# 2.0 TFSI - Very Strange Overheating Behaviour



## digitalfriction (Dec 13, 2019)

Hi,

I have a 2.0 TFSI Mk2 8J, I initially realised there was a problem when the car overheated when stood in a queue of traffic. Temperature stayed fine most of the time, only when in a queue would there be an issue.

Using VCDS I can test and prove that all the fans work, they speed up and slow down fine. Temperature readings also seemed gine.

This weekend I replaced both the thermostat, after radiator temp sensor, and put in fresh prestone coolant after a flush through.

First test run and the fans seemed to operate, day 2 and they did not.

I have then realised that the difference is if I have the internal cabin fans on or off. If I turn them off, the engine cooling fans do not operate at all....surely this cant be right???

Would really appreciate some help on this.


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## Hoggy (May 8, 2002)

Hi, If the aircon is ON the rad fans should run at low speed regardless of coolant temp, so it appears if the MK2 uses the same system as the MK2, the fans are not being switched on by the thermal switch in the radiator.
I don't know enough about the MK2 cooling system/fuses etc, so hopefully, someone will be along with more info.
Hoggy.


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## digitalfriction (Dec 13, 2019)

Hi Hoggy,

Yes, so with aircon on, the fans spin slow as expected, with aircon off and cabin fans on lowest setting, the engine radiator fans work perfectly. With aircon on and cabin fans on lowest setting, the engine radiatior fans work perfectly.

If at anytime i turn the cabin fans off, the engine radiator fans do not activate.


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## Hoggy (May 8, 2002)

digitalfriction said:


> Hi Hoggy,
> 
> Yes, so with aircon on, the fans spin slow as expected, with aircon off and cabin fans on lowest setting, the engine radiator fans work perfectly. With aircon on and cabin fans on lowest setting, the engine radiatior fans work perfectly.
> 
> If at anytime i turn the cabin fans off, the engine radiator fans do not activate.


Hi, As I said with A/Con ON radiator fans will run on low speed, but if coolant temps are high >98, radiator fans should run even if A/Con is OFF. Not sure how fans are controlled on the MK2 but I expect it is a thermal switch in the radiator.
Get a fault scan carried out.
Hoggy.


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## digitalfriction (Dec 13, 2019)

I dont think I am explaining very well,

The engine radiator fans work exactly how they are supposed to, they cut in at the right temperature, they run at the right speeds, and they turn off when they should. There are no faults logged in VCDS, and the 2 temperature sensors have been replaced, even though they read correctly.

The only issue, is that if I turn the inside, cabin fans off....the engine radiator fans get turned off too. (i.e. they never turn on even if temperature is high)


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## Hoggy (May 8, 2002)

Hi, I now understand, but sorry I don't know enough about the MK2 cooling system, I'm sure someone will have more info.
Hoggy.


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## Wolvez (Jun 22, 2018)

digitalfriction said:


> I dont think I am explaining very well,
> 
> The engine radiator fans work exactly how they are supposed to, they cut in at the right temperature, they run at the right speeds, and they turn off when they should. There are no faults logged in VCDS, and the 2 temperature sensors have been replaced, even though they read correctly.
> 
> The only issue, is that if I turn the inside, cabin fans off....the engine radiator fans get turned off too. (i.e. they never turn on even if temperature is high)


Completely normal for the fan to be off when AC is off if the temp is lower than 90 degrees C. If the car is overheating your water pump is probably faulty. It's a very well known issue on mk2.


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## digitalfriction (Dec 13, 2019)

Wolvez said:


> digitalfriction said:
> 
> 
> > I dont think I am explaining very well,
> ...


That would be right, if the car overheated all the time.

The issue here is that the radiator cooling fans only operate when the interior climate control fans are on (nothing to do with ac, temperature switches all work fine, fans all work)

Its like this, if interior fans are on, everything works perfectly, fans cut in and out at correct temperatures, and car does not overheat. If interior fans are off, car potentially overheats in traffic because radiator fans dont come on.


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## Wolvez (Jun 22, 2018)

Maybe you did not bleed the air when the coolant was replaced. Try bleedig the air by running the engine without the coolant cap and park the car on a steep hill with the nose down. Let it harm up and don't forget to turn the AC heater to max before installing the cap.


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## digitalfriction (Dec 13, 2019)

Wolvez said:


> Maybe you did not bleed the air when the coolant was replaced. Try bleedig the air by running the engine without the coolant cap and park the car on a steep hill with the nose down.


How does bleeding the cooling system affect the electrics? If the interior fans are on everything is perfect.

