Hi as mentioned in the title this issue is driving me insane !!!

2012 audi a6 c7, 2.0tdi 177hp automatic 

Car takes a long time to heat up to 90°(4 bars), when it eventually does it will drop down to (3 bars) when going down hill and engine isnt under load.

 

I have done the main thermostat 3 times (1 genuine audi, 2 non original), I have done the DSG thermostat 2 times (1 genuine audi, 1 gates), replaced the coolant temp sensor once too, the fans arn't running constantly either I've checked.

 

Oh and I've also done both coolant control valves, one on the passanger side at the windscreen and the other on the side of the gearbox.

 

Nothing has changed and the problem is still exactly the same, please tell me someone knows what I am missing here, I am at a complete loss and its driving me to the point I want to just get rid of the car, its heavy on fuel too as it takes a long time to heat up so it dosnt reach proper operational temp on short drives.

 

Ive seen many people have the same issues but changing one or both thermostats seems to sort most people out. And I have seem on some vw's have a one way valve somewhere on the system but no mention of it on my car.

 

I WOULD BE SO INCREDIBLY APPRECIATIVE IF SOMEONE KNOWS OR HAS AN IDEA OF WHERE I AM GOING WRONG

 

Thank you

 

15 minutes ago, Dave Mc said:

Car takes a long time to heat up to 90°(4 bars), when it eventually does it will drop down to (3 bars) when going down hill and engine isnt under load.

That data implies coolant is flowing through or past the thermostat. I can believe one faulty new thermostat but not three of them, so is it possible the stat is not seating properly in its housing? Does the car have a secondary stat somewhere in the circuit? (I have read of such a thing but never seen one myself.)

Hi cliff, yes it has a second stat behind the engine, I have changed that twice (1 original audi and a gates one) 

The stat not seated properly I wouldnt think could be an issue as it is inside the plastic housing, I didnt just replace the element itself but the entire housing each time 

Sum total of parts 

3 main thermostats 

2 secondary (dsg i beileve)

1 coolant temp sensor

Both coolant control valves 

 

Edited December 6, 2025Dec 6 by Dave Mc

Well coolant must be circulating, but I am blowed if I know how. Let's hope another member has some ideas.

3 hours ago, Dave Mc said:

Hi as mentioned in the title this issue is driving me insane !!!

2012 audi a6 c7, 2.0tdi 177hp automatic 

Car takes a long time to heat up to 90°(4 bars), when it eventually does it will drop down to (3 bars) when going down hill and engine isnt under load.

 

I have done the main thermostat 3 times (1 genuine audi, 2 non original), I have done the DSG thermostat 2 times (1 genuine audi, 1 gates), replaced the coolant temp sensor once too, the fans arn't running constantly either I've checked.

 

Oh and I've also done both coolant control valves, one on the passanger side at the windscreen and the other on the side of the gearbox.

 

Nothing has changed and the problem is still exactly the same, please tell me someone knows what I am missing here, I am at a complete loss and its driving me to the point I want to just get rid of the car, its heavy on fuel too as it takes a long time to heat up so it dosnt reach proper operational temp on short drives.

 

Ive seen many people have the same issues but changing one or both thermostats seems to sort most people out. And I have seem on some vw's have a one way valve somewhere on the system but no mention of it on my car.

 

I WOULD BE SO INCREDIBLY APPRECIATIVE IF SOMEONE KNOWS OR HAS AN IDEA OF WHERE I AM GOING WRONG

 

Thank you

 

Hi blocked heater matrix possibly have you checked the expansion tank to see if the silicate bag has broken in which case you will find the bag absent silicate, that will be floating round the water jacket blocking the heater core and any other small aperture it can find, the heater matrix is part of a bypass system for the coolant this ensures you get heat in the cabin as soon as the engine starts to warm, the silicate will also jam open thermostats.

Steve.

Hi Steve 

 

There is no bag visible in the coolant bottle but if the hearing matrix was stuck would this not slow coolant flow down and increase rise of temperature ? The heating in the cabin is fine and works very fast

 

If it is a bag lost in the system, is it just a case of stripping everything down until I find the remains of it or is there a common place like the water pump where it will be found ?

 

I'd forgotten about the bag of silica gel crystals that afflicted some cars. Thanks for the reminder Steve.

The point that Steve was making is that if crystals have been released into the coolant they will prevent the thermostat from closing, which would explain your symptoms. If that is the case it will be a devil of a job to flush them from the engine and radiator. Search Youtube for videos explaining the problem and what can be done about it.

How many miles does it take to get to 90?

Did it used to heat up faster?

How long have you had the car?

If either front and or rear are set to hot from a cold start, it will take longer to heat up.

Is the main radiator and both inlet and outlet hoses the same temp as it warms up?

When I got mine, I made them put a stat in, because I thought it was taking too long. 

When mine gets up to temp on the motorway, it stays there, does yours? 

Any update for us David? I see you are on line at this moment.

It was like this when he got the car about 7 years ago but mpg and a failed dpf brought to attention now 

Its my father's car and he just ignored it

The car prob takes 10km or so to get to temp on a motorway and when it gets to 4 bars if you reduce speed or traffic it will drop down to 3 bars, used to be worse but with all the new parts it has improved but its not perfect

I havnt done anymore with it since I posted this as im out of options and ideas 

I'd start by getting it to about 2 bars and check the rad and both hoses.

Might be worth using infra red camera or laser type temperature reading on varies coolant pipes to see what warming up and what isn’t, could monitor temp Into  heater matrix and temp out of it to detect possible blockage, also show up any possible coolant valves not operating as expected 

If you think you have poor mpg have you checked the live data for the varies coolant temp sensors and this would be what the ecu is using to adjust fueling 

I will check that tomoro, I think I have a infrared one in work that I can use, and I thought there was only one coolant temp sensor

 

As it stands now I thought or think I have replaced all components of the cooling system (some more than once) and that I was missing a one way valve or something, a blockage i would have thought would cause over heating rather than over cooling ? Cabin heat starts very quick and gets very warm so I dont know 

 

Has anyone an idea of where to get a cooling system diagram? Ive looked and keep getting alot of variant models and not very detailed, it is a 2012 a6 c7 2.0tdi 177hp with auto box 

 

Thanks again lads for getting back to me, really appreciate it 👍 

You may only have one temp sensor but it is common to have 2 on varies cars. Over cooling is something that we experience maybe once or twice a year up here in Scotland due to cold weather and partially covering grille is a quick fix. Is there signs that a previous owner has fitted a thicker core radiator or left some covers of an additional side mounted radiator maybe after a collision without realizing the impact it causes. Only thing I can think that would cause your symptoms 

Hi, 

I had exact same fault with mine. My temp sensor wasn't reaching op temp of 90c, even after new stat fitted. I had a slight coolant loss which I traced to rear coolant sensor on back of head. Took old sensor to motor factors (so as to buy correct sensor) and was informed that my sensor wasn't the correct one. Apparently they are colour coded, my one was grey when it should have been black. Fitted new black sensor (supplied with strong metal spring clip and not cheap black plastic one) bled coolant system and took for test drive.....The difference was night and day. Smooth rapid pick up, no lag/flat spot and  most importantly temp gauge reached 90c with 10/15 mins of driving. I now don't suffer from what people think is regenning, revs increasing to 1000rpm and cooling fans coming on. That was engine ecu being told engine was cold by incorrect sensor and was over fuelling to increase temperature.

Check the colour of your rear sensor, it could be just that simple a fix. My sensor cost me £14.50 from Europarts....nearly snapped my index finger fitting the metal clip though....it has never run so smoothly and used the least amount of fuel if now does or doesn't !!!!