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#1
22nd February 2018, 11:51 AM
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The aux fan should always be on unless the AC is in eco mode. Its lowest speed is also controlled by the radiator switch, if the AC controller hasn't already turned it on. Medium speed is controlled by the second contact in the radiator switch (which in my case switches on the main electric fan), and the top speed is controlled by the cluster in 'oh ****' mode
 Could be that the aux fan has failed - that's very common on the 4s & 6s. If its not running with the AC on then check to see if its seized. It can also take out the fuse under the passenger carpet if its failing but not actually dead yet. Add in a worn viscous clutch, and it could get quite toasty! 
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#2
22nd February 2018, 12:21 PM
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Thanks for all your replies. I will check the viscous fan. However wouldn’t it overheat at idle even more than when in motion if a fan had failed? My mechanic that did the thermostat left the car running for 15 minutes and it was running when I arrived. We had a chat, I paid up, got in the car and still at 90. Only once driving on a bypass (so not in traffic) did it begin the creep up. That’s the opposite to what my brain tells me when I think of how engine cooling works.
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#3
22nd February 2018, 12:38 PM
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Out of interest, why was the thermostat replaced? Or was it just part of a major service along with belts etc.?
__________________
2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions and extended leather. Ski hatch retrofit. Aftermarket reversing camera. DVB-T and XCarlink now removed as redundant. 2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation". (Even rarer than a D2 S8!) Oh, and a brand new engine at just under 30,000 miles on the factory one!  Finally: gone, but not forgotten..... 1998 D2 PF S8. Agate Grey/Platinum. Every option (I think) except electric rear seats, Tiptronic steering wheel, ski hatch, towbar & dimming door mirrors. e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course.  (internal dimming mirror added later)1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon. Added Auto-dim mirror, Leather seats, Trip computer, Cruise control, OEM Ford SatNav with CD changer. And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850! 
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#4
22nd February 2018, 12:42 PM
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Was my best guess at the overheating issue as looking around the internet it seemed quite a common problem. Only the thermostat was replaced.
One gem I think I’ve missed is that when it was idling in the garage the heater was all set to HI. Obviously once I started driving I turned it down. Was the aux pump just enough to keep it cool? Last edited by carl81; 22nd February 2018 at 05:11 PM.
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#5
22nd February 2018, 08:22 PM
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I have watched my Aux fan go into 'oh ****' mode, I am pretty sure it kicks in around 110C
I cant think of any other good explanation why the temp quickly goes back down after hitting 110C Also I am confident I confirmed my viscous clutch is not engaging, I suspect this is the combination which sends the car on a overheat cycle in warm traffic, when outside temps are around 27C and above. My thermostat is fairly new Audi original. Belt changed by pros 
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#6
22nd February 2018, 08:26 PM
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Quote:
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#7
22nd February 2018, 09:54 PM
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Quote:
On the bypass it's generating a hell of a lot more heat internally than just ticking over idling, that has to be taken away somehow. I recall when I first got an S8 and opened the bonnet after my first drive round how unbelievably hot it was under there and for how long, but that's normal. It sounds like something may not be 100% but nothing like as bad as you seem to suspect. At the end of the day if it all works as expected is there anything to worry about. e.g. Does coolant boil out of the expansion tank, does the engine run rough when cold or hot?
__________________
2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions and extended leather. Ski hatch retrofit. Aftermarket reversing camera. DVB-T and XCarlink now removed as redundant. 2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation". (Even rarer than a D2 S8!) Oh, and a brand new engine at just under 30,000 miles on the factory one! Finally: gone, but not forgotten..... 1998 D2 PF S8. Agate Grey/Platinum. Every option (I think) except electric rear seats, Tiptronic steering wheel, ski hatch, towbar & dimming door mirrors. e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course. (internal dimming mirror added later)1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon. Added Auto-dim mirror, Leather seats, Trip computer, Cruise control, OEM Ford SatNav with CD changer. And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
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#8
23rd February 2018, 06:47 PM
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Damn vcds couldn't get it to work. I think my cable is broken. Anyway see attached I didn't dare go any higher than that. It was running absolutely fine. I guess there is a simple test to see if the water pump has gone?
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#9
23rd February 2018, 08:15 PM
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Quote:
Does the overheating kick in around 88mph?!
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