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Fuel Pressure

Discussion in 'A3/S3 Forum (8L Chassis)' started by imported_VaulterTim, Dec 21, 2005.

  1. imported_VaulterTim
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    imported_VaulterTim Guest

    [Dec 21, 2005]
    Hi all,

    Posting this here since more people read this forum but it's also in the A4 section...

    Does anyone know if there is a non-return valve in the fuel line/system anywhere? I'm sure I remember reading previously that there is...

    Haynes manual also has a value for residual fuel pressure after a certain period of time... after engine is switched off... so that would also fit in with the previous theory.

    The reason I ask is because lately I have had problems starting my car which seem to be fuel starvation related.

    It will start then burble and cough for a few secnds then die. If I give it a little throttle on startup it still coughs but starts far more easily than when I don't. When it has started it runs fine.

    I have an aftermarket fuel rail fitted with a pressure guage attached so I am able to easily monitor fuel pressure at all times.

    When the ignition is turned on I get fuel pressure registered on the guage but after the initial priming of the rail, the pressure then drops off to almost zero... I am sure it didn't do this previously and even when the car had been standing a while there would be 'a' pressure registered on the guage.

    Any help/suggestions/ideas/feedback of personal experiences would, as always, be greatly appreciated.

    Cheers

    Tim
    #1
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  3. RichA3Turbo
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    RichA3Turbo ...Watching you! Staff Member Moderator

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    [Dec 21, 2005]
    [ QUOTE ]
    Hi all,

    Posting this here since more people read this forum but it's also in the A4 section...

    Does anyone know if there is a non-return valve in the fuel line/system anywhere? I'm sure I remember reading previously that there is...

    Haynes manual also has a value for residual fuel pressure after a certain period of time... after engine is switched off... so that would also fit in with the previous theory.

    The reason I ask is because lately I have had problems starting my car which seem to be fuel starvation related.

    It will start then burble and cough for a few secnds then die. If I give it a little throttle on startup it still coughs but starts far more easily than when I don't. When it has started it runs fine.

    I have an aftermarket fuel rail fitted with a pressure guage attached so I am able to easily monitor fuel pressure at all times.

    When the ignition is turned on I get fuel pressure registered on the guage but after the initial priming of the rail, the pressure then drops off to almost zero... I am sure it didn't do this previously and even when the car had been standing a while there would be 'a' pressure registered on the guage.

    Any help/suggestions/ideas/feedback of personal experiences would, as always, be greatly appreciated.

    Cheers

    Tim

    [/ QUOTE ]

    Tim,

    Have you checked/swapped the FPR? I believe this is the only thing that controls pressure (other than the pump itself)... Pretty sure there is nothing else in the system

    Rich
    #2
  4. imported_VaulterTim
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    imported_VaulterTim Guest

    [Dec 21, 2005]
    I haven't swapped it but the fuel pressure is fine at idle with vacuum connected and constant at idle with vacuum removed so I can only surmise that the FPR is not 'leaking'.

    I do have the original so will try that when I get time but... the one I have in there is only 7months old (adjustable Eurospec one) so I won't be too happy if it's failed already /ubbthreads/images/graemlins/frown.gif

    Cheers

    Tim
    #3
  5. dummi
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    dummi smoking a6

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    [Dec 21, 2005]
    Tim, the adjustable fpr's do not have a linear fuel curve, the spring in the adjustable fpr is 'crushed' on adjustment , you'd be best of with the stock one, or the bosch 5 bar. plug it in and let the computer unlearn the adjustable fpr and you'll be fine
    #4
  6. dummi
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    dummi smoking a6

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    [Dec 21, 2005]
    btw stock s4 or s3 is 4 bar if you can't find the 5 bar.
    #5
  7. imported_VaulterTim
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    imported_VaulterTim Guest

    [Dec 21, 2005]
    I'm not sure I understand what you mean there... 4bar is 4bar is it not? If the guage shows 4bar with the vacuum hose removed then the fuel pressure is, for all intents and purposes, 4bar.

    Just to avoid confusion... the FPR I have is not continually adjustable, it is adjustable to a set pressure that you can select by, well, 'adjusting' it! /ubbthreads/images/graemlins/grin.gif I don't think there is a spring in this one (maybe there is, I haven't dismantled it to find out), it has a nozzle on the end that the vacuum hose attaches to which can be screwed in or out and then locked off with another small nut.

    You can see it here

    As you rightly say, the standard FPR on A4's is a 4bar item (the S3 is 3bar, like all other A3's. I know that because I just removed one).

    I will pop mine back in as soon as I get time just to see if it makes a difference. I'm sceptical to say the least though!

    Time will tell I guess...

