Testing thermostat

[TALLANDREW]

So my coolant water has overheated several times this year, I thought it was just trapped air so I bled the system.

I then fitted a wagner oil cooler and swopped the coolant fluid.

I didn't drive much afterwards then It happened again recently when I was driving in heavy traffic to drop off some car parts. I concluded that I need to find out why and fix it.

Bit about the car. its a 2.0 CBEX 199ps 4 motion beach from 2019 with 200k km on the clock.

I did the old Chat GPT search and am working through the possibilities. (Read the sources to check.)

1 low water level_________________________ Was low when first noticed the overheating in the summer, probably due to boiling coolant. I topped it up and did a bleed. didn't drop afterwards.

2 low oil___________________________________Oil level is fine engine looses no oil.

3 air in the system________________________Bled system using VCDS twice, replaced fluid with a vacuum pump.

2 coolant leak____________________________ No visible leaks and performed a vacuum test when changing the coolant after fitting oil cooler.

3 malfunctioning reservoir cap___________Did a bleed and swopped this a few weeks ago after last overheat because it was cheapest easiest thing to check first. Not overheated since but didn't get stuck in traffic either.

4 faulty thermostat_______________________Using VCDS to monitor the sensors I brought the car upto temperature today on the Driveway took 1 hour outside temp was 4 degree. engine water temp reached 87 radiator temp had got to 40 and then radiator temp slowly rose to 60 to maintain engine at 90.

5 clogged radiator_______________________Coolant was very clean when I swopped it several months ago. I didn't do a flush though.

6 faulty fan_______________________________Ran fan remotely using VCDS test procedure

7 damaged/worn water pump ___________No graunchy noise coming from the pump

8 leeky head gasket_______________________Coolant doesn't have visible contamination no creamy buildup under the oil fill cap.

9 broken heat exchanger with DFP________Coolant doesn't have visible contamination and the level hasn't dropped since being refilled.

I'm getting very weak heating from the heater when outside temperature is low and auxiliary heater isn't used. I think this might be normal for a T6

My feeling is that the thermostat isn't operating correctly, I think my static test was a waste time and the only way to confirm if it is faulty is to put it into a water bath and watch it operate. Also with millage at 200k I should have just changed it when I fitted the oil cooler earlier this year. kind of kicking myself.

Any thoughts or things I have overlooked?

I have an thermostat and thought i would swop it as a precaution tomorrow morning.

 

[Dellmassive]

Has the water pump been replaced when the timing belt was done?

Assuming the timing belt had been done.

Can you see evidence that the water pump is pushing coolant around?

The engine has two coolant circuits with electric water pumps.

Is start by looking at those pumps to ensure coolant is being pumped.

 

[TALLANDREW]

the belt and pump have not been changed. When should that be done?

I’ll see if vcds has a test function for the electric pumps and listen for life. (-:

Thanks for the feedback.

 

[Dellmassive]

the belt and pump have not been changed. When should that be done?

I’ll see if vcds has a test function for the electric pumps and listen for life. (-:

Thanks for the feedback.

It was ..

service items:
timing belt and water pump (4 yrs or 120k miles...)

It now may have been to changed to 140k miles.(225k kilometers)

I've seen similar symptoms on a Vauxhall Astra years ago and the water pump impeller had cracked and come detached.... So it wasn't pushing the water round.... Which caused the water round the block to overheating and boil up.

 

[Dellmassive]

More info..

Thread 'T6 Service Intervals & Inspection, Flexible & Longlife'

30 Nov 2018

T6 Service intervals & inspections , Flexible & Longlife :

Im just posting this here to make it easier to refer back to in future . . .


source: Volkswagen Flexible Service Regimes (Longlife) | Volkswagen

********************************************************************************************

As a Volkswagen owner, you will already have experienced the pleasure of driving a car that's renowned for reliability. So when it comes to servicing it, why settle for second best? Visit your local Volkswagen Retailer or authorised repairer who will be ready to help you...

• Dellmassive
• 102ps 110ps 140ps 150ps 180ps 199ps 204ps 84ps caaa engine caab engine caac engine cxca cxeb engine cxec engine dellmassive diy oil oil change service due servicing
• Replies: 107
• Forum: Engine, Gearbox, Exhaust, Drivetrain

• 

 

[Dellmassive]

Post in thread 'Cambelt + Water Pump Replacement - Official guidance'

8 Mar 2022

well, its all done. . . . . . . .

