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wobbly idle

Discussion in 'A4/S4 forum(B5 Chassis)' started by pjw, Jan 9, 2011.

  1. pjw
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    pjw Member

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    for some reason my audi has a wobbly idle, especially when ive been driving and i dip the clutch. the idle is slightly rough but it does dip below the 1k mark to about 700rpm then sort of bounces up and down..

    has anyone got a suggestion of what this could be? ive just replaced the ignition system, but im thinking a vacuum leak? i cant see anything obvious, ive looked and anything without a jubilee clip etc ive replaced. just unsure of what to do next..

    i did see a post from audi regarding the ADP engine and a small hose from the throttlebody to the inlet causing this, but i cleaned it up and fitted new clips and it didnt rectify it.
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  2. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    I had this on the 1.8T. It would start then bounce up and down between 1000 and 5-600rpm before slowly settling down.

    It would also drop down to around 500rpm when approaching trafficlights or roundabouts when cold.

    Was no fault codes or anything, but mark suggested it could be the tank vent valve, so i swapped that with a spare i had handy and it fixed it.
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  3. pjw
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    pjw Member

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    Really? wow. What sort of price am i looking at for one?

    You mean the fuel tank?
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  4. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    You could try unplugging it, or pinching off the pipe to try and rule it out before replacing?

    On the 1.8T it sits above the airbox, and connects to a pipe that runs down into the wheel arch liner.
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  5. pjw
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    pjw Member

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    I think i know the one, when i was looking at vacuum pipes i remember seeing one that goes into the drivers wing. I was going to just to a throttlebody alignment with vagcom (good idea? i have had the inlet pipework off and replaced the battery in the last week) and audit the pipes again, i noticed that mpg is pretty poor at the moment and im hoping theyre related.
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  6. pjw
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    pjw Member

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    TBA sorted the wobble out a little and made it a little better, found the valve you were on about but the it started pouring down so not had chance to do anything with it. It has an electrical connection going to it though - should it not come up on the engine scan if its duff?
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  7. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Mine didnt.
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  8. nwmlarge
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    nwmlarge flat out

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    when mine did it i had to fix a vacuum hose which went under the intake runners, was attached to a valve around the 3rd one back.
    then it happened again in a different way and that time it was the maf, changed that and it was and still is fine.

    mines a 1.8t so may have a different vacuum routing to yours
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  9. pjw
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    pjw Member

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    MAFs just been changed and comes up with a correct reading on the sensors bit of vagcom. Ill try disconnecting this vacuum control valve, or at least take a trip to the scrappers this weekend. Cant seem to find any secondhand ones on ebay or anything. Ive had a look at the hoses but it could be anywhere, throttlebody seal, hoses etc etc.

    Could this cause poor mpg?
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  10. NATE6874
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    NATE6874 Member

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    Sounds like the problem I've been having for a couple of months now... After I disconnected the battery and changed the coolant. been running dodgy ever since. tried vagcom scan and no errors. tried the throttle body realignment and made no difference. The vacuum hoses do they have a small electrical solenoid attached that ticks?
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  11. pjw
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    pjw Member

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    Dont think it ticks. Thats probably your problem? Im going to the scrappers and try and replace mine, see if it makes any difference.
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  12. pjw
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    pjw Member

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    OK, ive cleaned my N80 vent valve thing in the dishwasher and it hasnt made a difference.

    One thing ive noticed is that when you look at the top of the butterfly where the throttlecable connects, i rev it up and then when it settles the actual metal arm thing on the top of the throttlebody wobbles - so its the actual throttlebody moving, not it letting air in by accident?

    The vent purge valve is a genuine bosch one but doesnt do anything, it doesnt tick or make a noise, and when i took it off i tried to suck and blow in either end and its blocked. Should it work one way? Im going to try the trick suggested on another post of running about with the petrol cap loosened.

    If i remove the vent valve so theres a big vacuum leak the engine is like a bag of spanners. I just want to say this isnt how it is usually - the idle is only wobbly by about 60rpm at idle and when it drops down after a rev it wobbles by about 200rpm for about five seconds. My main concern is that a 1.6 petrol is giving me about 25mpg at best, and im hoping whatever this problem is when i sort it i can see normal mpg again.
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  13. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    If the TB is the same arrangement as the 1,8T, there is a big plastic cam which the throttle cable attaches to , and in the middle of the plastic cam is a metal segment.

