Hi all, sorry to jump straight in asking for help on my first of day joining. I’ve got to the point of frustration and desperation, as I’ve had this issue pretty much since purchasing the car, but thought it was a simple coolant top up.

The car itself has far exceeded my expectations in performance, economy and drive ability.

I’ve had a 2015 Octavia VRS hatch for about 10 months. In those 10 months It’s had a new clutch, rear wheel bearings, discs, pads ect and thermostat. All that apart from the clutch I’ve done myself. To me it’s general ware a tare and maintenance.

The issue I have is the coolant level is normal when cold but virtually every other day when I go to start it up I get the low coolant warning. When I check the level the expansion tank (which is new and without the bag) is empty, when I release the pressure the coolant rushes back into the expansion tank to between the min and max marks and sometimes above the max. That’s why I changed the thermostat. But unfortunately the problem is not resolved.

It’s not losing any coolant and there is no water in the oil. It’s got me a bit stumped for solutions. Any help or advise will be much appreciated

Thanks in advance

Hello, welcome to the forum.

It is likely that you have an airlock in the cooling system - this might have been caused when the thermostat was replaced.

I believe that there is a special bleeding procedure using VCDS - although it may be possible without using the tool.

14 hours ago, Warrior193 said:

Hello, welcome to the forum.

It is likely that you have an airlock in the cooling system - this might have been caused when the thermostat was replaced.

I believe that there is a special bleeding procedure using VCDS - although it may be possible without using the tool.

Hi

Thanks for your response. It was like this before I changed the thermostat, which is why I changed it, hoping it would solve it. But no such luck. I’m going to look into the pressure test you mentioned and go from there. However I checked the level this morning and released what little pressure there was and the level was well below the min mark so I topped up with about half a litre, think I will see how it goes for now. Also I have noticed that the running temp was around 90 before I changed the thermostat and since, it’s seems to be hovering around 70 or under.

If the engine is now actually running cool, it is possible that the new thermostat is faulty (or jammed open) What happens to the engine temperature after a longish, hard run?

One point to note, the indicated temperature on the instrument panel gauge does not necessarily indicate actual true temperature - OBD11 or VCDS plugged into the diagnostic port will show actual temps.

Cold running is not great for the engine.

editors note, moved to octavia mk3 forum. Welcome @Mullbers

9 hours ago, Mullbers said:

Hi

Thanks for your response. It was like this before I changed the thermostat, which is why I changed it, hoping it would solve it. But no such luck. I’m going to look into the pressure test you mentioned and go from there. However I checked the level this morning and released what little pressure there was and the level was well below the min mark so I topped up with about half a litre, think I will see how it goes for now. Also I have noticed that the running temp was around 90 before I changed the thermostat and since, it’s seems to be hovering around 70 or under.

This VCDS scan of a Polo coolant Temp will give some idea of your actual temperature. You can see that with a true/actual temperature of ~ 75C then the coolant gauge will indicate 90C, it will remain showing this 90C until or if the actual coolant temp reaches ~ 107C and then start displaying the true temperature again. (I've obviously not verified this). As noted in another post, it does look as if the new stat is either stuck open/partially open, very unlikely IMO, or is the incorrect one, allowing coolant to by pass into the rad.

My Polo has got two coolant stats set to different opening temps but I don't think they are interchangeable

VW Polo 191C561 02092024

Coolant Temp

VCDS(true temperature)/Car coolant Gauge

52 /52

53/55

56/58

58/62

60/65

63/69

64/72

66/74

68/78

69/82

71/84

73/86

74/88

75/89

75/89

Edited 24 August, 202524 Aug by Johngerard