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Car sometimes doesn't start after being driven earlier

IrnBro Sep 17, 2020

  1. IrnBro

    IrnBro Registered User

    Hi, my 2007 A6 3.0 TDI has developed an annoying problem.

    For the first trip of the day, the car starts fine and takes me wherever I want to go. However, after driving to wherever and doing whatever, upon returning to the car it sometimes refuses to start for around 30 minutes to an hour. After this time has elapsed, it then returns to being able to start normally. So I can potentially get stuck somewhere for an hour which is not ideal.

    Other information;

    - For the first fail, engine cranks and it cranks strongly, but engine doesn't start, and after 4-6 cranks it stops. It cranks more times than it would if the car were to start normally. For subsequent attempts to turn the engine on, it cranks fewer times before "giving up." The cranks don't get weak, they maintain the same level. I dont make a habit of trying to turn it on multiple times after the first fail.

    - Around 8 months ago I put petrol in a totally empty tank, and have had a number of problems since then. I fix them as they pop up but I assume this is related to that (since all other problems have been related to that.)

    - I have taken it to an independent garage to obtain fault codes, but no relevant codes appear, and the garage are struggling to figure out what it could be. They think starter motors, but since the problem has not appeared with them, it feels like they are guessing.

    - I have tried to ascertain a pattern in the type of drive that seems more likely to cause this fault, but dont have a huge sample size. It might be that longer drives make it more likely to occur since 3/4 times it's happened there has been an above average amount of driving. However it also occurred once with a short drive.

    Thanks for any help
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  3. Retroman

    Retroman Audi A3 2010 Sportback 2.0 TDI 170 (CBBB engine)

    Unlike diesel, petrol is a strong solvent and so would release a lot of gunk. If the diesel fuel filter hasn't been changed for a while - I would do so as the petrol may have caused it to be dirtier (thereby restricting flow) than normal.

    I've known a mistaken petrol fill damage glow plugs but that would show up on a cold start.

    Engine management sensors which are starting to fail (eg ones attached to a hot engine like a cam position sensor) sometimes start to suffer from "heat soak" i.e. malfunction when getting very hot - although I would expect an error code or dash warning light.
    IrnBro likes this.
  4. B5NUT

    B5NUT Well-Known Member VCDS Map User

    I had a very similar fault with my A6, it was very hard to start after the engine had got up to temperature. In the end I replaced the injectors and that cured the issue, they were also weeping fuel and there would be a lot of white smoke when the engine was warm and the car was at idle.

    It also won't be the glow plugs as they only kick in if the engine is cold.
    IrnBro likes this.
  5. IrnBro

    IrnBro Registered User

    @B5NUT and @Retroman
    Thanks for your replies
    I called up my garage to inform them of your replies but before I had a chance they told me they had identified the crank sensor as the issue. Both it and starter relays have been replaced in my car, which seems to be functioning now, although I havent taken it on any extended drives to be sure that the issue is resolved. However, it does seem to start up a lot more decisively than it used to.

    Thanks for replying anyway, I will bear your comments in mind for any future issues which will probably arise.

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