    The metal segment is the end of the actual throttle butterfly shaft, and is connected to the idle control valve as well. As a result you would kinda expect it to move, as the ICV is messing with the idle.

    25mpg would point to a different issue imo. Perhaps a MAF sensor on its last legs or a dodgy thermostat/CTS. What do the fuel trims look like in vagcom?
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  14. pjw
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    I dont know, i did actually try and look at the fuel trims using rosstechs guide but it wouldnt connect to the channel number. Think ill give it another go and see if theres an alternative. I did manage to view the MAF, lambda readings etc so i must have done something wrong. I owe you a beer sometime mate.

    Would the duff MAF and CTS not come up on a DTC scan though? Im sure i read something about the MAF reading 80% of your BHP, might give it all a check over.
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  15. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    if you were in the area to view the maf and lambda you were in the right bit to view the fuel trims, just need to keep flicking thru until you find them.

    The MAF outputs "grams per second" of airflow, and in general this translates pretty linearly to horsepower with a 1.25 multiplier.

    IE a peak MAF figure of 80g/s would mean 100hp.


    to get a decent idea, you can do a log of the MAF readings on a third gear pull, from 3000rpm to the red line, and then view the log to see the highest recorded MAF value

    CTS might appear in a scan, but MAF often doesnt. The reason for this is that it doesnt pack up completely, they just start reading low. Say instead of 80g/s it might only output 60 or 70 even though 80 is actaully flowing thru it into the engine.
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  16. pjw
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    I just went out and the g/s of the MAF registers 80% of the Hp. I did a redline run and it went up to about 80gs, which is about right. The coolant temperature doesnt show properly on the page 02 (it showed 2.0), but if i flick through it shows as 87.5 degrees, which is right for the gauge and right for the car being warm.

    For some reason i cant access the fuel trim screen 032, it comes up with cannot access when at idle and when the ignitions on but not running.

    The only one thing that struck me as odd was it showed "12.0 BTDC" which implies the timing is retarded? Or is this done from the knock sensor?

    It also shows that the TankVent is on, which indicates it must be working. Its getting more than annoying now but i dont want to just go throwing sensors in it and spending a fortune if i can work out which needs sorting.
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  17. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Timing is almost always before TDC. 12 degrees is about normal for an idling engine, mid range on cruise and you'll probably see over 30degrees.

    the block numbers are ECU specific, so 032 might be normal for the motronic unit used in a 1.8T but not neccesarily the same as the ECU used on your 1.6.

    If it says Tank Vent on and the vent valve isnt ticking then its duff.
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  18. pjw
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    Ok thanks for the info. Its 100% not making a noise at all, and ive listened to it cold and hot.

    Off to the shops i go!
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  19. pjw
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    Just to be sure ive taken the valve apart and stripped it down, and aside from the coil inside there isnt much that looks worn, (i only say the coil as i cant actually get in to see, but everything else is back together). Still makes no sound at all. Is it supposed to be loud?

    Could there be anything else? I think im going to change the CTS just to be sure.
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  20. pjw
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    pjw Member

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    Just to confirm (as ETKA says theres two senders) the dash is showing the right temp when hot - 90 - so is this sensor the one for the ECU?

    [​IMG]

    I just want to make sure ive got the right part, i remember buying a sensor for an XR3i a few years ago and it only registered to 84 degrees - the thermostat opened at 89!
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  21. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    If the dash is showing 90 and Vagcom is showing 90 then the CTS will likely be fine.

    The sensor you've highlighted does look like the correct part though.
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  22. NATE6874
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    I cleaned the MAF with electrical cleaner. it's made no difference.
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  23. pjw
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    pjw Member

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    Just went to the scrappers and got an N80 valve off an A6. Sorted it right out. Aragorn, YTMND! I have a spare now too which i might put up for sale, ended up with two.

    Ticks away nicely now though, was deffo the problem.
    Last edited: Jan 22, 2011
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