    Cheers for the feedback /ubbthreads/images/graemlins/smile.gif

    Tim
    #6
  8. dummi
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    dummi smoking a6

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    [Dec 21, 2005]
    in conventional fpr's and adjustables they have a spring, with the vacumm it alters rate of pressure build up , regulated by the spring, if you crush the spring it will respond at a different value, this is what you are in effect doing. to get 5 bar from a 3 bar you would need to increase the spring compression, in a 'non adjustable' it simply has a spring with a higher value of compression, in an 'adjustable' the spring is compressed, i hope that is simple enough for you

    i have an s3 engine here with a 4 bar, my a6 s4 engine has 4 bar, my last a6 had a4 engine with a 3 bar as stock, thats what i'm going from.
    #7
  9. imported_VaulterTim
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    imported_VaulterTim Guest

    [Dec 21, 2005]
    [ QUOTE ]
    in conventional fpr's and adjustables they have a spring, with the vacumm it alters rate of pressure build up , regulated by the spring, if you crush the spring it will respond at a different value, this is what you are in effect doing. to get 5 bar from a 3 bar you would need to increase the spring compression, in a 'non adjustable' it simply has a spring with a higher value of compression, in an 'adjustable' the spring is compressed, i hope that is simple enough for you

    [/ QUOTE ]
    Gotcha.


    [ QUOTE ]
    i have an s3 engine here with a 4 bar, my a6 s4 engine has 4 bar, my last a6 had a4 engine with a 3 bar as stock, thats what i'm going from.

    [/ QUOTE ]

    Interesting... and thinking about it, I actually think the older-engined A4's are 3bar although I'm probably wrong there too! Not sure about the A3/S3 arrangement now, food for thought...
    #8
  10. johnmv55
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    johnmv55 Member

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    [Dec 22, 2005]
    You need to monitor the fuel pressure AFTER you have turned the ignition off, The car should be able to maintain it's fuel pressure for a set time, Porsches used to have to maintain half the working pressure (2 bar/30psi) for I think 1 hour after turning the engine off, If the fuel pressure falls away with a stopped hot engine the fuel will boil in the fuel injectors/rail, and the car will run like a dog on the next start up untill all the vapour etc has dissapeared and pure fuel is again present.
    #9
  11. Grant
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    Grant Well-Known Member

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    [Dec 22, 2005]
    [ QUOTE ]
    btw stock s4 or s3 is 4 bar if you can't find the 5 bar.

    [/ QUOTE ]
    Are you sure?

    I thought the S3 and TT was a 3bar? People usually upgrade to 4bar.

    What benefit would I see in changing to a 4bar system?
    #10
  12. dummi
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    dummi smoking a6

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    [Dec 22, 2005]
    [ QUOTE ]
    [ QUOTE ]
    btw stock s4 or s3 is 4 bar if you can't find the 5 bar.

    [/ QUOTE ]
    Are you sure?

    I thought the S3 and TT was a 3bar? People usually upgrade to 4bar.

    What benefit would I see in changing to a 4bar system?

    [/ QUOTE ]

    well i definently have a 4 bar on my s3 engine, not sure about all variants

    benefits allow mapper to be more flexible maybe get you 4-5 hp more out of a bolted on k04 on a 150 or 180 1.8t engine,

    the adjustable you have Tim you would need a custom map with it to make it work, aftermarket maps are based on the non-adjustables that come stock. if you are going to have a remap soon at that time you could bolt the adjustable back on. to get your fueling back to normal for now put the stock one back on
    #11
  13. imported_VaulterTim
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    imported_VaulterTim Guest

    [Dec 22, 2005]
    [ QUOTE ]
    You need to monitor the fuel pressure AFTER you have turned the ignition off, The car should be able to maintain it's fuel pressure for a set time, Porsches used to have to maintain half the working pressure (2 bar/30psi) for I think 1 hour after turning the engine off, If the fuel pressure falls away with a stopped hot engine the fuel will boil in the fuel injectors/rail, and the car will run like a dog on the next start up untill all the vapour etc has dissapeared and pure fuel is again present.

    [/ QUOTE ]


    I have monitored the pressure after switching off the engine... as I said in my initial post it drops off to almost zero when it has been standing and even when I have turned the ignition on, after the initial priming of the system it will fall off again. The Haynes manual states 2.5bar for I think, 30mins.




    [ QUOTE ]
    [ QUOTE ]
    [ QUOTE ]
    btw stock s4 or s3 is 4 bar if you can't find the 5 bar.

    [/ QUOTE ]


    Are you sure?

    I thought the S3 and TT was a 3bar? People usually upgrade to 4bar.

    What benefit would I see in changing to a 4bar system?

