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  #1  
Old 16th April 2019, 07:21 AM
madcjd madcjd is offline
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Default Coolant temp dropping.....

Doing my usual, twice monthly, drive between Lancashire and Heathrow.....
Driving home on Sunday evening, I hit a terrible jam on the M6 (no shocks there), which added about an hour to my journey.
Not long after that sitting around I happened to notice my coolant temp gauge had just gone slightly to the left of 90 deg - it always sits dead centre once up and running, so this was a first.
I continued to watch the temp fall to 70 and then work it's way to midway between 50 and 70 deg.
I pulled into the Knutsford services and give her 5 minutes to sit with ignition off and then inspected the engine bay.
I shone a torch into the header tank and the coolant was still pink and uncontaminated (I think I was half expecting an oil cooler fault chucking oil into the coolant!)
Nothing else obvious to see either.
So, I set off again (carefully) for the last 70 miles and initially the temp climbed a bit, but then resumed to the 50-70 range.
My initial thoughts were thermostat stuck open or temp indication (sender).
The next day, yesterday, did some local driving and the coolant got up to temp and sat nicely at 90.
I think the next motorway run will tell the truth. I have priced up a new thermostat, but, then it was mientioned it could be the pump at fault?
Any thoughts out there?
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  #2  
Old 16th April 2019, 07:28 AM
madcjd madcjd is offline
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Thinking about it, if the pump was an issue it would be 'getting hotter' rather than cooler.
So forget that ........
I think it will just be the thermostat - £60 approx from TPS, with housing.
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Current:
Night blue - 2005 D3 4.2 TDi LWB
My pride and joy - 1991 ur 20v quattro (owned since 2003).
BMW 645 (visiting the darkside!)
Previous cars
A4 cab 3.0 Quattro petrol
T5 Caravelle exec 2.5 (2006)
A8 D2 3.7 quattro sport (2001)
A8 D3 4.2 quattro (2003)
A6 4.2 quattro avant (1999)
A6 2.8 quattro (1998)
Audi Coupe 2.6E (1993)
Carlton 3.0 GSI
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  #3  
Old 16th April 2019, 07:46 AM
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moltuae moltuae is offline
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Does it seem like it's getting up-to-temperature? (ie engine feels hot, cabin heating still blowing hot). I had similar a similar issue a few years ago that turned out to be just the temperature sensor.
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The Citroen Era: '84 BX 1.6 RS --> '89 BX 1.9 DTR Turbo --> '94 XM 2.0L Turbo --> '96 XM 2.0L Turbo Exclusive --> '00 Xantia Activa 2.0L Turbo
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Old 16th April 2019, 02:35 PM
madcjd madcjd is offline
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Yes - all appears normal.
I had an A6 a few yrs back and that had a failed coolant temp sensor - the car drove like a dog or even failed to start.
It was cheap as chips to replace.
Because the A8 is driving normally, I think I've discounted that, but, I'll keep it in mind- what's more, it's cheaper than the thermostat.
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Current:
Night blue - 2005 D3 4.2 TDi LWB
My pride and joy - 1991 ur 20v quattro (owned since 2003).
BMW 645 (visiting the darkside!)
Previous cars
A4 cab 3.0 Quattro petrol
T5 Caravelle exec 2.5 (2006)
A8 D2 3.7 quattro sport (2001)
A8 D3 4.2 quattro (2003)
A6 4.2 quattro avant (1999)
A6 2.8 quattro (1998)
Audi Coupe 2.6E (1993)
Carlton 3.0 GSI
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  #5  
Old 16th April 2019, 02:54 PM
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moltuae moltuae is offline
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Mine drove fairly normally too, if I recall correctly. I suppose it depends on how the sensor fails. If the sensor tells the ECU the engine's hot whet it's cold, I suspect that might cause cold-starting issues due to lack of enrichment. However, if the sensor tells the ECU the engine's cold when it's hot, it might cause the ECU to over-fuel the engine initially, but I suspect that feedback from the oxygen sensors and fuel-trim will compensate for that to some extent.
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------------------------------------------------------
2002 FE S8 Ebony Black Pearl
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------------------------------------------------------
Cars Owned:

