I don't agree with your Temperature of the Coolant going up to 205F. The OEM VW Thermostat is a 87C Thermostat which is about 188 degrees Fahrenheit. The Thermostat should start to open at that Temperature and be fully opened at around 195F.
I do have a Scangauge which does real live Data and it usually does climb to about 196 or even 200 at times when it is extremely HOT out but it rarely ever climbs above 200. And yes, I have changed the Thermostat every Timing Belt Service.
Second for this Beetle and always with the OEM VW Original Thermostat. I don't trust those aftermarket ones out there.
So maybe your Scan Tool is off a bit or perhaps mine but if a Thermostat is rated at 87C or 188F it should stand to reason that it shouldn't climb much more than 195F before it is fully open or there is a problem with the Thermostat. Perhaps a Soft fail and it's not opening all the way?
Engine Temperature 101!
So here goes, I have 3 different scan tools, Ultra Gauge & Fluke IR and K style thermometers. All devices read very close to reach other. There is not an issue with my tools being off. Additionally the engine temperature is actually reported via ECU to the ODB stream within the car anyway, it is not provided raw to the scan tool.
My car has had the coolant sensor replaced with the green top sensor and I have correlated that the coolant sensor and ambient temperatures are close or agree for cold start situations.
The thermostats do no start to open at their rated temperature, they stat to open before their marked temperature and then open a fixed amount that is typically their nominal operating point (temperature rating usually on the box and maybe stamped on the actual thermostat), they can physically open beyond the operating point as the engine temperature increases to allow more coolant to circulate. This behavior is pretty typical of most thermostats.
Additionally, even thermostats with weak springs will open to the same point at their nominal operating temperature, not further. What usually happens with "soft" thermostat is the spring gets weak and they start to open too early allowing engine coolant to circulate through the radiator too soon. Almost every "soft" failed thermostat I have tested will usually open to the same point as their nominal operating temperature opening distance. The only real deviation from this is a thermostat that for some reason does not close properly allowing coolant to continually circulate and this does not occur very often from what I have seen.
Now on the next major myth and misunderstanding, by even "seasoned" mechanics, that needs to be clarified is what is the proper engine operating temperature. I can assure you it is NOT the thermostat operating temperature and it typically not even within 10F of the thermostat operating temperature. It is usually somewhere between 15-20F above the thermostat operating temperature. This confusion is why so many cars on the road have "chronically" cold running engines.
Engines will rarely run even close to the thermostats operating temperature. This is due to the location of the temperature sensor within the engine. Most modern vehicles have the temperature sending unit located somewhere in the cylinder heat, usually in the output path of the coolant as it leaves the cylinder and is being directed toward the radiator.
Because of the location of the temperature sending unit, it is actually measuring the coolant temperature after the thermostat has opened and as the coolant has extracted heat from the engine and cylinder head(s). This is done pretty much by design so the ECU can more closely monitor combustion chamber temperatures to more closely control emissions, fuel mixture and ignition timing. As I think most of use recall, combustion chamber temperatures rise rather quickly as the fuel mixture leans out and prolonged exposure to extremely lean conditions can cause engine damage if the combustion chamber temperatures continue to rise too high. So measuring the engine coolant temperature as it leaves the cylinder head had multiple benefits.
Here is the basic benchmark I have come up with over the many years of working on cars/trucks and other engines. Once the ambient air temperature is about 70F, you cannot easily determine if a thermostat is weak. Using a garden hose and cooling then engine radiator you can super cool the radiator and simulate a cooler environment, but this does not always work a 100%.
I do have one car where the coolant temperature sensor is located directly on the thermostat housing and below 70F this car will report the engine coolant at the thermostat operating point, but this car also has a 79C/175F thermostat and I can tell you that because of the nature of this specific engine and where the temperature sensor is located, you can determine a bad thermostat very quickly if you know what to look for. Also on this car, even a 10F drop in operating temperature will be very noticeable in fuel usage and performance. And I can tell you that the thermostats last on average 2 years at most. There is very little room for error on this engine with the cool temperature that is runs.
You can expect that most engines at a minimum should register at least 10F+ above the thermostat operating temperature. A more likely window is the engine should consistently run 15-20F higher than the thermostat operating temperature. If your engine is not operating consistently at least 15F+ above the thermostat temperature, you likely have a "soft" failing thermostat due to a weak spring.
Well what I can tell you is almost all modern engines are designed to run typically 200-210F based upon where the temperature sensor is located. Unfortunately engine coolant temperature is not always the best way to judge an engines temperature as compared to oil temperature. Oil temperature along with engine coolant temperature is a much better indicator of overall engine temperature.
If you only change your thermostat with timing belt and water pump changes, you likely are only on your 2nd or 3rd thermostat in a 13 year period. I can tell you from all the cars I look at and from a sampling of problem thermostats I have played with, any thermostat over 3 years old is really not reliable in my book. Likely the spring is weak and the thermostat is opening too soon and causing a cold running engine.
You are telling me that your engine temperature only varies by as much as 12F MAX and this is only on "extremely hot" days. You obviously have not seen that engine coolant temps have a hard time maintaining anything tighter than about 10-12F window under most ideal circumstances anyway.
In closing what I can say base upon the information you are reporting, your thermostat is in a "soft" fail state due to a weak spring. I would expect your engine to be running in the 200-205F range, not so close to the thermostats operating point. But you have made it quite clear the problem is with my car and the numbers I am stating. Well the numbers I am stating are not only from my car, buy many of my cars and other similarly configured cars with thermostats that are either new or less than 2 years old that are within the expected operating window that I have found to be "the norm".
I know you will disagree and do not want to hear anything I have to say, but unless you can get a handful of similarly configured cars to report in on what their "nominal" operating temperatures are, especially with a thermostat that is less that 1 year old, only then you will likely see what I am saying is in fact correct.
My point all along has been to attempt to educate everyone on what the proper engine operation temperature should be and that the VW/Audi idea of only replacing the engine coolant sensors for a CEL with a engine coolant code is VERY much not the correctly resolution!
Engine coolant temperature threads are just like oil threads, way too many opinions and not enough facts.
What I can tell you is at least with engine coolant temperatures you can talk about them all day long, but until you actually get a scan tool an read the real time value know where the benchmark is, there is nothing but hot air!
chill chill we get the message 200-205 F temperature.:crazy: last time I checked mine it was running between 199-202, fluctuating slow. The ambient temperature was 70-80f Texas heat