Timing Belt & Water pump job - 50K miles so early (but 4years old so due according to VWCV UK on time basis (204ps DSG))

We have the Factory Extended warranty for years 4&5.

So I took ours to a main dealer as I have factory warranty and didn't want any funny business with any future claims.

That was yesterday.... Needed an engine mount *(highlighted last time) which was oos..... Which was ordered over night and installed this morning. - so now its the next day.

++++

Had a haggle with the SA and got prices too (march 2022):

£750 inc vat...

• Dellmassive

• 

 

[Dellmassive]

The cam belt , water pump thing has been changein the UK the other year to match the longer intervals that the Euro garages used....

Ie 130k miles.

 

[DXX]

The shrouded water pump is a really bad idea, you may find that the pump shroud has seized and is restricting flow, better than the jamming event that can cause timing belt failure.
If you are pulling the water pump you may as well do the timing belt service.
IMO best replace the pump with a non shrouded version as on older T6 engines

 

[TALLANDREW]

Just happily running the bleed process with vcds. I can hear the pump on the rear left and bottom right happily wizzing away.

I’m trying a different bleed process to the one described on the vcds website.

I’m running the low pressure circuit first. VW have this tip as a footnote in one of the learning documents.

I went ahead and did the thermostat to rule that out. Annoyingly The old thermostat worked fine just a little sluggish compared to a new one.

You need some balls to crack open the housing without a spare part to hand. It turns to the right and there’s a pin on top locking it. It opened without releasing the pin or breaking it. You need the car in the service position to do the job.




Service position



I guess that leaves the belt driven water pump as next.

Going to drive it a bit first in the hope that a better bleed is what it needs.

Thanks for the service info and tip on using the older style pump. I’ll need to check that out.

 

[mmi]

Saturday,13,December,2025,14:08:40:46589 VCDS -- Windows Based VAG/VAS Emulator Running on Windows 11 x64 VCDS Version: 25.3.2.0 (x64) HEX-V2 CB: 0.4671.5 Data version: 20250822 DS365.0
www.Ross-Tech.com
VIN: WV2ZZZ7HZKH142084
License Plate: Mileage: 99260km-61677mi Repair Order:

It's MY19

Address 01: Engine (J623-CXEC)

tip on using the older style pump. I’ll need to check that out.

I'm afraid the "shrouded pump" occurrence has turned upside down - the newer CXEB/CXEC engines (built after 2.10.2017) are without.




Source: IFI CP2 Message
121-013

 

[TALLANDREW]

thanks for the part number.

after the thermostat exchange the problem has changed, the issue changed from overheating to underheating. In despair I dropped the car off at VW this morning and begged them to look at as soon as possible.

The engine water heating is now non existent, it got to 50 degree's on a 50km test drive yesterday, the fan is running all the time at 19%.

Both engine temperature sensors read the same 50, So I think they are not the problem. strangely the oil temp is 80 degrees.

The new thermostat is fitted and was tested before fitting. The temperature sensor on the radiator exit reads 20, the inlet pipe on the top of the radiator is warm but not 50. I don't think the water is cooling down by going through the radiator.

I tried driving with the heater turned off so the car isn't cooling through the heater circuit. that had no effect.

I am there for thinking the fan must be cooling the engine down because the fan is running despite the temperature readings from outside or the engine. weirdly stop start is also active despite outside temp of below 4 degrees (I thought this was deactivated at low temperatures). I can activate and change the fan outputs with VCDS and there are no error codes. As the fan is a pretty expensive part i am reluctant to just order it and try swopping it.

I just don't have the experience to diagnose this which I why I took it to VW.

looking at the water system there are a lot of valves and gates which could also be faulty or open and might be allowing the engine to cool itself, just too many possiblities.

I did deactivate the bleed process adaptation before doing my test drive, otherwise that would be open until i had driven 150 km.

in conclusion its time to throw the towel in.

 

[TALLANDREW]

Just to finish this topic.

The thermostat needs to be changed with the thermostat housing because it isn’t easy to reseat the thermostat once it’s removed.

A new housing and pre installed thermostat seems to have solved my issues

 

[TALLANDREW]

Oh dear, I’m going to have to also admit I think that febi 106524 thermostat doesn’t fit that engine. I thought I triple checked it and on some sites it says it’s compatible but I just went back to Febi’s website and they don’t list a thermostat for the sg vans. With the febi thermostat fitted in the old housing I can blow air through. With the old original one I can’t. So maybe with a different manufacturer it might have been possible to just change the thermostat.