Thanks for the reply.

So you say that your car reaches 90 Celsius after 20 min highway driving no matter what.

Is this the temp gauge reading on the car or have you watched live data readings from Vagcom/OBD11 etc. I have noticed that with my OBD11, it shows lower readings than my cars temp gauge by about 5-10 degrees.

Also is your car also a VRS model, because it has really open front grilles that let a lot of air through to cool the engine.

Will check that hose going to the radiator. So if the thermostat is fully closed the radiator hose should be cold and the coolant temp under 92 Celsius which is the opening temp for the thermostat. When the coolant temp gets over this 92 degree mark the thermostat opens and the hose to radiator would get warm, right?

Can't really say is the webastos heating weaker than normal, havent noticed anything, but will try that what you said, that heat the car with webasto and then try the radiator hose before starting.

I have a 1,5 hour drive later today with mostly 80-100 km/h limits and the ambient temp is going to be 5-7 Celsius.

I will monitor the coolant and oil temps with OBD11 to see how they behave.

I'm starting to think that I may have a partially stuck open thermostat, the spring in it getting weak perhaps.

Another question just out of curiosity, what kind of oil temps are you all getting when its colder weather like below +10 Celsius? Especially in the diesel engines?

I usually never get the oil temp over like 90-93 Celsius with normal 50-100 km/h driving for 20-30 minutes, if I dont have the windguard in place at the lower grille.

With the windguard in place I can get the oil temp over 100 Celsius and even to 112 Celsius max if driven hard.

That makes me wonder that should I keep the windguard in place for longer time of the year and take it off after the ambient temps get over 7-8 Celsius regularly.

The reason for this is to get the oil temp over 100 to get rid off condensation in the oil...dont know if under 100 degrees is enough to get it off?

Also, does a faulty thermostat have an effect to the oil temperatures in some way?

2 hours ago, Jo88Mak said:

Also, does a faulty thermostat have an effect to the oil temperatures in some way?

Only in so much that a failing open thermostat will make the engine run cooler.

Yes it reaches 90 C (by the gauge) way before the 20 minutes are done, typically half way (5 mins of city drive and 5-10 mins of highway, depending on weather etc.)

For other reasons I have seen the temperature been close to 90 when gauge says so, but I have not checked/monitored what VCDS says the temp is when the gauge first time reaches 90 C. But I am quite confident it is not 75 C. I had webasto problems and when webasto had heated the coolant to 70 C (VCDS reading) the gauge was very close to 70 C.

I have a Scout and it has basically open bottom honeycomb grill similar to VRS. VRS might have taller bottom grill since scout has that extra bumber thing, not sure. The top grill is half way open (lower part of it). I dont believe this is the reason and was not this a recently developed problem?

Yes when thermostat opens like it should (87-92 C I think) the hose to radiator warms up. Should be cold before that.

I think you would have noticed difference in webasto preheating performance if you had stuck open thermostat. But as you said it might be partially open or open early.

In current +5 C weather with masks (top and bottom) I end my 20 highway drive with 110 C oil. I just took them of and now the oil is 95 C in end of the drive.

I usually put on and remove the bottom mask when I change summer/winter tyres. Then typically from december to march I also have the top mask on. If I wud only do city driving I would keep both masks for the winter tire season. However, it is not necessary and many do not do it. But I believe it is worth it in long run.

There is a oil cooler where coolant flows on other side so cooler coolant would cool the oil more.

1 hour ago, Skodaudi said:

For other reasons I have seen the temperature been close to 90 when gauge says so, but I have not checked/monitored what VCDS says the temp is when the gauge first time reaches 90 C. But I am quite confident it is not 75 C. I had webasto problems and when webasto had heated the coolant to 70 C (VCDS reading) the gauge was very close to 70 C.

I admire your faith.

Ask yourself this question, have you ever seen the temperature gauge indicate a steady temperature between 75° and 90° or between 90° and 110°?

I bet it has always risen or fallen fairly quickly through these ranges, quickly as in damped but not instantaneously, faster than the engine could actually change through that temperature range.

I know it shows 90 C in range of something like 75-110 C.

But yes if you use webasto to heat the coolant for long time and turn ignition on the gauge may go ie. to 80-85 and stay there. Also it has stayed and moving between 90-110 while having fun in snow for too long.

I dont believe it is as bad as here is always pointed out. And as I said, while I have been looking at something else with VCDS, I have seen it to be close to 90, less or more, while operating and gauge at 90 so I know it is not typically 75 C as in OP's case.

Thanks for the info.

I have been in sub zero temperatures with the heater on full driving on very light throttle as always and usually have to go up a steep hill or give it some beans to help the gaage over the 75/80° mark to jump up to the 90°.

On one of those occasions only after the temperature gauge had got to its fiddle factor 90° I think I was driving very slowly on a back road looking for somewhere and the load on the engine did not create enough excess heat to keep up with the heater, I noticed the temperature gauge needle fall very quickly (but nonetheless damped) to 60 or 70°c, I knew why it happened, there could have also been some hysteresis on the thermostat.

It was the first time in driving VAG vehicles since 1998 that the gauge needle has behaved in a relatively true manner!

