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  1. #1
    Active Member Two Rings
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    Faulty thermostat or faulty engine coolant temperature sensor?

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    Hi I am trying to find out where the fault lies in my car. I have an Audi A4 2.4 V6 B5 1998 with climate control.

    A few weeks ago when motorway driving the temperature gauge dropped to about 70 degrees C - I ignored it assuming the cold weather was the reason.

    Then a couple of days ago the temperature gauge stopped moving altogether and now stays on minimum.

    I think my engine is running a little rough and sometimes when I switch the engine on when it is still warm it has a little trouble starting - not a nice clean start like normal. It starts perfectly every time when cold.

    Then the blowers packed in but came back a day later - i think this may be a separate fault. The blower motor may be on it's way out.

    When the blowers started working again i immediately turned the heat up to full blast and the air that came out was very hot just like normal.

    I have been told that my thermostat is stuck open, but why would i get lovely hot air coming through if the thermostat is at fault? When i done the test it was on a cold engine and i immediately started driving - within minutes the hot air was coming through.

    So two possible faults but which is it?

    Thanks in advance

  2. #2
    Active Member Two Rings Corzee's Avatar
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    Mar 27 2012
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    Thermostat just controls coolant flow to the engine. If the thermostat was bad, it should have failed in the open position, which just means it will take your car slightly longer to get up to operating temperature. It wouldnt cause you to run bad.

    CTS however has a two-fold purpose. It controls the coolant temp gauge, (which would explain it not working properly) and is also provides high(?) priority temp/mixture info to the ECU, which would explain you running rough on starts. Correct me if Im wrong guys.

  3. #3
    Established Member Two Rings Buckle's Avatar
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    I don't know which it is, but changing a thermostat doesn't take more than maybe 2 hours, and doesn't cost more than $50. So why not start there? It's cheaper than buying a new engine.
    Delta Sigma Phi '05

  4. #4
    Veteran Member Four Rings walky_talky20's Avatar
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    ^The thermostat is behind the timing belt on the OP's car, so it's actually not a very quick job. Definitely something you don't want to do unless you have to (or are already in there for the t-belt/water pump).

    Based on the running/starting problems and good heat in the cabin, I would lean heavily toward a bad coolant temp sensor. The sensor is very cheap, and very easy to replace as compared to the thermostat. So even if I was leaning toward the thermostat being faulty, I would still probably advise you change the CTS first just in case.

    Another tell-tale sign here is the oil temp. If the coolant temp shows cold, but the oil temp shows warm - that is usually indicative of a bad CTS. The oil runs through an oil/water cooler. Generally speaking, when the t-stat fails open, the coolant runs cold which causes the oil to run cold as well. This usually means that neither gauge moves very much. If, however, the coolant gauge shows cold, but the oil temp gauge reads warm like normal - you probably have a bad sensor.
    ^Don't listen to this guy, he's not even a mechanic.
    2001 Laser Red A4 1.8TQM, 5-Speed Swapped, APR 93, Splitter, electronic oil pressure gauge, B6 Sport 17's, ST Coilovers,

    "You need to know what you are doing...Its like a rainbow jungle in there." - Redline380

  5. #5
    Active Member Three Rings
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    Well

    My money is on your coolant temp sensor

    Coolant temp sensor registers the temperature of the engine to help making better fuel/air mixture for the engine. According to your description about the heater and the sensor gauge, it seems like your sensor is dead. if your thermostat is stuck open, heat is going to blow luke warm and it will not warm up properly but your needle will go up at least a quarter of the way, not stuck at the lowest position.

    Low Coolant Temp registered to ECU via Coolant Temp Sensor -> ECU thinks engine is cold -> ECU tries to help warm up by making the mixture richer -> Rougher run in some conditions and also fuel mileage issue

    Stuck Open Thermostat -> Longer time to warm up-> ECU thinks out side temp is cold -> ECU also again tries to help warming up the engine by making richer mixture -> Higher fuel consumption.