    [/ QUOTE ]


    well i definently have a 4 bar on my s3 engine, not sure about all variants

    benefits allow mapper to be more flexible maybe get you 4-5 hp more out of a bolted on k04 on a 150 or 180 1.8t engine,

    the adjustable you have Tim you would need a custom map with it to make it work, aftermarket maps are based on the non-adjustables that come stock. if you are going to have a remap soon at that time you could bolt the adjustable back on. to get your fueling back to normal for now put the stock one back on

    [/ QUOTE ]


    Again... I am sure as standard that all the A3 range has a 3bar FPR as standard... lots of people will upgrade to the 4bar but that will not yield any performance gains at all simply by plugging it in... That will, at worst, make the car run rich but the ECU will more than likely correct the fueling automatically anyway.

    It is sometimes required to increase the cars' fuel pressure if a tuner has exhausted the duty cycle of the injectors installed in the car. So rather than running the injectors at 100%(or greater) duty cycle at standard system pressure you can raise the pressure and back off the duty cycle thus making tuning easier and the fuel system more flexible and easier to manipulate.


    So... moving on from our little side conversation /ubbthreads/images/graemlins/wink.gif... back to the initial problem!

    The car ran fine when I first installed the adjustable FPR... has done for 7months... it has only recently started it's antics. It has now also gained kangaroo petrol characteristics /ubbthreads/images/graemlins/burningmad.gif Same lumpy startup and fine at constant throttle at mostly any speed but it has gained a nasty hesitation (on/off/on/off etc.) if the throttle is depressed, hard or soft. This seems to go away after a while though.

    I'm starting to think the MAF may be at fault but that still does not account for the lumpy startup scenario. I've yet to swap the FPR's to solidly ascertain whether or not my adjustable one has developed a fault so the two MAY actually be unrelated... but I doubt it.

    It could also be a clogged fuel filter I guess...

    Also, I had to put normal crappy unleaded in it a week or so ago, albeit only £5-worth (because I practically ran out and the Optimax was off /ubbthreads/images/graemlins/frown.gif) but I hated doing it and it's been in the back of my mind ever since.

    Any more ideas gents?

    Cheers

    Tim
    #12
  14. dummi
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    dummi smoking a6

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    [Dec 22, 2005]
    well, failing any of those cures above, check the sensors on vagcom, one of my lambda sensors went some months ago and idle was very lumpy
    #13
  15. johnmv55
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    johnmv55 Member

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    [Dec 22, 2005]
    If the fuel pressure is not holding thats whats wrong with the lumpy starting, fuel pumps normally have a built in non return valve, also your fuel pressure reg could be passing, if either of them is leaking and it will run like a pig after being left.

    A blocking fuel filter normally shows first at the point in the rpm range were max torque occurs,

    my money's on the fuel reg !!!!!!!!!
    #14
  16. imported_VaulterTim
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    imported_VaulterTim Guest

    [Jan 23, 2006]
    *Update*

    I finally traced the fault - definately a lack of fuel pressure, i.e pressure not being retained after the engine has been switched off.

    Pressure should be maintained at 2.5Bar for 10minutes after switching off the engine. I have no pressure at all immediately after switching off the engine, as indicated by the fuel pressure gauge I have fitted to the fuel rail (which I have also found to be reading low as it happens, not much help!)

    In last few days I have traced this fault even further to a damaged O-ring seal on the end of the FPR which was letting fuel leak past and therefore not allowing fuel pressure to be maintained... I duly swapped the damaged O-ring for a new one but to my disappointment the fuel pressure still did not hold /ubbthreads/images/graemlins/confused.gif

    So... I removed it yet again and found that the new O-ring had also been damaged! /ubbthreads/images/graemlins/burningmad.gif

    This seems to be due to a rough or sharp edge on the inside of the fuel pressure regulator adapter which tears off a small piece of the O-ring every time the FPR is installed. This seems to be unavoidable and regardless of how gently I insert it, how much I lubricate it or how many precautions I take, no amount of care can ensure that the O-ring is not damaged. I have repeated the process numerous times to the same effect.

    I've e-mailed ATP (who I bought all these bits off) to see what their response is but tomorrow I am going to try chamfering the inside edge of the smaller FPR Adapter opening (the one that accomodates the tip of the FPR) once I have ascertained how far into the housing the tip of the FPR sits... I need to remove everything first!

    So, problem traced which makes me feel a little better but it now needs to be resolved... [​IMG]

    Cheers

    T



    Here are a few pictures...


    Eurospec FPR1

    [​IMG]



    Eurospec FPR2

    [​IMG]



    O-ring closeup

    [​IMG]



    OE and Eurospec FPR Comparison

    [​IMG]



    FPR O-ring Detail1 (if you can see it!)

    [​IMG]



    FPR O-ring Detail2

    [​IMG]



    FPR O-ring Detail3

    [​IMG]
    #15
  17. darnich
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    darnich Member

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    [Jan 23, 2006]
    Don't know if that helps but I had similar problems from a dodgy coolant temp sensor....
    everything was back to normal immediately after its replacement
    #16

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