Is this a faulty thermostat, or just my imagination and the winter?

[peter-h]

I have a diesel 180HP, 2012. Had it since new. Great car, never gives any problems. Wonderful to drive. Never had the VW "ECU fix", of course

The engine temperature normally goes up to 90 and sits there. This winter I've noticed it takes ages to get there and it doesn't get there unless you go up the A23 at 70mph. As soon as you come off the fast road and go into country lanes, it drops to 80 or 70. This is with outside temps of 0 to 5 degC. I really do not remember ever seeing this before. The heater works seemingly fine. The temp never goes above 90, presumably because the fan comes on then.

Googling suggests it is a faulty thermostat, £50, and ~£200 labour. I took it to the garage I normally use. They did some tests and said it all seems ok. They said there are no error codes in the system. They spent £150 and wanted a total of £850 for fitting a £80 genuine VW thermostat. They said it is 5hrs labour (at £125/hr) because it is behind the engine. So I told them to forget it and paid them the £150.

Does the above make sense?

It is not a VW dealer. I do not want to go to one because they have sometimes done the VW cheat fix without authorisation. And if they do, you have no comeback because they are authorised by VW. They told me this. Then I would have to pay some £400 to have it reversed and I know VW made it harder and harder, even concealing it was done by faking the version numbers.

I do have an ODB2 cable and VAG-COM, on an ancient laptop, and used this to do some tweaks like changing the central locking behaviour, years ago.

[Snug335]

Hi,
I have a 140bhp diesel (2009) which I’ve owned for almost a year and a half now so driving it through its second winter. With the outside temperatures you’ve described (0-5C) my Scirocco takes longer than the summer months to get to 90 on the gauge tho my car does appear to remain at 90 once there regardless of what roads I’m driving on - 50mph/ city driving for example. I’ve had the climate control set to 26C on average and the heated seat on this winter too.

A thought is your thermostat maybe slightly open - could a Coolant flush help? I was surprised by the Rates charged by your Garage. I guess a lot depends on what part of the Country you’re in. I have a trusted Independent Mechanic I have used for over 10 years and his Rates are less than half what you are quoting above. I get my Servicing and MOTs done with him too. (I’m in the East of Scotland). Full receipts with VAT paid.

It’s good that your heater is working fine despite the drop in engine temperature and equally good that your engine isn’t overheating with the thermostat stuck closed.
I hope you get a resolution.

[1.4_tsi]

The water temperature gauge on VW Group cars (and I'm guessing others) in effect "lies" when it tells you the temperature is being maintained at a precise 90 degrees. This is pretty much impossible to achieve, and is no doubt intended to reassure the driver that all is well, as well as enhance the impression of "Vorsprung durch Technik". I find the oil temperature gauge more useful, because it doesn't lie when the engine temperature rises and falls from 90 degrees.

If your coolant temp gauge is wandering below 90 degrees after reaching normal operating temperature, I'm guessing that the thermostat is seizing partly open, most likely due to corrosion. Since it is a purely mechanical component, there's not much that can be done to reliably test it in situ. I was surprised by the 5 hour labour estimate, until I watched this video (not sure if this is the correct engine type):

https://youtu.be/-DsbYr6lSrw

Not a job I would want to DIY, or entrust to anyone other than a VW specialist with good experience of these engines

[peter-h]

Interesting video - thank you for posting it.

Looks like a couple of hours' work. And yes this garage is charging a very high rate. I found someone who is setting up on his own (used to work for a dealer) at £60/hr.

Yes mine is the 2.0 TDI engine.

I am sure the temp gauge lies - that is standard in software (my field of work). But I so clearly remember the gauge sitting right at 90 during basically all driving, slow and fast, and this behaviour is so obviously different.

[1.4_tsi]

Yeah, the coolant temperature guage wandering from 90 definitely points to a fault in the engine cooling system. Pity the thermostat is so difficult to access. On my Mk2 Scirocco, it could be popped out in seconds with the coolant by undoing two screws

[peter-h]

I am pleased to report it was the thermostat. £80 for the genuine VW part and 4hrs' labour.

Now the engine goes straight to 90C and sits there, just like it always did.