Thermostat

[Minstadave]

Now we know what car it is I can tell you this is not normal.

VAG coolant temp gauges do not give a direct readout, in fact the gauge will read as 90 over a range of temps. The gauge should read 90 consistently once upto temp, variances around normal operating temp will not be visible on the dashboard, it should be stuck at 90. If its stays below 90 on a decent run or not getting to 90 you most likely have a thermostat that's stuck partially open, the other thing to check is the coolant temp sensor, it's not likely to be this but this is much cheaper and a DIY job so it's worth ruling out.

 

[Jono8]

Now we know what car it is I can tell you this is not normal.

VAG coolant temp gauges do not give a direct readout, in fact the gauge will read as 90 over a range of temps. The gauge should read 90 consistently once upto temp, variances around normal operating temp will not be visible on the dashboard, it should be stuck at 90. If its stays below 90 on a decent run or not getting to 90 you most likely have a thermostat that's stuck partially open, the other thing to check is the coolant temp sensor, it's not likely to be this but this is much cheaper and a DIY job so it's worth ruling out.

The guage on the dashboard does get to the 90mark in 10 minutes at the very maximum and stays there consistently. The only time it drops is coasting downhill in gear for over a mile. It never stays below 90 or doesnt get up to 90 on the dashboard.

As my earlier post says i have got the exact readings from the climate control computer and it varied bwteen 75-78 degrees the whole way home from work today. I will check to see what it goes down to when going down that hill on my way to work.

 

[Dangerous Dave]

When you say at a stand still at 2 degrees ambient it was 79 degrees, did you mean 89?

I was driving around 50-60mph showing 79c with an outside temp of 2c. My aircon is also disabled under 4c so the front fans wouldn't have been running either.

 

[Fuzz]

Most thermostats open at 78°C So expect it to hover in this area.
The ECU will consider anything over 70° as normal running temp.

 

[Jono8]

Most thermostats open at 78°C So expect it to hover in this area.
The ECU will consider anything over 70° as normal running temp.

Everywhere i have been today it has moved between 75-78. No more, no less. It will be interesting to see how low it goes going down the hill tomorrow.

 

[Matt82]

78* seems low in my experience

looking on the usual parts websites, the 2.0 tfsi stat should be 87*

 

[Clarkey]

reading from the cc, is that cylinder head temp, or heater matrix temp? I would expect the heater temp to be a little lower.

 

[Matt82]

the obc on these things can report some ecu data, like the obc tricks on the e36

it will be reporting the ecu's water temp sensor

 

[Jono8]

Hm, interesting. so it is 10 degrees off basically? Does that mean it is not closing fully, or that the thermostat could be dodgy/ not opening at he right temperature somehow?

 

[Matt82]

opening too early it would seem

 

[Jono8]

opening too early it would seem

What would cause that?

 

[Matt82]

its a typical failure, something to do with the wax i remember reading once

im not sure if you remember the story about driving my M3 home after i bought it, ice on the inside of the windsreen by the time i got home? well when i bought a new stat for that i tested the new one compared to the old one. the old one fully opened in luke warm tap water

 

[Matt82]

this

if you can figure that out

 

[Jono8]

So stats can lose their ability to open at the correct temperature then yes?

Presumably, as it is already running at a low temperature ( although within an acceptable range for the temp guage on the dash to be where it should be), the downhill coasting part just tips it below the correct working range causing the slihgt drop of the needle. Does that sound plausible?

hm, looks like i might get it done soon then to be on the safe side.I can get the OEM VAG part for ~ £30 it seems. I will have to talk to my local VAG specialist to see how much they will charge me in labour to fit it.

 

[Dangerous Dave]

I don't think it running around 79c is a problem a quick google search shows other owners with running temps in the same region . The engine has 4 or 5 temp sensors for coolant I will check these with VCDS and see how this correlates to the temp reported on the climate panel.

If your thermostat is broken so is mine !

 

[Brookert]

The view from the Audi TT forum is that coolant temp shouldn't vary much below 90. It can go up to 100 in traffic (then the fans kick in, as per usual), but it shouldn't go below 82. 82 is the lower threshold for a 90 reading on the temp gauge, and the stat is a 82 degree open one I think?


If you've got the tools, it's generally very easy to replace.

 

[Jono8]

The view from the Audi TT forum is that coolant temp shouldn't vary much below 90. It can go up to 100 in traffic (then the fans kick in, as per usual), but it shouldn't go below 82. 82 is the lower threshold for a 90 reading on the temp gauge, and the stat is a 82 degree open one I think?


If you've got the tools, it's generally very easy to replace.

Hm odd. My climatronic display was giving 75-78 readings but the gauge was bang on centre.

With regards to it being easy to replace, i have read it is a bit of a bugger in these and the alternator needs removing to gain access to it. Not something i really fancy doing myself. I cant find a clear and good guide to do it anyway.

 

[MatteH Oxford]

It's not likely a failed 'stat is opening at a lower temperature, more like it's not closing fully any more and letting coolant through permanently.

78* seems low in my experience

looking on the usual parts websites, the 2.0 tfsi stat should be 87*

That's probably the temperature that the 'stat is fully open. The only way to properly test is to remove it and check to see if it is fully closed when cold.Then stick it in near boiling water and observe the temperature it starts to close at using a thermometer in the water; by the looks of it 87c.

IIRC mercury thermometers are rather accurate, usually within 1c over their entire temp range.

A point to note is that it's normal in a diagnostic to check the temperature sender resistance, supply voltage/earth & the gauge operation first before removing the thermostat, unless you have a good solid indication of it failing i.e. no heat from the interior heater or the the car overheating.

 

[Matt82]

It possibly is, I'm just telling you that in my experience, if the stat says x then the temp will be very close to that

Its an 82* stat in my golf, at a steady cruise water and oil temp both stick at 82

Find out the temp of the brand new stat, if its much higher than 78 and you care enough, change it. Simples!

 

[MatteH Oxford]

Given how thermostats generally work, for them to open at a temperature cooler than they are rated for would mean that they have got more efficient as they've aged.

I think that is rather unlikely.