I really don't know how else I can explain it......

Cabin fans on, car fine. Cabin fans off, radiator fans do not turn on when the temperature reaches 97

I will create a video and upload to youtube or something, that will explain it easier.


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## Wolvez (Jun 22, 2018)

digitalfriction said:


> Wolvez said:
> 
> 
> > Maybe you did not bleed the air when the coolant was replaced. Try bleedig the air by running the engine without the coolant cap and park the car on a steep hill with the nose down.
> ...


If there is air in the cooling system the coolant temp sensor might not work properly. What's the temp on coolant temp sesnor 1 & 2?


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## Wolvez (Jun 22, 2018)

Does the computer detect the engine is overheatIng?


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## digitalfriction (Dec 13, 2019)

Wolvez said:


> Does the computer detect the engine is overheatIng?


Yes, if I let it get that high. Temperature sensors are all reading fine, with the interior fans on, the engine fans work fine, which means the temperature sensors work fine (and I took them all out and tested the resistance at varying temperatures), and are being read, waterpump is fine, thermostat is fine.

Its like the interior fan switch is acting like an on/off switch for the whole radiator fan system


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## cas5259 (Mar 17, 2020)

If you're overheating at idle or low engine speed, it sounds like a coolant flow issue. Does revving the car in neutral bring the temp down? That should increase flow and decrease coolant temp. If not, like a few others said, take a look at the coolant pump. A not hot radiator input hose can also suggest that coolant flow is low. Input hose should be hotter than the output.

Squeeze the radiator hoses as well. Do they feel full of fluid? If not, you may have air in the system.


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## digitalfriction (Dec 13, 2019)

Hey, thanks for the help, I don't know how else to explain it, but what I am saying doesn't seem to be going across lol.

The car overheats if the radiator fans don't turn on, the ONLY reason the radiator fans do not turn on and cut in and out as they are supposed to...is if I turn the interior climate control fan knob to OFF. So I can 'fix' the problem by keeping the interior climate control fan knob at the lowest setting, but I doubt very much that is how Audi designed it.


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## Wolvez (Jun 22, 2018)

If AC is off, The ECU will only turn on the fan after reaching normal operating temp.

When AC is On the fan is always on.


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## digitalfriction (Dec 13, 2019)

Wolvez said:


> If AC is off, The ECU will only turn on the fan after reaching normal operating temp.
> 
> When AC is On the fan is always on.


I agree, and thats exactly what happens when AC is on. But when the interior FAN (nothing to do with AC) is off, the engine radiator fan doesnt operate.

I appreciate the help, but what im saying is clearly not being understood, I will get the video to demonstrate.


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## Wolvez (Jun 22, 2018)

Did you bleed the air when you change the coolant? If you just fill the coolant bottle without squeezing the lower radiator hose air will be trapped in the cooling system. If there is air trapped on the cooling system coolant will now flow properly. The coolant sensor connected to the upper radiator hose will not detect hot coolant properly. Watch this video to understand what I'm saying.


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## digitalfriction (Dec 13, 2019)

Wolvez said:


> Did you bleed the air when you change the coolant? If you just fill the coolant bottle without squeezing the lower radiator hose air will be trapped in the cooling system. If there is air trapped on the cooling system coolant will now flow properly. The coolant sensor connected to the upper radiator hose will not detect hot coolant properly. Watch this video to understand what I'm saying.


Ok, so I know what airlock is, and I know how to bleed systems. Can you explain how an airlock disappears when I turn the cabin fan on, and then reappears when I turn it off? Because that is the only way an airlock would make sense in this situation. As I have said 4 or 5 times....it all works perfectly if the cabin fan is on....


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## Iceblue (Jul 20, 2018)

Can you explain in detail the test you ran to determine whether the fans switching off at the rad occurs at the same as you turn off the interior fan. I assume at idle whilst stopped. How hot was the temp guage when you switched off the interior cabin and how long did you wait for the rad fan not to come on after that.


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## Wolvez (Jun 22, 2018)

[smiley=argue.gif]


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## Iceblue (Jul 20, 2018)

:lol: Thought it may help for your diagnosis. I would not know what is wrong with it


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## digitalfriction (Dec 13, 2019)

Well.....lol, I completely give up now

I have gone out to take a video, and now the radiator fan kicks in when its supposed to, even when the cabin fan is turned off. I am totally lost, so all I can say for sure is, yesterday, I stopped the car, left it idling, and the engine radiator fan did not kick in at all, today it does.

