My car went overheated due to the thermostat being stuck open, and I replaced it with a new Febi thermostat with a new o-ring, but it is open at 84 °C; the older one was 87 °C. The car is still overheating!

After replacing the thermostat, I removed the timing belt and inspected the water pump. I noticed the plastic impeller was loose and not properly gripping the other side's pulley. To address this, I purchased a new water pump with a metal impeller. However, when it arrived, it turned out to be a different type of impeller than shown in the product picture.

After fitting, bleeding the coolant 100%

After this, the temperature would rise very slowly to 72°C, whereas before it would normally reach 90°C (halfway on the gauge). I wondered if more air was trapped, so I bled the system further. The temperature reached 74°C after 30 minutes of driving in 0°C weather. Strangely, after stopping at Sainsbury's for half an hour, the temperature quickly rose to 90°C five minutes after setting off again. The next day, the car again stayed at 74°C and not 90°C, but then after 20 minutes with the engine off, it reached 90°C within 10 minutes of driving afterwards.

Was something wrong with the design of the Impeller effect the flow of water? My old pump is in the first picture, and the new one that I have on now is in the second picture.

Has the new Thermostat develop a fault already?

I can’t think what is wrong with my car.

If it’s a DSG replace that stat also IMO

8 hours ago, 43576 said:

My car went overheated due to the thermostat being stuck open, and I replaced it with a new Febi thermostat with a new o-ring, but it is open at 84 °C; the older one was 87 °C. The car is still overheating!

After replacing the thermostat, I removed the timing belt and inspected the water pump. I noticed the plastic impeller was loose and not properly gripping the other side's pulley. To address this, I purchased a new water pump with a metal impeller. However, when it arrived, it turned out to be a different type of impeller than shown in the product picture.

After fitting, bleeding the coolant 100%

After this, the temperature would rise very slowly to 72°C, whereas before it would normally reach 90°C (halfway on the gauge). I wondered if more air was trapped, so I bled the system further. The temperature reached 74°C after 30 minutes of driving in 0°C weather. Strangely, after stopping at Sainsbury's for half an hour, the temperature quickly rose to 90°C five minutes after setting off again. The next day, the car again stayed at 74°C and not 90°C, but then after 20 minutes with the engine off, it reached 90°C within 10 minutes of driving afterwards.

Was something wrong with the design of the Impeller effect the flow of water? My old pump is in the first picture, and the new one that I have on now is in the second picture.

Has the new Thermostat develop a fault already?

I can’t think what is wrong with my car.

I presume you mean the car overheated because the stat stuck shut (not open).

You don't say what year your car is but my 2019 Polo (manual, non DSG Gearox) has two thermostats to promote quick warm up.

The VAG cars coolant temperature for many years now (at least 25) will read a steady 90C once the coolant temp reaches 75C and will not read higher unil/if the actual coolant temp exceeds something like 107C, you can see my coolant indicates 90C when the actual temperature reaches 75C.

I very much doubt if that new stat would develop a fault so soon.

Edit: If your gauge showed 74C then the actual coolant temp was probably more like 66C.

VW Polo Petrol Jan 2019

Coolant Temp

VCDS/Car

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

75/90

Edited 4 January4 Jan by Johngerard

10 hours ago, gav_is_con said:

If it’s a DSG replace that stat also IMO

It is manual.

9 hours ago, Johngerard said:

I presume you mean the car overheated because the stat stuck shut (not open).

You don't say what year your car is but my 2019 Polo (manual, non DSG Gearox) has two thermostats to promote quick warm up.

The VAG cars coolant temperature for many years now (at least 25) will read a steady 90C once the coolant temp reaches 75C and will not read higher unil/if the actual coolant temp exceeds something like 107C, you can see my coolant indicates 90C when the actual temperature reaches 75C.

I very much doubt if that new stat would develop a fault so soon.

Edit: If your gauge showed 74C then the actual coolant temp was probably more like 66C.

