A8L D3 Temp Sensor or Thermostat?
 

So I have been noticing that my A8 takes forever to get to normal operating temperature. Sometimes will never reach it even after 20 minutes of driving. Recently it did show a code but after it was cleared it has not shown again and it still is very slow to warm up. My question I have, is it the temperature sensor or the thermostat? Also where are any of these located as I can't seem to see them anywhere and the coolant hose looks like it goes to the back side of the engine but can't see anything else. Anyone's help would be appreciated, especially any kind of diagrams showing where things are located and part numbers for each. Thanks

Based on your symptoms, I'll say your thermostat is stuck open.

What year and Engine code/engine type. Your location (North America or Europe etc) will also help.
That said, if it is a North American V8, 2004-2006, You probably have engine code BFM.

For that particular engine, The thermostat is on the face of the engine, behind the timing belt above the water pump. The timing belt has to be removed per the service manual. Quite an involved job that requires some specialized tools. The service manual or Alldata access is highly recommended for this.

+1 on all of Mister Bally's responses.

Sorry should have put that in my otigorig post. I have a 2008 BVJ 4.2 fsi engine. North American model. I think it's on the back side of the engine but read there might be 2 thermostats. Thanks for the help

So, the FSI V8 honestly is little documented on this board. Sure there are some posts, but just never a lot of enthusiast owners who wrenched on them. Besides the FSI injection, a lot of the ancillaries are different than the much longer year history of the port V8 that preceded it. It puts a further premium in my opinion on getting a decent manual for it. Manuals seem like horse and buggy these days relative to slam bang YouTube videos or whatever for some owners I guess, but more the way to go to get consistent answers. alldata has the online subscription ones that aren't too expensive.

On the FSI's--like most all the newer turbo motors that followed from there--they moved the ancillaries to frankly a Rube Goldberg sort of gear drive set up. The timing belt (which drove the water pump on the BFM V8) went away and got replaced by the cam chain drive way at the back of the motor. All done to make front to back motor length yet more compact. As part of this, the water pump moved from the front of motor like the earlier BFM's over the the side (US driver's) coupled w/ the gear drive stuff. The thermostat is near it. Places it low down on motor on that U.S. drivers side, so belly pan would come off to access it. Cutaway illustration below with thermostat in lower right corner. No guarantees, but link to ecs site for what should be it and then what it looks like. https://www.ecstuning.com/b-genuine-...t/079121115bk/ Oddly, I think it is the same one as for the D3 S8--V10 but again the gear drive stuff and also (early) FSI. Discounted dealer like audiusaparts or the other online ones will usually actually be cheaper, plus will confirm fitment by VIN.

Given 4.2FSI is so thinly covered on this board, you may find more luck looking at posts for either the Gen 1 Q7 4.2 or the C6 A6 4.2. Both used the FSI motor, and it was rolled out there and used for the whole generation for any that had the V8; later got superseded by 3.0T in Audi's generally. Know that while the RS4 (and R8) 4.2's arose in same timeframe, they are a different implementation in some ways. Thus better to use C6 A6 and Q7 as reference points when you don't find anything easily for D3 A8 4.2 FSI.https://cimg1.ibsrv.net/gimg/www.aud...760a97d98e.png

https://cimg3.ibsrv.net/gimg/www.aud...db717c838c.png
https://cimg5.ibsrv.net/gimg/www.aud...99733189e4.jpg
https://cimg6.ibsrv.net/gimg/www.aud...48930479c7.jpg
https://cimg7.ibsrv.net/gimg/www.aud...aaa3069bea.jpg
https://cimg9.ibsrv.net/gimg/www.aud...e451e7f650.jpg
https://cimg1.ibsrv.net/gimg/www.aud...f1f6e7d833.jpg
Hello R2B,
As MP stated, I believe your thermostat is NOT in the back of the engine, but is located at the front lower driver's side as pictured.

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Thank you for the pictures, I plan to look as soon as I get home. Looking online i should be able to replace only the thermostat itself or is the thermostat built into the plastic housing?

See links.

https://www.europaparts.com/thermost...9121115ba.html

https://www.ebay.com/itm/Genuine-W01...97.m4902.l9144

https://www.ebay.com/sch/i.html?_fro...tro%20L%204.2L

I have And Audi A8 4.2 TDI 2006 and i have 2 Thermostat. the normaly one was not helping me. Its was the one close at the oil cooler. Part number: 4E0 121 113
https://cimg3.ibsrv.net/gimg/www.aud...c11e02f58c.jpg

I could be wrong but I think that picture is of the inline transmission oil cooler.