TDi Owners: Check your thermostat is working correctly here.

dj_efk

A2OC Donor
United-Kingdom
There's been a lot of talk around failing / sub-optimal thermostats recently and I thought it would be helpful if I create this post in a new thread ahead of a long journey in Audrey - with a known-good OE thermostat - and measure the warm-up temp at 3 minute intervals so that those of you experiencing a slow warm-up time and wondering if this is normal can compare your temperatures to mine using CDIS or VCDS to check.

Test Conditions:

Ambient temp: 12°C (according to the outside temp measurement on the dash)

Route Description: 8.5 miles of flat B roads at up to 40mph, then undulating motorway (including the steep uphill section of M40E after High Wycombe), M25N (again some fairly undulating steep terrain for a motorway) at 60mph or so.

Relevant car notes: Deleted EGR (so slightly slower than standard warm-up), six speed gearbox, 60mph is ~1750 RPM. Car is lightly loaded with just myself driving. Oil level is full (relevant as the oil and coolant temps impact on each other). The climate unit is set to 21°C and on auto.

Measured Coolant Temperatures:

Start: 15°C
3 mins: 38°C
6 mins: 49°C
9 mins: 62°C
12 mins: 74°C
15 mins: 78°C (this is when I joined the M40)
18 mins: 84°C
21 mins: 88°C

Notes:
As can be seen, as the temp got to mid-70s degrees then the rate of increase slowed down and stayed slow while the oil gradually started to catch up and as the thermostat began to open as it should somewhere in the 80s.

The needle was pointing to 90°C around the 16/17 minute mark (dash reading was 81 or perhaps max 82 °C); as has been well documented elsewhere it stays at 90 even when the true temp is ~8/9°C below - and presumably above - this.

I also had to wait at some lights for over a minute between 3 & 6 minutes, otherwise warm-up would have been a little faster.

I can do another one of these if the temp drops again as I’m told it might. Meanwhile I hope this helps someone.
 
Last edited:
There's been a lot of talk around failing / sub-optimal thermostats recently and I thought it would be helpful if I create this post in a new thread ahead of a long journey in Audrey - with a known-good OE thermostat - and measure the warm-up temp at 3 minute intervals so that those of you experiencing a slow warm-up time and wondering if this is normal can compare your temperatures to mine using CDIS or VCDS to check.

Test Conditions:

Ambient temp: 12°C (according to the outside temp measurement on the dash)

Route Description: 8.5 miles of flat B roads at up to 40mph, then undulating motorway (including the steep uphill section of M40E after High Wycombe), M25N (again some fairly undulating steep terrain for a motorway) at 60mph or so.

Relevant car notes: Deleted EGR (so slightly slower than standard warm-up), six speed gearbox, 60mph is ~1750 RPM. Car is lightly loaded with just myself driving. Oil level is full (relevant as the oil and coolant temps impact on each other).

Measured Coolant Temperatures:

Start: 15°C
3 mins: 38°C
6 mins: 49°C
9 mins: 62°C
12 mins: 74°C
15 mins: 78°C (this is when I joined the M40)
18 mins: 84°C
21 mins: 88°C

Notes:
As can be seen, as the temp got to mid-70s degrees then the rate of increase slowed down and stayed slow while the oil gradually started to catch up and as the thermostat began to open as it should somewhere in the 80s.

The needle was pointing to 90°C around the 16/17 minute mark (dash reading was 81 or perhaps max 82 °C); as has been well documented elsewhere it stays at 90 even when the true temp is ~8/9°C below - and presumably above - this.

I also had to wait at some lights for over a minute between 3 & 6 minutes, otherwise warm-up would have been a little faster.

I can do another one of these if the temp drops again as I’m told it might. Meanwhile I hope this helps someone.
Not familiar with the diesels, where is the temperature sensor in the cooling system?
Mac.
 
I'll join Mac on the question.

The first thing I checked is the temperature of the radiator and hoses that lead to and from it. Cold tubes told me that the thermostat indeed was working correctly. Of course, I did it after comming home.

My clima is set on +19 and it is usually set to windscreen defogger with 3 bars of blowing.
When the ambient temp is below +5, in 22km from my home to work and vice versa, the coolant temp needle doesn't move a muscle. When clima is off, the needle barelly moves by the end of the trip.
If the needle has moved when the clima is off, it goes right back down if I turn it on. The engine can idle for hours, it will not affect the needle.

Have you tested it with lower or negative ambient temp? I noticed a breaking point at +5, as I wrote above.

The position of the sensor is important in this case because if it is somewhere far from the block or head and the water pump has failed, the self circulation of the cooland sends false info to the sensor as it is slower and the coolant may cool down significantly before reaching the sensor though it does its job on cooling the motor. If the sensor is right next to the heat source, that means either the engine is incapable to heat itself in winter, not to mention the cabin, or the car had history of using tap water and the walls of the block just look like the inside of a teapot that may seriously cause the engine to overheat under non-granny driving in summer.
Though in this case the coolant would be chalky. Now it's nice and clear.
 
Not familiar with the diesels, where is the temperature sensor in the cooling system?
Mac.
in the outlet from the block immediately below the tandem pump in the pipe that then runs towards the bulkhead and cabin heater circuit, possibly with the Webasto plumbed in-line if the car is so-equipped. It is therefore present within the short circuit that circulates before the thermostat opens to allow return flow from the radiator.
 
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