VW Polo Petrol Jan 2019

Coolant Temp

VCDS/Car

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

75/90

My car is a BKD 2.0TDI 140BHP 2009, I don't remember seeing two thermostats. I will get another thermostat and put in hot water to test before installing it. I will update later, thanks.

The thermostat was installed after I tested it in hot water at 88 °c, and it started to open. The old thermostat was not faulty.

I fitted a new O-ring, with no sealant used. After a 10-mile drive, the temperature never reached 90 °c, so it was the same old story- meaning it wasn't the thermostat.

When the car has been idle for about 5 minutes, the temperature gauge drops to 65 °c, and the cooling fan is not on.

The radiator is full, and the top hose from the radiator to the side of the cylinder block and to the matrix heating feels hot, but flat, as if there's air in the system. I bled it three times using my funnel tool to release air no more bubbles. Are the top hoses supposed to have pressure, or is it normal for them not to?

Edited 7 January7 Jan by 43576

Above, you show the coolant temp as 57.6C and you show the Coolant temperature cooler output at 39.6C.

Is the Coolant temperature cooler, the radiator??, if so then that points to the coolant (maybe) being cooled from 57.6 to 39.6C but may only be one of the air venting pipes that has a very small flow through it into the top of the rad.

Just to clear up your original statement "My car went overheated due to the thermostat being stuck open, and I replaced it with a new Febi thermostat with a new o-ring, but it is open at 84 °C; the older one was 87 °C. The car is still overheating!"

Was it overheating? and did you check this original stat's opening temperature (87C) as well?.

Before my car was overheating in traffic, the thermostat was stuck closed. I tested it won't opened so I know it's faulty. I replaced a new thermostat, not OEM, but I did not test and fit it straight away. The issue appears cooler than before. I have bleeding everywhere.

I thought it might be a new thermostat fault, so I bought an expensive OEM thermostat, but it did not make any difference. The hose on the side to matrix feels flat but hot, expect more air, or is it normal?

Not sure what you mean by flat, what do you get when you open the coolant header tank cap, you normally just get a gush of air/vapour.

I will show you picture the flat pipe tomorrow.

Can you ensure, with a vernier calipers or whatever that the 2 replacement stats are dimensionally the same as the the original failed shut one.

Update: I am deaf, and my mum heard a bubbling noise from the engine. It is definitely air trapped somewhere, and I found the top hose from the matrix heating too low pressure when I opened a big gush of water came out, and I quickly put it back, the hose became full pressure. Now the temperature rises and stay 90c on the gauge. My diagnostic tool is now at 79 °C with an outdoor temperature of 4 °C, much better than before. The maxtric top hose should have a bleed valve, as it would have made my life easier.

Very good, what you might do now sometime is to drive the car normally and ensure the diagnostics see a temperature of at least 86/88C, the stat opening temp, it goes fully open a ~ 98C so would expect around 90/93C (actual temp) under normal driving conditions, it might rise to the stat opening value while idling but may not at these low ambient temperatures now.

I can't understand, I have bought a bleeding tool kit, like a cup, put on the top of the coolant tank realse the bubbling. Why wasn't it bleeding 100% from the matrix? I don't get it.

I wouldn't be 100% happy until you do that test above (let the heater off), one would have thought that if you have/had driven it after installing this stat and because the top hose was apparently air locked that you should get overheating again, I've changed the coolant in a few cars of your era and I used to just let the cover off the header tank until the coolant was good and hot, sometimes, after a drain down and a cold engine, only 1/2 the coolant could be added but as soon as the stat opened then the remainder could be poured in.

Thanks I will try this.

Actually, that little plastic tank is a Degasser, even with the stat shut, you should see a small flow of water entering this tank, remove the cover and watch it.

Any update re the above?., did you get the stat to open??.

I smell a fumes out of the back, I see the EGR cooler gasket have black carbon through the gap. So next weekend have to remove to inspect if required a new gasket or whatever, by the way my coolant system runs fine and lovely hot not too hot like in the summer.

Fab.