experience a big increase in oil quantity. So I do not understand dealers, who always willing to change hpfp. Though it is not going to solve a problem.

P.S. Recently had a call from my dealer. He is going to replace high pressure release valve. When asked about low pressure dropping when cruising on highway, he said that it is because of bad release valve as it is not allowing to build up pressure, therefore lpfp has a lot of job. Well that sounds logical. By the way, I will try to swith to rs4 limiting valve. We will see, hope that will help.


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## Uber-A3 (Feb 23, 2002)

have you logged lpfp sounds like intank is going out. This is a problem and VW revised the old pump. I had to replace mine it was dipping down to 2.6 bar under demand.


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## tautvydasv (Mar 13, 2007)

Well we will see. I still have couple of months of warranty left, so now they will change the high pressure release valve.

However, I really think that there might be a problem with intank lpfp aswell. Also I will change lpfp fuel filter, maybe that one is completely clogged. However when the release valve is broken, it is like a hole in your fuel rail, therefore lpfp might be not supplying you with fuel in that case.

By the way mine is not very old, it is 2008 pirelli edition.


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## Uber-A3 (Feb 23, 2002)

tautvydasv said:


> Well we will see. I still have couple of months of warranty left, so now they will change the high pressure release valve.
> 
> However, I really think that there might be a problem with intank lpfp aswell. Also I will change lpfp fuel filter, maybe that one is completely clogged. However when the release valve is broken, it is like a hole in your fuel rail, therefore lpfp might be not supplying you with fuel in that case.
> 
> By the way mine is not very old, it is 2008 pirelli edition.


What is your rail pressure just cruising and high demand? The lpfp feeds the hpfp so unless the demand is high for the hpfp there is now way the relief valve is sending it all back. The relief valve is in the high pressure rail so if the hpfp isn't being requested at a high rate the lpfp should have no problem keeping up. Also makes more sense that the lpfp is going bad rather than 2 relief valves. You need to log lpfp duty cycle and pressure, this will tell you for sure.


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## CoNGLoMeRaTeGTi (Apr 4, 2007)

Uber-A3 said:


> What is your rail pressure just cruising and high demand? The lpfp feeds the hpfp so unless the demand is high for the hpfp there is now way the relief valve is sending it all back. The relief valve is in the high pressure rail so if the hpfp isn't being requested at a high rate the lpfp should have no problem keeping up. Also makes more sense that the lpfp is going bad rather than 2 relief valves. You need to log lpfp duty cycle and pressure, this will tell you for sure.


That is false. The FRP relief valve only opens when the FRP exceeds 120, 130, or 135 bar depending on which valve. During partial throttle daily driving the valve remains sealed and the LPFP does minimal work. This fuel system is returnless!


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## tautvydasv (Mar 13, 2007)

CoNGLoMeRaTeGTi is 90% right! You have fuel returning when after going WOT you release accelator. But in general it is really returnless, that is why a hole in it can bring your lpfp pressure from 4-5bar to 1,8 bar.

In general I am not able to meet any requested pressure  So it is the fault of high pressure release valve. 

For example on idle I have only 7 bar instead of 20 with some misfires, though the lpfp pressure is around 7 bar  If I press on accelerator I get the requested fuel pressure, but it is only when standing. However when driving and I have injectors opening more ms due to load, I get problems I hardly go, over 70 bar, though I am requesting 129,9 bar 

It will be clear after I change that valve, it will be done on friday. I will put in the rs4 valve, that is rated at 135 bar. Hopefully this will live longer as it will have a little bit less work


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## jpiwko58 (Dec 25, 2008)

tautvydasv said:


> CoNGLoMeRaTeGTi is 90% right! You have fuel returning when after going WOT you release accelator. But in general it is really returnless, that is why a hole in it can bring your lpfp pressure from 4-5bar to 1,8 bar.
> 
> In general I am not able to meet any requested pressure  So it is the fault of high pressure release valve.
> 
> ...


what are your mods>? you have a HPFP mod? Because if so it'll stress the cam follower that much more. well even w/ out an after market one you're still going to put alot of stress on the cam follower... Not saying it's not worth it but just letting you know. Good luck bro.


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## tautvydasv (Mar 13, 2007)

Cam follower is not a problem here. It is usually a problem in states due to using not certified oil and bad quality fuel (tfsi engine is one of the most engines diluting oil with fuel..). I use Castrol LL3, which I think is even unavailable in states.

In case of follower fault there is different behavior and of course different sound from engine. 

I have autotech fuel pump kit, it is really good I have changed my cam once after 20 000miles of using it with the pump. Though it looked practically the same as before when modifying the pump.


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## tautvydasv (Mar 13, 2007)

Today my dealer has changed pressure limiting valve. Also after some modifications(I have K04 car), I have installed bsh intake. And I am back to more than normal  By the way I have persuaded them to use audi rs4 valve. We will see how this one will hold.


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## Uber-A3 (Feb 23, 2002)

CoNGLoMeRaTeGTi said:


> That is false. The FRP relief valve only opens when the FRP exceeds 120, 130, or 135 bar depending on which valve. During partial throttle daily driving the valve remains sealed and the LPFP does minimal work. This fuel system is returnless!


I know that the valve is mechanical and only opens when the limit is reached. I meant that unless the valve was stuck open the car would have little problem achieving the requested at cruising highway speed. I never said it returns it to the tank. it returns it to the low pressure side of the pump.

Good to see you fixed your issue!


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