The Audi Era: '97 A8 4.2 (Ming Blue) --> '96 A8 4.2 QS (Dark Green) --> '02 FE S8 (present)
The Citroen Era: '84 BX 1.6 RS --> '89 BX 1.9 DTR Turbo --> '94 XM 2.0L Turbo --> '96 XM 2.0L Turbo Exclusive --> '00 Xantia Activa 2.0L Turbo
The Banger Era: '76 1.2L Lada VAZ-2101 (Ruski Fiat 124) --> '80 1.7L Morris Ital HL, finished in Ermine White and Rust
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  #6  
Old 16th April 2019, 06:38 PM
Tiburon3 Tiburon3 is offline
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Quote:
Originally Posted by madcjd View Post
Doing my usual, twice monthly, drive between Lancashire and Heathrow.....
Driving home on Sunday evening, I hit a terrible jam on the M6 (no shocks there), which added about an hour to my journey.
Not long after that sitting around I happened to notice my coolant temp gauge had just gone slightly to the left of 90 deg - it always sits dead centre once up and running, so this was a first.
I continued to watch the temp fall to 70 and then work it's way to midway between 50 and 70 deg.
I pulled into the Knutsford services and give her 5 minutes to sit with ignition off and then inspected the engine bay.
I shone a torch into the header tank and the coolant was still pink and uncontaminated (I think I was half expecting an oil cooler fault chucking oil into the coolant!)
Nothing else obvious to see either.
So, I set off again (carefully) for the last 70 miles and initially the temp climbed a bit, but then resumed to the 50-70 range.
My initial thoughts were thermostat stuck open or temp indication (sender).
The next day, yesterday, did some local driving and the coolant got up to temp and sat nicely at 90.
I think the next motorway run will tell the truth. I have priced up a new thermostat, but, then it was mientioned it could be the pump at fault?
Any thoughts out there?
Your thermostat is faulty, it's a very typical issue.
Cheers
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  #7  
Old 17th April 2019, 10:19 PM
H-M3 H-M3 is offline
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Let us know how you get on as my one takes lot longer to reach normal temperature these days.
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Old 19th April 2019, 10:25 AM
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snapdragon snapdragon is offline
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https://www.ebay.co.uk/itm/AUDI-A8-4...-/332302365216

4E0121113 This is the usual culprit, Audi call it a Coolant Regulator.
I don't know about 4.2TDI location though but seems to be present on many VAG models, from Golfs to Bentleys, including our A8 3.0 TDI, W12 and others. Usually it's beside the gearbox where the coolant pipes enter it or behind the radiator near the bottom.

When it fails, coolant can bypass the main thermostat and run through the radiator and autobox even when cold.
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Last edited by snapdragon; 19th April 2019 at 10:29 AM.
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Old 19th April 2019, 11:15 PM
H-M3 H-M3 is offline
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Quote:
Originally Posted by snapdragon View Post
https://www.ebay.co.uk/itm/AUDI-A8-4...-/332302365216

4E0121113 This is the usual culprit, Audi call it a Coolant Regulator.
I don't know about 4.2TDI location though but seems to be present on many VAG models, from Golfs to Bentleys, including our A8 3.0 TDI, W12 and others. Usually it's beside the gearbox where the coolant pipes enter it or behind the radiator near the bottom.

When it fails, coolant can bypass the main thermostat and run through the radiator and autobox even when cold.
Cheers Snap. I assume these are perishable items that need servicing/replaced after a while. Anyone done on theirs?
__________________
Current cars

Audi A8L 4.2TDI Silver 2006 £290 tax
Factory options-Phone in armrest now BT. Hybrid DTV. Voice command, Xenon plus (no cornering lights WTF), DRL, Soft closure, auto boot, Folding and tilting side mirrors, Front & rear heated seats, Rear elec blind, 19 alloys, Bose surround, Black leather & alacantra, Paddle shift.

List to do: Redo rear tint, RVC, FL rear lights, FL speedo (bought) 20 alloys. Rear seat entertainment..... Any donations welcome

Seat Ibiza 1.4tdi Eco 2008 Silver (£0 Tax)

NO VCDS rely on Marcin & Neeldub
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  #10  
Old 20th April 2019, 03:40 PM
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snapdragon snapdragon is offline
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Quote:
Originally Posted by H-M3 View Post
Cheers Snap. I assume these are perishable items that need servicing/replaced after a while. Anyone done on theirs?
They seem to fail more than the main thermostats but are cheaper and often easier to replace.
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