On older cars you could always see the swings in temperature and if you had something with an inherent cooling weakness like my Triumph Stag you would drive always with one eye fixed on the needle, so well did I know the micro-deviations that the day I was accelerating from the last roundabout before my home 3/4 of a mile away I saw the gauge go up a tiny bit quicker than normal, I stopped the engine immediately and saw that the water pump was leaking, I had bought it with blown head gaskets, warped heads and seized head studs, stopping when I did saved it from a second occurrence.

Ok so a little update.

I drove today for 1,5 hour straight with most of the time at average speed of 100-120 km/h...also did some hard accelerations aswell.

I was monitoring the coolant and oil temps with OBD11 the whole journey.

I preheated the car with webasto for 25 min and the ambient temp was 7-8 Celsius the whole time.

So when i started my journey the coolant temp was according to OBD11 and also the temp gauge about 70 degrees Celsius. O

Sorry about the earlier post, dont know how to edit it.

Ok so a little update.

I drove today for 1,5 hour straight with most of the time at average speed of 100-120 km/h...also did some hard accelerations aswell.

I was monitoring the coolant and oil temps with OBD11 the whole journey.

I preheated the car with webasto for 25 min and the ambient temp was 7-8 Celsius the whole time.

So when i started my journey the coolant temp was according to OBD11 and also the temp gauge about 70 degrees Celsius. Oil temp wont show under 50 degrees on Maxidot.

I had my A/C set at 22 Celsius. The coolant temp lowered a bit at the beginning to close to 60 but started to rise back after 5 min drive. Webasto was off when driving also.

So the highest coolant temp that OBD11 would show was 76 degrees Celsius and it would stay at 75 almost the whole drive when "warmed" up. The cars temp gauge would show 90 at this point.

Oil temp was at highest about 96 and would stay between 87-93 most of the time. Also noticed that the Maxidot shows exactly the same temp for oil as does OBD11.

So what do you guys reckon? Do I have a faulty thermostat? Should the real coolant temp be more than 75 degrees, in my opinion, yes.

Also would it be better to still use the windguard to get the oil temp higher, over 100 degrees?

Did you try the radiator hose?

I wud have expected 100 C oil with such drive but I dont know should it get there. Do you have wind guard for top grill? I guess that would help enough and not block too much in any case.

And why you dont use Webasto while driving until the temp gauge is at 90? I think it is better to get to operating temp asap. I often keep webasto on until gauge 90 and then few more minutes, especially on city driving.

15 hours ago, Jo88Mak said:

So what do you guys reckon? Do I have a faulty thermostat? Should the real coolant temp be more than 75 degrees, in my opinion, yes.

Mine also but it depends on where the temperature sender is in the coolant circuit, is your car a DSG by any chance? They have a second DSG thermostat which often fail.

Mk3 diesels dont have a dsg thermostat like mk2. And mk3 petrol vrs has a valve instead of a thermostat.

Edit. The temp sender is in the head, after it has absorbed heat from the head. I also dont see 75 C to be reasonable temp there.

Edited 29 March, 20251 yr by Skodaudi

My car is a manual.

Hmm, maybe the coolant temp sensor could be faulty? Not an expensive part to replace also like the thermostat isnt.

Perhaps interesting:

42 minutes ago, Skodaudi said:

Perhaps interesting:

Hmm, interesting. So maybe i should try to change the coolant temp sensor first...its a cheap part and also easy to replace.

If I remove this part will I lose a lot of coolant and do I need to bleed the coolant system afterwards?

Also it would be a good idea to measure the coolants real temp with an IR-thermometer and see what the real temp is. Dont have one of my own though.

Also heres the diagram for my cars coolant system.

Hello everyone!

A little update to this topic.

We have had our first very warm summer days here in Finland and I remembered this temp issue that i wrote about at winter time.

So yesterday it was almost +30 degree Celsius outside temperature and i was driving for about 1,5 hours with driving speed 80-120kmh most of the time.

I was watching my coolant temp with OBD11 and after the car had warmed up, the coolant temp would stay between 80-85 Celsius most of the time with a maximum temp reading being 87. The temp gauge on the dash was showing 90. And the oil temp was between 95-105.

I havent yet changed the thermostat or the coolant temp sensor. Both still original from the factory and the car has now 218tkm on the clock.

What do you guys think? Are my temps on the cold side and is my thermostat faulty, partially stuck open perhaps?

A little update to this topic.

So I changed my thermostat on the weekend and also flushed my coolant system with distilled water to get the old G13-coolant off the system and filled it with new G12 Evo-coolant.

The issue that i was having with the engine running a bit cold in my opinion was the cause of the original thermostat stuck partially open, so the coolant didnt ever warm up over about 85 Celsius even when there was hot summer days with outside temps at 30 Celsius and driving on motorway at 140kmh. Also the oil temp hardly never reached over 100 Celsius in Summer.

So after changing the thermostat the coolant temp reaches 92 Celsius when the outside temps at the moment in September are at between 6-15 Celsius and the oil gets to just about over 100 Celsius.

Also the car gets up to temp faster in my opinion.

So problem solved👍

Edited 1 September, 20251 Sep by Jo88Mak