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Audi A6 Estate 2.5TDI 2000reg intermittent start problems

Khi Jun 27, 2010

  1. Khi

    Khi New Member


    I'm a noob to this forum, but I'd really appreciate any adive or help you knowledgeable people could give me.

    I've recently bought a 2000 reg Audi A6 estate, and it's pretty obviously I've been done by the last owner. I'll try to describe the syptoms as best I can.

    When I met them to buy it, the engine was running, so I turned it off to let it cool for a bit. Started and stopped it while I inspected the vehicle but could find nothing too untoward. I then drove 5 hours from Wales back to Brighton with no issue (I once stopped and turned the engine off to refuel at a fuel station).

    We drove it on and off the next couple of days with no issue, then suddenly one morning it wouldn't start. It turned over repeatedly, however wouldn't fire. The battery eventually ran pretty low so I had to stop trying. A few days later we hookep up the jump leads to a mates car, and it turned over for aaages, then suddenly fired! I turned it off, then started it again no worries. I waited till late that night to go out and start it once more, and again it started no problem. A couple days later we drove it around, put loads of diesel in it, filled the screen wash etc, then parked it again. The next day, once again it wouldn't start.

    It is booked to go into the garage, but I'm trying to think of everything I could to help the guy find the fault.

    There is a clear line that connects to the pipe from the tank, and is before the fuel filter, this showed occasional bubbles going through it, which made me think there was an air leak in the pipes coming from the tank. I looked all over the pipes but couldn't find any diesel weeping out of any joints. I filled the car with diesel, and the bubbles appeared to stop, which made me think maybe it was something in the tank causing bubbles (apparently the tank is split into two, and a small pump balances the levels in the two tanks, sometimes an issue with this pump can cause the side that feeds the engine to run dry). However the day after filling the tank is when it wouldn't start for the second time.

    Other factors include:
    - There is white smoke coming from the exhaust when it does run, I don't know if diesels are the same, but it makes me think it's running lean (again fuel supply issue?).
    - There is a high pitched whine coming from the engine when driving around - I can't figure out if that's just the turbo or something else
    - There doesn't appear to be any power issues when driving, it seems pretty pokey and drives ok when it's going
    - On the two occasions it refused to start, it happened when starting from cold (ableit it's mid summer at the moment). The first time it ticked over for a couple of seconds, then shuddered and stopped. The next time it was ticking over for a while, while I faffed around with seatbelt etc, then when I went to pull away it stopped
    - Both times it failed to start the car was facing uphill - although it did start on other occasions in the same spot
    - I have a receipt from the last owner saying the fuel pipes, fuel filters and oil filter were recently changed (makes me think they were trying to eliminate air leaks)

    Reading all around the forums etc I've found many suggestions, including
    - Faulty coolant temperature sensor
    - Faulty fuel pump (although why would it run fine sometimes but not others?)
    - Air leaks in the pipes

    The car doesn't go in to the garage until the 6th July, so anything you guys could suggest would really be appreciated.

    I've obviously been royally done over by the last owner. :(

    Thanks in advance for taking the time to read this! Any other info I can give I'm happy to crawl around the car/engine and find out for you if it helps you to diagnose.

    Cheers guys
  2. Khi

    Khi New Member

    Ooh also another suggestion was faulty immobiliser?
  3. fjtwelve

    fjtwelve Moderator Staff Member Moderator

    Go to the diesel forum on here, theres an extensive post on the white smoke issue, likely to be your crankcase breather is blocked
    As far as intermittent starting from memory there are posts on this problem on VWaudiforum.co.uk and a number of reasons why this can happen both mechanical and electrical.
    You need to find someone local with a VAGCOM he know how to use in exchange for some drinking vouchers. This may throw up faults which will point you in the right direction
    Be prepared to travel to get to someone who really knows this engine well, adamss24 is a frequent poster on the diesel forum and is in London.
  4. bigfruit

    bigfruit New Member

    Hi Khi.
    I also have a 2.5 TDI V6 Audi A6.
    I once had the same problem.
    You have to check if all the ingition plugs are working. If so, it could be air in the system. It can get in though the diesel return hoses. If those are ok, check the retaining valve that is mounted on the diesel filter.
    So to conclude, if the car only has a hard time starting after it´s park for a long time, it can only be the ignition plugs or air in the diesel circuit.

