UK-POLOS.NET - THE VW Polo Forum

You raise a very fair point about questioning the diagnosis. It's true that in many workshops, parts get thrown at a car before finding the real issue.
However, to answer 'to what' the system is responding beautifully: it's responding to heat soak.

On a highly-strung 1.4 TSI, running at 133 km/h generates massive internal heat. The moment you slow down to a stop, airflow drops to zero while that heat radiates out, pushing the coolant briefly to 108°C. The system responded beautifully because the ECU instantly saw this, slammed the fans to 86%, and dropped the temperature by 8°C in under two minutes. That is active, working thermal management.
The factory stats on the EA111 CAVE are mechanical (87°C for the head, 95°C for the block), but they have to manage a very complex dual-circuit design.

Since the car is currently at the workshop having tests done, they are the ones who can actually pressure-test the system, check for air blocks, or see if a thermostat is lazy. Let's wait and see what the professionals find on the physical car!

The humble two stage thermoswitch in the side of the rad does some thermal management even if not as precise as the above, as long as the engine is not switched off, maybe the twin charger has a auxiliary electric pump to deal with this and circulates some water.?

What are your views.... on any of these engines (not just the twin charger), if the jacket stat failed shut, does any circulation take place around the cylinders and if not does the ECU detect this by some means and flag a alarm, the twin charger's rad temperature is monitored which will make it easier, but mine does not have a rad temperature sensor as far as I know/saw.

You actually guessed right! The 1.4 TSI Twincharger does have an auxiliary electric coolant pump. It’s controlled by the ECU to keep fluid circulating through the turbocharger after the engine is switched off (or under high thermal load) to prevent heat soak from baking the oil inside the lines.

As for your question about a thermostat failing shut, modern ECUs are extremely paranoid about this... even on cars without a dedicated radiator sensor, the ECU doesn't just look at a single temperature, it monitors the rate of temperature increase from a cold start.
If the cylinder head temp rises way too fast compared to the engine load/time map, the ECU knows there is no circulation, triggers limp mode, and flashes the red warning light before actual damage occurs.

Modern thermal management is definitely a maze compared to the simpler, bulletproof setups of the past.
Let's see if the workshop finds any of these sensors or stats acting lazy.

Oh for the days of the fail open vacuum filled thermostats on the air cooled VWs, not quite foolproof but almost except that the flaps/linkage jammed shut as the flaps were spring loaded to be wide open.

Modern cars may be infinitely more efficient and cleaner, but they have certainly stripped away that raw, mechanical romance.

Our 'ancestors' did not have these modern OBD anxieties. The connection tools and data logs for their locomotion systems were way more organic.

The ultimate F2F system.