Apologies, its frustrating the heck out of me, and I cant explain it :-(


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## Iceblue (Jul 20, 2018)

Thats great news and if you have any more issues follow Wolvez's advice


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## Hoggy (May 8, 2002)

Hi DF, Nothing worse than intermittent problems. 
What was the coolant Highest temp when fans were not cutting in. Could it have been a false high temp indication?
Hoggy.


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## digitalfriction (Dec 13, 2019)

Hoggy said:


> Hi DF, Nothing worse than intermittent problems.
> What was the coolant Highest temp when fans were not cutting in. Could it have been a false high temp indication?
> Hoggy.


The highest I saw before turning off was 112, but I suspect it has gone higher because prior to changing the thermostat and sensors the red over heat warning came up and the temp gauge was pretty far over to the right.

I am going to do some more testing, im not convinced its resolved, and certainly dont trust it yet lol


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## Hoggy (May 8, 2002)

digitalfriction said:


> Hoggy said:
> 
> 
> > Hi DF, Nothing worse than intermittent problems.
> ...


Hi DF, At 112+ I would have expected steam from the reservoir vent, was that happening?
Hoggy.


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## digitalfriction (Dec 13, 2019)

It wasn't exactly spewing steam, but yes, it was really hot, couldn't touch anything on the engine, and you could smell the hot coolant sort of smell before even opening the bonnet. The on dash temp gauge sits at 90 normal, then has a marker for 115 and for 130, I turned off when the needle was at the 115 mark, previously when the red alert came up the dial was close or on 130. I don't think it is a false reading, and if it was a false hot reading, wouldn't the fan be on all the time?


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## Hoggy (May 8, 2002)

digitalfriction said:


> It wasn't exactly spewing steam, but yes, it was really hot, couldn't touch anything on the engine, and you could smell the hot coolant sort of smell before even opening the bonnet. The on dash temp gauge sits at 90 normal, then has a marker for 115 and for 130, I turned off when the needle was at the 115 mark, previously when the red alert came up the dial was close or on 130. I don't think it is a false reading, and if it was a false hot reading, wouldn't the fan be on all the time?


Hi, Temp indication is just that, only an indication & doesn't control the fans.
Where the thermostat & the coolant sensor OEM items or "OEM quality" after-market items.
The MK1 TT has many coolant probs using after-market cooling system items. 
Hoggy.


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## digitalfriction (Dec 13, 2019)

Ok, so more testing...

Today I took my son to school, and drove back home, about a 40 min drive in total, so car was fully up to normal temp. I left it running on the drive for a few mins, no fans. I grabbed the laptop and VCDS and looked at the temperatures, even took a video which I can post up shortly...111C engine outlet, and 113C radiator outlet, opened bonnet and its roasting hot, and no fans.

So I turned off engine, and tried the VCDS output test...no fans. I went and checked the relay and its clicking fine when I run the output test. I took the relay out and tested it with a meter to check the contacts were actually connecting....they are.

So I unplugged the main plug on the fan unit, it has 4 wires, 2 thick, 2 thin. I get a constant 12v between the 2 thick wires, a constant 12v between one thin wire and the thick negative (but the engine is hot so this could be due to that), and when I run the output test, the other thin wire goes from 0.3v to 3.04v and back down again.

So, I am now thinking that the 2 thin wires are signal wires to the fan unit, and they tell it to run or not, the 2 thick wires are the feed for the fans. So there must be some sort of control module on the fans themselves? And I reckon my problem is that that unit is stuck, worn, intermittant, or maybe now just dead?

Does anyone have a schematic, or know what voltages I should be getting at the connector? Can anyone think of any further tests that are worth doing, or does my conclusion that I need a new fan unit sound right?

Thanks again

UPDATE: So an hour after the above, I go to car again to pick other kids up, as soon as I start the engine the fans ramp up to 100% and then down to off...temperature gauge showed 70c....never noticed the fans come on again after that for the whole journey.

Totally lost


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## Hoggy (May 8, 2002)

Hi, I think you will have to use a meat thermometer to compare the actual coolant temps with the temperature indication you are getting. As soon as it's safe to remove reservoir cap use a meat thermometer & compare with the VCDS readings.
Where the sensors OEM.
The under-bonnet heat feels tremendous even at 90 degrees
Hoggy.


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## Wolvez (Jun 22, 2018)

digitalfriction said:


> Ok, so more testing...
> 
> Today I took my son to school, and drove back home, about a 40 min drive in total, so car was fully up to normal temp. I left it running on the drive for a few mins, no fans. I grabbed the laptop and VCDS and looked at the temperatures, even took a video which I can post up shortly...111C engine outlet, and 113C radiator outlet, opened bonnet and its roasting hot, and no fans.
> 
> ...


