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Strange rough idle wont go away

Jenno007 Apr 30, 2014

  1. Jenno007

    Jenno007 Well-Known Member

    I'll try keep this as short as possible whilst giving all the details.

    I have a 1999 A3 AGU. It is experiencing a rough idle. Car shakes quite a bit whilst idling which is driving me nuts (the revs fluctuate a little but nothing serious). I have cleaned the MAF and throttle body. I have checked for leaks and started replacing the rubber hoses with silicone.

    I recently got this code
    16500 - Engine Coolant Temp. Sensor (G62): Implausible Signal
    P0116 - 35-10 - - - Intermittent

    I have been trying to use my vag com device to find the issue however the AGU doesn't support blocks 31 and 32 which would help identify a leak. However I found something a little unusual. In group 7 the Lambda and Voltage fluctuate wildly (consistently up and down) whilst the car is idling (took a picture if that would be any help to you). I'm not sure if this is normal but it didn't seem right to me. Can anyone confirm this?

    Can anyone recommend any blocks I should log that would give me a clear indication of what could be causing the problem. Or any other sensors / things to check.

    Otherwise I'm going to have to start replacing things (I was thinking in this order)
    Coolant tempt sensor > Thermostat > Spark Plugs > Fuel Filter > ???

    I would GREATLY appreciate any help what so ever! Thanks guys
  2. pj123

    pj123 Member

    Faulty CTS is a common cause for rough idle, you had a fault on the CTS so replacing it may well fix the issue.

    Note cleaning MAF does not necessarily mean that the MAF is not a fault. Log using VCDS to check you get the required flow rate.
  3. Jenno007

    Jenno007 Well-Known Member

    I did some logs checking the flow of the maf and I got a flow of around 190 (I forget the units now) at the red line and that seems reasonable to me based on what I've heard. I'd be interested to know why the CTS can cause a rough idle but I've ordered a new one and a new thermostat just to be safe.

    Thanks for the suggestion!

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