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thermostat + CTS sticky in the FAQ - helpful?

Discussion in 'A4/S4 forum(B5 Chassis)' started by Tom_B, Oct 22, 2013.

  1. Tom_B
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    Tom_B Member

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    [Oct 22, 2013]
    I just finally got round to replacing the thermostat on the 1.9 having owned the Car for just over a year and it had the classic wandering temperature guage symptoms = would not warm up beyond 80 deg and then would cool down on a run + Oil temp guage showing same temps and sometimes just not show any temp reading at all. When scanned with VCDS lite it showed a code on the Coolant Temperature Sensor (CTS) which I replaced with a genine VAG item. THe temp guage now read all the time but still wandered. Replaced thermostat and car now comes up to 90 deg oparating temperature within 10 minutes and the oil temp also reaches 90 deg shortly after. First run out driving 20 miles to York and back the DIS shows 60 mpg which it has never done even on careful motoway runs when 56 mpg is the best it has done.

    As problems with wandering temps are a very frequent question being asked on the forum would it be helpful to have a sticky on the FAQ section that Scott set up?

    so possible content could be:


    • Wandering temps are a very common symptopm on B5s especially as they are all now more than 12 years old.
    • check for a CTS fault using VCDS - the lite version will pick up fault codes.
    • If car shows wandering temps on gauges most likely to be lazy thermostat
    • replace thermostsat with genuine VAG part
    • replace CTS if necessary with genuine VAG part - don't forget to order the 'o' ring as VAG don't give you one
    doing the work on the 1.9

    • replacing the thermostat on the 1.9 is very simple and takes about 20 mins all in - the thermostat is just below the alternator - follow up the radiator hose to be sure where it is. Two allen headed bolts hold it in.
    • you will lose some coolant - so have some distilled water and G13 to replace lost coolant ( dont mix G13 with anything else)
    • The CTS is located at the back of the engine block and is held in with a plastic circlip - only disconnect the exhaust recirculation pipe and with a bit of wiggling you can get the old CTS out - I found that getting the CTS with the connecting plug in place easier than trying to get the the plug off in situ
    • put it all back together and check for leaks - run the engine up carefully to make sure you have enough coolant in the system.

    doing the work on the v6 2.5 TDi
    • replacing the CTS is simple because it is at the front of the engine block and is accessible with the cover off. A bit of wiggling will get you there.
    • Thermostat - much more complicated - I have not done this myself so I dont know what it involves - usually done with a timing belt change? PaulFs thread about fitting a remote thermostat seems to imply that its such a pig of a job its easier to do the remote thing. Those who know more can you add in what you know?

    Has anyone come across some good pictures to illustrate?

    When those who are more expert and knowledgable could comment on whether what I have written is correct then I can cut and paste into a new thread which Scott might add to the FAQ if he thinks helpful.
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  3. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

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    [Oct 22, 2013]
    The CTS fault you had probably wasnt actually a faulty CTS.

    Everyone makes that mistake. In my experience the CTS almost never fails.

    What happens is the ECU notices the temperature signal wandering around, and decides that the reading doesnt make any sense, so flags up a fault code. But ofcourse the readings actually perfectly accurate, as the fluctuations are caused by the thermostat!
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