    Sorry about my poor english, but I´m from Portugal.
    Hope I helped.
    See ya ;-)
  5. Gavin0478

    Gavin0478 Active Member Team Daytona Team V8 VCDS Map User quattro Audi Q7 S-line owners group TDi

    think he means glow plugs not ignition plugs.
  6. Khi

    Khi New Member

    Hey thanks for the posts guys. Just to update you, the issue is still present, but we know a bit more now.

    There was a clear hose going into the filter, which showed a big air bubble, so "bigfruit" you were right, it's obviously air in the system. Many of the fuel pipes have now been replaced, and a genuine VW filter has now been installed (previously it was a ford filter for some reason!?), and the system primed. The car was running fine on a long test drive, and on tickover. It was then left facing uphill again, and the next day an air bubble had formed again! I then got the mechanic to do pressure tests in each section of the fuel system - it was found that the system seemed to hold pressure everywhere except the fuel pump itself. However, since carrying out this test, the car has been parked facing uphill for ages (almost a week now), and starts every time. The clear bit of hose (now installed after the filter) has no sign of any bubbles. It's really flummoxed me as to how it would leak one day, then the next hold fuel in the system with no air for a week following (when all that happened was a pressure test - unless the pressure test moved a seal somewhere).

    What we're planning to do next is a vacuum test, rather than pressure test, as apparently some seals will hold pressure one way, but not a vacuum the other (and vice versa). I'll vaccum test each section of the fuel system, and see if it holds vacuum. Also put a clear bit of hose directly into a container of diesel and plug the other end straight into the inlet of the fuel pump. We'll run it for a while then turn it off and let it sit - see if any bubbles appear in that bit of clear hose (removes all other variables/parts of the fuel system). We'll prob do a vacuum test on the pump as well.

    Anyway any further suggestions would be welcome. I've read in other posts that lifter pumps can be a culprit in these circumstances, however apparently these audis don't have a lifter pump, they have a pump in the tank, but it does something else internally within the tank (which I don't yet understand, working on that one!).

    We also desperately need the car next weekend, so until we find the root cause, I'm tempted short term to install a diesel primer pump bulb into the system so at least we can get going again if air gets into the system...

    PS Engine is code is AFB
    Last edited: Jul 22, 2010
  7. Watty

    Watty ..... (_!_) Sarah's A3s #1 Fan

    I've recently taken my cover off and have also noticed air bubbles coming through the short length of clear pipe, but mine starts and runs perfectly and I think this is common. When you boot it, it will run full of fuel with no air, but on tick over and low revs, it only drags through little diesel, most is recirculated...., so I don't think that's the issue.
  8. Khi

    Khi New Member

    It's true, I could see occasional bubbles when it was running, however when it was refusing to start the bubble went from a bubble to a completely empty pipe. No diesel in it whatsoever - a dribble of diesel would appear every now and then, but that was it!
  9. Watty

    Watty ..... (_!_) Sarah's A3s #1 Fan

    Sounds like the fuel pump then, possible on it's way out if it's itermittent. See what the garage say when it goes in..
    Last edited: Jul 24, 2010
  10. The Slug

    The Slug Active Member

    Sounds like fuel pump to me too...goodluck hope it's gets sorted out soon
  11. Khi

    Khi New Member

    We're pretty sure it's the fuel pump now. The guy at the garage had a vacuum pump attached to the outlet of the pump, clean diesel with no bubbles was going into the pump, and there were bubbles in the diesel as it came out of the pump.

    None of you guys could help in identifying the right part number for the pump that should be used on this car could you?

    Audi A6 Avant estate V6 2.5TDi
    2000 reg
    engine code: AFB

    I'm worried that the pump that's already on there was possibly the wrong pump for the car anyway, so would be worried about sourcing another pump with the same number!

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