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Starting issue on 2003 8L S3

Dan Blowes Nov 23, 2018

  1. Dan Blowes

    Dan Blowes New Member

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    Sorry I know this is one of my first posts, but I know that this forum is friendly and helpful, and im lookimg forward to being part of this community for long term ( I love my 8L S3) but I need some assistance and I feel someone else may have had this issue. I have a intermittant starting issue on my stage 2 8L S3 (BAM), seems fuel related, from cold start it will crank and crank and just hiccup, but not always start. No codes stored, all sensors are okay (have changed crank sensor, CTS, tried another good MAF), no air leaks, checked fuel pressure and on first prime only gets 1 bar to the rail, upon key off and reprime back up to 3 bar and starts fine. Once engine is off, pressure drops like a stone from the rail (inlet side, not return), checked FPR and all okay (tried a new 4 bar as well with same issue) What would you expect rail pressure wise on first prime? Car has a DW uprated pump, is there a check valve on the pump unit in the tank to maintain pressure in fuel line? Any help would be great as it's driving me crazy, thank you everyone.
     
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  3. Stuart B

    Stuart B Well-Known Member

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    how have you scanned for faults? with VCDS or a generic reader?

    it does sound like a crank position sensor issue from threads I have seen popup on ASN and TTF over the last few years (apart from the hiccup at the end)

    but you clearly have all the meters etc to see a fuel pressure change - so I will back away :D
     
  4. Dan Blowes

    Dan Blowes New Member

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    Yes mate, no codes whatsoever, did some logs whilst cranking, and that was all good, I have a feeling it is to do with the fuel pump that maybe the check valve is knackered or the pump isn't quite installed right, this is then allowing the fuel to drain back to the tank, so the first prime is just filling the lines and the rail, but not to a high enough pressure to allow starting.

    Sent from my SM-G960F using Tapatalk
     
  5. Stuart B

    Stuart B Well-Known Member

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    is there a pump on the passenger side too linked to the main swirl pot behind the driver.

    I dont suppose the fuel filter is fitted backwards is it?
     
  6. Stuart B

    Stuart B Well-Known Member

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    also someone on here previously fitted the fuel pump pipes around the wrong way but I dont think the engine started at all then..
     
  7. Dan Blowes

    Dan Blowes New Member

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    I changed the fuel filter and is definitely round the right way, I believe there is only one pump and that pulls fuel via a tube from one side of the tank to the other to fill the swirl pot reservoir, the issue happens regardless of fuel level

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  8. Stuart B

    Stuart B Well-Known Member

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    I think you do electrically plug in the passenger side tube into the swirl pot electronics when you fit the pump, but if it happens all the time surely unrelated to that side anyway.
    I didn't think there was a check valve just pump->inline filter->fpr->fuel rail and it primes for about 5 seconds when you put the ignition on, as when I changed my fuel filter it took about 4 or 5 tries to get the fuel to start the car.

    I might be tempted to check the fuel pump is sealed correctly as the pump is fitted into the existing swirl pot and has o rings top and bottom if I remember rightly.

    I think you need an expert (not me) as you already have done all the basic stuff I would have tried, I am sure there'll be someone here with some advice.
     
  9. <tuffty/>

    <tuffty/> Badger 5 Edition...Its all about the flow... Staff Member Moderator Audi S3

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    This happen at any fuel level more often than not?

    Its been known for this issue to manifest on half a tank or less and be fine above that... this would suggest the fuel is draining back from the pumps swirl pot..

    As its had a pump change it may suggest that the seals may not have been installed correctly or the balance connection might be loose..

    Might be worth checking the pump installation...

    <tuffty/>
     
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  10. Dan Blowes

    Dan Blowes New Member

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    It does seem to manifest itself worse when below half a tank, I'm going to whip the pump out as the conclusion I came to was the swirl pump bucket is emptying so the priming is really just filling that rather than sending fuel to the front. I know the pumps good as it is only about 1.5 years old and is the uprated DW pump

    Sent from my SM-G960F using Tapatalk
     
  11. Dan Blowes

    Dan Blowes New Member

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    Fixed!!! Completely un-fuel related in the end, was the Forge dump valve getting stuck, making the car have a massive air leak so it wouldn't fire, changed it out and now no issues at all and the car is more responsive

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  13. <tuffty/>

    <tuffty/> Badger 5 Edition...Its all about the flow... Staff Member Moderator Audi S3

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    DOH!

    <tuffty/>
     
  14. Stuart B

    Stuart B Well-Known Member

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    thats weird there was no VCDS fault? it usually shows a "pressure difference between the something and something - Check DV!!" <-- I cant remember the exact fault. I had this with my OEM 710N on the S3..
     
  15. Dan Blowes

    Dan Blowes New Member

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    There was no codes stored, but I think that is because I have a cold side relocation. Car has been 100% perfect since I removed the forge valve and goes so much better

    Sent from my SM-G960F using Tapatalk
     

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