The fan control module is built-in to the large fan on the left side. The right fan is controlled by the left fan.


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## digitalfriction (Dec 13, 2019)

Hoggy said:


> Hi, I think you will have to use a meat thermometer to compare the actual coolant temps with the temperature indication you are getting. As soon as it's safe to remove reservoir cap use a meat thermometer & compare with the VCDS readings.
> Where the sensors OEM.
> The under-bonnet heat feels tremendous even at 90 degrees
> Hoggy.


So currently I have one AUDI sensor and one OEM non-Audi stamped sensor in, but I have a spare AUDI stamped one. All the sensors have been taken out and tested for resistance at varying temperatures of water, they all read the same. I honestly have no doubt the temperature readings are correct, the engine gets crazy hot, I cant touch the radiator pipes, and you can feel the heat coming off it. Admittedly I am more experienced in old school cars, but I am sure the engine is 'really' overheating when it says it is.


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## digitalfriction (Dec 13, 2019)

Wolvez said:


> digitalfriction said:
> 
> 
> > Ok, so more testing...
> ...


That is what I was hoping to hear, thanks  I am going to get a new fan unit and fit that, I have a feeling the controller is having issues. I might pull the fan unit out in an hour or so and see if there is anything obvious.


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## Wolvez (Jun 22, 2018)

I misunderstood what your trying to say. I assumed when overheating occurs turning the AC will turn on the radiator fan on. If output test can't turn it on, it will also not turn on when you turn the AC on.


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## digitalfriction (Dec 13, 2019)

Wolvez said:


> I misunderstood what your trying to say. I assumed when overheating occurs turning the AC will turn on the radiator fan on. If output test can't turn it on, it will also not turn on when you turn the AC on.


No, you were right, and 2 days ago, turning on the AC would indeed start the fans on slow, and if the engine hit temperature it would run the fans on full speed, yesterday it seemed to only work when the interior fan was turned on (I still cant explain this) and today, it seems I cant make it work at all, no matter what I do.

I have just ordered a replacement fan unit, its not OEM, but it was cheap enough that I can prove that it resolves the problem, and then if needs be I can get an OEM one.

Sorry for being so confusing in my replies, I normally don't post or ask many things, I have worked on and built cars as a hobby for over 20 years, and don't often get completely stumped, but this really confused me and it seems the issue and test results change each day.


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## ZephyR2 (Feb 20, 2013)

Have you seen this post by MT-V6 in another thread about an unrelated matter -

_Some systems are linked in interesting ways. With the climate control module disconnected, the engine cooling fan with run constantly. I assume it normally uses the climate module to measure ambient air temperature or similar?
_
See thread here -
https://www.ttforum.co.uk/forum/viewtop ... &t=1969641


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## digitalfriction (Dec 13, 2019)

ZephyR2 said:


> Have you seen this post by MT-V6 in another thread about an unrelated matter -
> 
> _Some systems are linked in interesting ways. With the climate control module disconnected, the engine cooling fan with run constantly. I assume it normally uses the climate module to measure ambient air temperature or similar?
> _
> ...


I hadn't, but that is interesting. I had started to wonder if even changing the car stereo had done something, it certainly throws up enough errors in VCDS when you remove the stereo and disc changer. I really should look to see about coding them out of the system as I now have an android unit.

It does kind of make sense that with how canbus systems work there could be a chain of devices, if a or b isnt there then default to condition x, or something along those lines.


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## digitalfriction (Dec 13, 2019)

I just wanted to post an update in case anyone was following this issue. Changing the main fan (with the embedded controller) appears to have resolved all the issues. My theory as strange as it sounds is that the controller was overheating and acting intermittantly, then eventually failing completely.

The new fan is behaving much better, regular, quiet, low speed fans to keep the temperature in check, I have not been able to overheat the car since 

Thanks for everyones input and words of advice.


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## Tassos (9 mo ago)

digitalfriction said:


> I just wanted to post an update in case anyone was following this issue. Changing the main fan (with the embedded controller) appears to have resolved all the issues. My theory as strange as it sounds is that the controller was overheating and acting intermittantly, then eventually failing completely.
> 
> The new fan is behaving much better, regular, quiet, low speed fans to keep the temperature in check, I have not been able to overheat the car since
> 
> Thanks for everyones input and words of advice.


Hello mate 
I have exactly Tha same problem as you
I have change the main fun to
New ater pump
New thermostat
New water temp sensors 
And again the same


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