Car: 2015 Octavia Scout, manual gearbox, 2.0 TDI (CRMB), 230000km

Hi,

My octavia has an issue. It takes a really long time to reach operating temps. Yesterday driving at 90 km/h (+5 min city traffic first) and 10°C outside, it took me almost 15 min before I had reached 90°C coolant temp and ≈ 75°C oil temp. Driving speeds below 50km/h I can barely get the needle to move.

Oil temp also has a hard time to go above 80-85°C going 80-90km/h, but driving in highway speeds I can reach 90-110°C depending on outside temp.

Also if I engine brake and roll down a long hill, the coolant temp drops ≈ 5-10°C. This happens even in the summer, with temps around 20-25°C.

So, my main hypothesis is a broken coolant thermostat, since I have no error codes and the fault has just appeared without leaks, noises or any other symptom. Water pump was swapped last year, hopefully it's not that. One thing i'm not sure about is if I have the easily accessable DSG thermostat under the intake, even though I have a manual gearbox?

What do you people think, should I try having a mechanic swap the thermostat (the main one that's hard to reach)? My guess is it will cost a couple hundred euro, so that's why I'm asking here first if it could be something else.

Edited 10 hours ago10 hr by Haxel

Hi.

I had the same symptoms in my 2015 2.0 Vrs Tdi manual gearbox, last year with about 225kkm back then.

The car took really long to reach operating temp and i logged my car with obd11 while driving, my coolant temp never got over 82Celsius even in summer so my thermostat had stuck open.

I changed by myself a new OE Gates thermostat and also emptied the whole coolant system of the original G13 coolant and changed it to the new G12 Evo variant.

The filling of the new coolant and getting air out of the system was pretty fiddly because you have to use something like Vagcom or obd11 to run all the electric waterpumps in the system, my car has four pumps: the main water pump driven by the cambelt, it has an electrially operated "sleeve", an auxiliary heating pump, charge air cooler pump and my car has factory webasto, it has also its own.

There is also a procedure in vagcom and obd11 where the car runs the air purging process in the main coolant circuit and the secondary ones. You really need to do these so no air is left in the system.

I would say almost certain that in a manual car there is only one thermostat in the engine.

Also you could check that after a hard motorway drive your upper radiator hose should feel hot, dont burn your hand and if your fan is on watch for your fingers.

Also you could check if your coolant temp sensor is giving logical readings if you have access to an obd tester.

Here is the coolant circuit of 2.0 Tdi

5 minutes ago, Jo88Mak said:

Hi.

I had the same symptoms in my 2015 2.0 Vrs Tdi manual gearbox, last year with about 225kkm back then.

The car took really long to reach operating temp and i logged my car with obd11 while driving, my coolant temp never got over 82Celsius even in summer so my thermostat had stuck open.

I changed by myself a new OE Gates thermostat and also emptied the whole coolant system of the original G13 coolant and changed it to the new G12 Evo variant.

The filling of the new coolant and getting air out of the system was pretty fiddly because you have to use something like Vagcom or obd11 to run all the electric waterpumps in the system, my car has four pumps: the main water pump driven by the cambelt, it has an electrially operated "sleeve", an auxiliary heating pump, charge air cooler pump and my car has factory webasto, it has also its own.

There is also a procedure in vagcom and obd11 where the car runs the air purging process in the main coolant circuit and the secondary ones. You really need to do these so no air is left in the system.

I would say almost certain that in a manual car there is only one thermostat in the engine.

Also you could check that after a hard motorway drive your upper radiator hose should feel hot, dont burn your hand and if your fan is on watch for your fingers.

Also you could check if your coolant temp sensor is giving logical readings if you have access to an obd tester.

Thanks, these are great suggestions! Now that you mention it, I have removed the webasto (or aux. heating) waterpump and never bled the coolant system after that. Maybe that could be the issue instead of an open thermostat?

I also own an OBD11 but I've never looked at the readings for the coolant or oil temp, will try that. I might also try and bleed the system, see if that helps.

If I decide to change the thermostat I'm gonna let a mechanic do it. I've spent the entire summer working on my cars this summer in my driveway, and draining coolant is so messy lol. But I'm glad you managed to do it yourself!

12 minutes ago, Jo88Mak said:

Here is the coolant circuit of 2.0 Tdi

Thanks, I onle see one thermostat here. I guess the other thermostat would show up in the DSG schematic