    Since your gauge is not even working, I will just replace the CTS it doesnt cost more than $20 but changing it might require you to take the back end of the intake manifold off(throttle body +vacuum hose stuff) in order to gain better access or move all the hoses out of the way and work from there... either way I've seen DIY on 1.8T ones but I dont think ive seen one for V6 (which I have) but It should be the same procedure, it will be just a bit harder to get around.
    P.S. There are some ghetto fix (I mean GHETTO) for stuck open thermostat. you just simply place a cardboard box infront of the radiator to kill its heat transferability to help warm the engine up. ghetto right? fix the thermostat if it is stuck open.
    1996 Audi A4 Quattro 2.8 12V

  6. #6
    Registered Member One Ring
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    if you do change it you might as well do the timing belt and water pump

  7. #7
    Active Member Two Rings
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    Wow, so glad i registered with this forum. Thanks for the replies, yeah the oil temperature does reach normal and stays there. Doesn't really fluctuate and get's to normal temperature in the usual time expected.

    I previously called the audi dealer and they quoted me 37 for the sensor, do you think this is a fair price or is there somewhere else i can get genuine parts?. I just didn't want to order it in case it turned out to be the thermostat. I'll go ahead and order the sensor when they open tomorrow.

    Does anybody know where this sensor is located on the 2.4 V6 model? And is it a job I could do or are there any how to's I could work from?

  8. #8
    Senior Member Three Rings Artiemas's Avatar
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    North Branford, CT

    Quote Originally Posted by plugin View Post
    Wow, so glad i registered with this forum. Thanks for the replies, yeah the oil temperature does reach normal and stays there. Doesn't really fluctuate and get's to normal temperature in the usual time expected.

    I previously called the audi dealer and they quoted me 37 for the sensor, do you think this is a fair price or is there somewhere else i can get genuine parts?. I just didn't want to order it in case it turned out to be the thermostat. I'll go ahead and order the sensor when they open tomorrow.

    Does anybody know where this sensor is located on the 2.4 V6 model? And is it a job I could do or are there any how to's I could work from?
    http://www.ecstuning.com/Search/Cool...sor/ES2535414/ 5.99 is always better. The job is pretty easy, mostly plug and play. Though, I'm not sure if the 2.4 v6 is any different from the 2.8 when it comes to bay configuration.

    http://www.audiworld.com/tech/eng108.shtml

    That's for the 2.8.

    It's a little tricky on the 2.8s, while the 1.8 is literally to the point where my 2 year old nephew could do it.

  9. #9
    Veteran Member Four Rings walky_talky20's Avatar
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    The 2.4 should be significantly similar to the 2.8. The engines are basically identical.
    ^Don't listen to this guy, he's not even a mechanic.
    2001 Laser Red A4 1.8TQM, 5-Speed Swapped, APR 93, Splitter, electronic oil pressure gauge, B6 Sport 17's, ST Coilovers,

    "You need to know what you are doing...Its like a rainbow jungle in there." - Redline380

  10. #10
    Active Member Two Rings
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    Yeah that does look tricky, and I've seen a how to with pics for the 1.8. Still can't find anything for the 2.4 v6 maybe there's not as many of these cars around or something. My engine layout looks different when compared to your 2.8 but that doesn't mean they won't be in the same or similar place. The wife has the car just now so I will check tomorrow. I'd like to do this myself but not if it's going to waste an entire Saturday - I remember being told that the brake master cylinder was a piece of cake to change on this car and I was still working on it 6 hours later!!

  11. #11
    Active Member Two Rings
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    So for anyone with the 2.4 v6 version wondering about their ects it is in the same place as the 2.8. I was able to remove the sensor without removing the air duct fron the throttle body which is just as well because i was not able to fit an allen key into the small space to remove it. Trying to get the new sensor and o ring to fit back in was a bit of a pig but i got there in the end and everything works as it should.

  12. #12
    Established Member Two Rings nemick's Avatar
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    Great job!! Is this forum the dog's dangly bits, or what?
    Neil M.

    2004 allroad 2.7T TIP

    2001 A4 1.8TQ, TIP

  13. #13
    Veteran Member Four Rings .Mad Hatter.'s Avatar
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    Late to the party but I will jump on the CTS bandwagon. Mine went bad years ago and now I suspect my mk4 of having a bad one too.

  14. #14
    Veteran Member Four Rings J Ozzie's Avatar
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    Ii have this cold reading problem.. replaced the CTS and still the same... so my guess is the Tstat
    Jamie
    2001 Audi A4
    Vancouver Audi Club.ca

    where's our resident intoxicated lobster Canadian - Thomas

  15. #15
    Veteran Member Four Rings .Mad Hatter.'s Avatar
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    One of the key things to indicate the CTS is that the heat blows hot.

