COOLANT TEMPS UNDER OBD

[ciclo]

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!

[Johntheo1]

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.

[ciclo]

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.

[Johntheo1]

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.

[ciclo]

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.

[toadJo]

I am still lost, my mechanic didn't check s**t and said if you want bring it on monday and i can replace the termostat, i asked him what else could it be , a hose etc, he said nothing i checked everything before replacing the pump and the radiator, i am looking for a new shop currently. From my understanding of the situation the only logical thing would the thermostat and the thermostat housing.
I have no coolant loss, oil isnt milky and the hoses arent hard when the car is cold, also coolant color is deformed so a head gasket leak is impossible i believe.

[toadJo]

So, today i went to another mechanic shop from a friend that only does VAG group cars, Turns out 1 of the 2 thermostats doesnt open at all. How did they find out? They drove the car like i did and 1 hose at the return side was staying cold. Shop said i need to replace the whole plastic housing with both thermostats and do a drain and fill.

[Johntheo1]

A faulty stat was high on my list of suspects, it's surprising they don't have a sensor stuck in the block which would soon show if there was a closed stat on the cylinder block instead of relying on all their fancy footwork, ironically it was the comfort gauge that drew your attention to it, was any alarm flagged up at any time?
You might ask/find out which stat has failed, I am assuming its the cylinder one.

[ciclo]

Glad you found a VAG specialist who gave you a clear path forward.
Replacing the entire plastic thermostat housing is definitely good preventative maintenance on the EA111, as those housings are notorious for warping over time.

Just to clarify one technical detail for future readers following this thread:
- Based on the OBD logs shared earlier -holding 105°C–108°C at 133 km/h and dropping to 100°C within two minutes of stopping-, the thermostat could not have been completely seized shut. On a high-output 1.4 TSI Twincharger, a totally blocked cylinder block stat would cause an immediate overheat (+125°C), trigger the red warning light, and boil over within minutes of highway load.
- What the shop likely noticed is a lazy/partially sticking stat, or simply the secondary circuit (95°C block stat) staying closed during a light test drive while the primary head circuit (87°C) was doing all the work, which is actually normal behavior under low load.

Either way, putting a brand-new OEM thermostat housing on a 2011 CAVE engine is never a bad investment.
Thanks for your report for the forum, it will be very useful for future readers. Enjoy the car and your inner peace!

[Johntheo1]

Looking at the numbers again, its difficult to see if the cyl or jacket stat was only partially open. At a average speed of 118.75KPH (avg over 2 minutes) the coolant temp was 106.25C, oil temp 99.48C, Rad out temp 90.3C, highest fan speed 41%
At that moderate speed the engine was probably only running at a very modest probably ~ 40/50BHP, but the cylinder head temp was over 106C, if the cyl head stat is OK and begins to open at 87C and is fully open at 102C then the only reason I can see for this is that the jacket water was extremely hot and transferring heat by conduction to the head. If the jacket stat was OK then the cylinder head stat mustn't be opening sufficiently to give a coolant temp of probably ~ 95C ish at that modest engine output. Just wonder if the comfort gauge hadn't gone above its comfort zone (assuming it has one), and had not been seen, would there be any post on here re this problem.