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  • Anyone know what this sensor is?

    Just like the title says. It’s located at the back of the valve cover on the passenger side. I’m not sure it’s supposed to be there.

    Vehicle is a 1990 Audi coupe Quattro with the 2.3 20v


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  • #2
    It's an air temp sensor. It's used by the fan relay, in conjunction with water temp from the MFTS, to run the fan after turning off the engine and protect the alloy head from distortion.
    It's definitely supposed to be there.

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    • #3
      Yup, engine ambient air temp sensor.
      Panthero Coupé quattro 20vt
      Indigo ABY coupé
      Imola B6 S4 Avant

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      • #4
        Should be a temperature depending switch instead of a sensor.
        '95 Audi S2 Avant
        '92 Audi Coupe T89 quattro 2.3E

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        • #5
          Thanks guys! I’m dealing with a issue where the car won’t return to idle. I was wondering what it was. I found out I don’t have a coolant temp sensor so I ordered one of those from oreillys. Hope that will fix the issue. If I have any more issues along the way you’ll probably see more threads made.


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          • #6
            Check the idle switch on the throttle body.
            Check it works and that it engages when the throttle closes.
            Panthero Coupé quattro 20vt
            Indigo ABY coupé
            Imola B6 S4 Avant

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            • #7
              Does it return to Idle eventually?

              The idle control valve can slow down if it is dirty or even stick solid especially if it has been left standing for a while.

              A good blast out with solvent, ensure the slide valve moves and has no surface damage.

              A slow return to Idle may also be an excessively rich mixture. I have had exactly this recently while trying a whole load of things to get my fuelling right. I had a set of injectors on for test which were making it run rich. The lambda control system struggled to bring things back into spec but did do eventually. Revs would come down, loiter around 1500rpm then come slowly back to 900.

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              • #8
                Originally posted by steve briance View Post
                Does it return to Idle eventually?

                The idle control valve can slow down if it is dirty or even stick solid especially if it has been left standing for a while.

                A good blast out with solvent, ensure the slide valve moves and has no surface damage.

                A slow return to Idle may also be an excessively rich mixture. I have had exactly this recently while trying a whole load of things to get my fuelling right. I had a set of injectors on for test which were making it run rich. The lambda control system struggled to bring things back into spec but did do eventually. Revs would come down, loiter around 1500rpm then come slowly back to 900.
                It does return to idle eventually or sometimes dies. I was thinking a coolant temp sensor would help since it needs to know how cold the engine is to know how much fuel to add. Since there’s not even one connected that could be causing an issue.

                I can try cleaning out the idle control valve. The car has been sitting inside of about 4 years but that valve was never connected to the car. It was just sitting in a box. I couldn’t understand why the car wouldn’t idle, then I found that idle control valve and now it idles so I was pretty happy about that.


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                • #9
                  Originally posted by Error404 View Post
                  Check the idle switch on the throttle body.
                  Check it works and that it engages when the throttle closes.
                  That’s the green wire right? It does engage when the throttle is closed.


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                  • #10
                    The ECU uses a temp sender in the back of the head

                    S2 Coupe 3B Project


                    Ur quattro restoration

                    S2 Avant

                    Boost is the new rock and roll!
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                    • #11
                      Originally posted by newsh View Post
                      The ECU uses a temp sender in the back of the head
                      Ok. Well mine was gone lol. The car has a long history behind it. The head was rebuilt 4 years ago and nobody put the sensor back in.


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                      • #12
                        Originally posted by HONDATUNING776 View Post

                        Ok. Well mine was gone lol. The car has a long history behind it. The head was rebuilt 4 years ago and nobody put the sensor back in.


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                        Ah!

                        S2 Coupe 3B Project


                        Ur quattro restoration

                        S2 Avant

                        Boost is the new rock and roll!
                        sigpic

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                        • #13
                          Doesn't the coolant just flow out without the sensor?

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                          • #14
                            Originally posted by Bowie69 View Post
                            Doesn't the coolant just flow out without the sensor?
                            Yes unless the hole has been plugged

                            S2 Coupe 3B Project


                            Ur quattro restoration

                            S2 Avant

                            Boost is the new rock and roll!
                            sigpic

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                            • #15
                              Originally posted by Bowie69 View Post
                              Doesn't the coolant just flow out without the sensor?
                              It’s currently plugged. We didn’t know a sensor went there at the time.


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