  16. #16
    Veteran Member Four Rings J Ozzie's Avatar
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    Quote Originally Posted by .Mad Hatter. View Post
    One of the key things to indicate the CTS is that the heat blows hot.
    and goes cooler as the temp drops.. mine only sees the middle when its warm out.. now that its like 7c mine is always in the C both oil and coolant
    Jamie
    2001 Audi A4
    Vancouver Audi Club.ca

    where's our resident intoxicated lobster Canadian - Thomas

  17. #17
    Veteran Member Four Rings .Mad Hatter.'s Avatar
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    But is your heat hot? If the heat is hot when the water temp. gauge shows cold; it's the CTS. If you have no heat then your car won't warm up, and it's the T-stat.

  18. #18
    Active Member Two Rings
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    Ok It's been a couple of weeks since I changed the CTS and I am still getting a reading on the instrument cluster when the car is hot or cold so no problem on that side of things.

    HOWEVER

    I still seem to be having trouble starting the car when it is warm. It starts first time everytime when cold but when warm i could crank it for maybe 4 or 5 seconds and then give up and apply some gas to help it along.

    Another symptom is that when warm the engine idles rough. Well maybe rough is too strong a word. It's just a little bit noisy - you get the odd noise here an odd noise there - it doesn't purr.
    When cold it purrs really sweet - it makes 1 continuous noise

    I am still on the original fuel filter so that is getting changed in a few days time to see if that helps cure the problem.

    The thing is I read on another forum that even though the temp gauge on the cluster shows as ok, the readings going to the ecu may still be duff and so may be causing my problems. Is there a way to check the temp readings being fed to the ecu by the sensor without vagcom? Could I use the climate control diagnostics to find out?

  19. #19
    Active Member Two Rings
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    Bump.

  20. #20
    Active Member Three Rings
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    Macon, GA

    Climate Control Diagonostic has capability of monitoring coolant temperature.. i think... I think ive used that before.. but it didnt work right for me.
    1996 Audi A4 Quattro 2.8 12V

  21. #21
    Active Member Two Rings
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    Yeah i've used it before on channel 49. Apparently the coolant sensor has 2 parts. 1 feeds the ecu and the other feeds the information to the cluster temperature gauge.
    What I've yet to find out is if the climate control diagnostics gets its info from the part the ecu uses or the part that the gauge uses. I suspect it's the latter, but don't have confirmation yet.

  22. #22
    Veteran Member Four Rings seanj130's Avatar
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    Thermostat... You are going to have to do it at this point. I know it will be a pain in the ass on your motor.
    Best regards,
    Sean.

    Paying someone to install parts and bragging about it being fast, is like watching someone
    bang your wife and being proud to raise their kids.

  23. #23
    Active Member Two Rings
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    Quote Originally Posted by seanj130 View Post
    Thermostat... You are going to have to do it at this point. I know it will be a pain in the ass on your motor.
    Even though i get hot air through the vents ok?

  24. #24
    Veteran Member Four Rings J Ozzie's Avatar
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    Quote Originally Posted by .Mad Hatter. View Post
    But is your heat hot? If the heat is hot when the water temp. gauge shows cold; it's the CTS. If you have no heat then your car won't warm up, and it's the T-stat.
    i just remembered we have 2 of em...

    I replaced the Coolant flange one but not the lower :(
    Jamie
    2001 Audi A4
    Vancouver Audi Club.ca

    where's our resident intoxicated lobster Canadian - Thomas

  25. #25
    Active Member Two Rings
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    Quote Originally Posted by J Ozzie View Post
    i just remembered we have 2 of em...

    I replaced the Coolant flange one but not the lower :(
    Yeah the heat is is definitely hot.

    As for the CTS well it was replaced a few weeks ago but I wasn't aware there was 2 of them. The one I replaced was the same one as shown here -

    http://www.audiworld.com/tech/eng108.shtml

  26. #26
    Registered Member One Ring
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    I have a 2.4 12v 1997 avant
    My engine has a new thermostat
    the oil runs up to temp normally
    the analogue water temp gauge has be schizophrenic for a while and now it does not move at all from its cold position
    my climate control number "51" secret code temperature responds as it should, running up to 90deg and staying there
    which coolant sensor do i replace, the one at the firewall end of the engine or at the radiator end?
    Or am I looking at the wrong water temp code on the climate control?
    Love Peter



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