COOLANT TEMPS UNDER OBD

[toadJo]

Hello,Around 3 weeks ago i was cruising at the higway and my coolant needle moved, i went to the workshop and they said i need a new pump and a radiator, i replaced both.
Now when i drive i check the coolant temps through an OBD app, but i can't tell if the problem has solved or not coolant always fluctuates between 95-105, shouldnt the water be stable around 97-98?
Is 100+ on water safe, should i go back to the workshop to check more stuff ( the needle hasn't move since) am i overthinking things.I have like 30 logs of normal driving through the OBD that i can show.
(Polo GTI 6R, 1.4TSI CAVE EA111)

[RUM4MO]

I'm not sure that you should expect the coolant temperature to be controlled as well/tightly as 97>98C, the cooling system, as well as thermostats, has a 2 speed radiator cooling fan - or even 2 single speed cooling fans, so the coolant will end up rising before being controlled by the fan speed switching or fan switching in the case where there are 2 cooling fans, and then getting pulled back down and the fan speed switched or the second fan switched off - and so the process will continue - all normal way of working I'd think.

Edit:- this just shows that most of us don't actually try to find out what coolant temperatures to expect when our cars are being driven and everything is working okay.

I had a post cat Lambda probe issue/failure with my wife's 2015 VW Polo 1.2TSI 110PS, ie B1 S2 - electrically it didn't seem to be "broken" but it had probably reached the point in its life where it was failing to work as intended - so maybe I could have tried cleaning it - but I didn't, I just bought a new genuine VW Group B1 S2 (Bosch) Lambda probe, fitted it and the problem as in logged fault, disappeared.

Anyway, while I was using VCDS with the engine running, I started checking a few other field/channel values - Radiator Outlet Temperature Specified 98C, Radiator Outlet Temperature Actual -40C !!! This was during the summer period in Central Scotland, hummmm. I immediately ordered up a coolant temperature sensor and reported this issue on a Volkswagen Forum - the answer I got was "this engine in that car does not have a sensor that detects/monitors the radiator outlet coolant temperature - so ignore that reading" - so it looked like VW Group messed up and allowed a field/channel value to be viewed where it had no associated sensor input, so it was just reading its "window low limit" - not very clever or useful!

[ciclo]

Within tolerances. Temperatures are correct.

I fully agree regarding the accuracy of some diagnostic software, even if certain specific signals -which might be required for some models due to particular equipment- are involved.

[toadJo]

Happened again yesterday, it only occurs under stay highway cruising needle moved obd 108 on coolant and 93 on radiator outlet, i left it at the shop that replaced my pump and radiator, idk what will they say needs replacing now

[ciclo]

It's within tolerances, you still went back to the workshop? I bet the mechanics there are already hiding their tools when they see your Polo pulling up.

Jokes aside, here is the quick science lesson on why your car is perfectly fine and you should stop staring at the OBD screen before you hit a lamppost:

1. Modern ECUs are smart (smarter than the needle): The car doesn't target a single flat number like 97°C. When you are cruising on the highway under low load, the ECU purposefully lets the temps rise to 100°C - 105°C. Why? Because hotter engines have less friction, better efficiency, and lower emissions. If you step on it, the ECU will proactively drop the temp back to the low 90s to protect the engine.
2. The "Lying" Needle: The reason your dashboard needle hasn't moved from 90°C is because Volkswagen programmed it that way. It's a "buffered" gauge. It stays dead center anywhere between ~75°C and ~110°C specifically so drivers don't panic over normal operational fluctuations. The needle is happy, you should be too.
3. Physics: Your cooling system is pressurized and filled with coolant, not pure tap water. It won't boil at 100°C; its boiling point is well over 120°C. 105°C is well within the safety margins for a EA111 CAVE engine.

Unless you are losing coolant or the dashboard starts screaming at you, your car is fixed.
Do your mechanics a favor, close the OBD app, and just enjoy the GTI!

PD: It is my greatest hope that I have not hurt your sensitivity , lately the temperature rises very quickly in these circumstances.

[toadJo]

Maybe i wasn't clear or idk much about my polo , under highway speeds 120-140 with an ambient temp of 12c the needle shouldn't move nor the water hit 108c with a new pump and a radiator.

[toadJo]

Time Phase Timestamp Engine Coolant (°C) Radiator Outlet (°C) Engine Oil (°C) Car Speed (km/h) Fan Speed (%)
1 Minute Before Peak 00:23:36 106.0 90.3 98.3 121.0 31.2%
During Peak (Start) 00:24:37 108.0 92.3 104.0 101.0 41.1%
During Peak (End) 00:25:38 108.0 93.2 106.2 0.0 (Stopped) 56.5%
1 Minute After Peak 00:26:38 102.0 91.9 106.6 68.0 86.0%

[ciclo]

Fair point! If the needle is actually moving past 90°C and hitting 108°C at 12°C ambient, you are right to be concerned.
Since the pump and radiator are brand new, there might just be a stubborn air bubble trapped in the system or a faulty thermostat.
Show the workshop your logs, and ask them to do a proper vacuum re-bleed. Better safe than sorry!

[ciclo]

Those logs are excellent data, thank you for sharing! Looking closely at the numbers, your cooling system is actually performing beautifully and doing exactly what it's programmed to do.

Notice that your highest temp (108°C) happens right after cruising at high speeds (121 km/h), which is normal, as the engine is working hard against wind resistance.
The moment you stopped (0.0 km/h), the ECU instantly ramped up the fans from 41% to 86%, dropping the coolant temp down to a safe 102°C in just 60 seconds.
Your oil is at a perfect 106°C under load, and the delta between engine coolant and radiator outlet is spot on.
If the dashboard needle briefly twitched near 108°C, it's just the buffered gauge crossing its threshold.

Rest assured, your new pump and radiator are doing an amazing job.

[toadJo]

Timeline,Time,Speed (km/h),RPM,Engine (°C),Radiator (°C),Oil (°C),Fan (%)
-2.0 Min, 00:22:37, 118, 2765, 99.0, 86.3, 91.4, 18.6
-1.5 Min, 00:23:08, 133, 3106, 105.0, 87.4, 94.3, 11.1
-1.0 Min, 00:23:36, 121, 2549, 106.0, 90.3, 98.3, 31.2
-0.5 Min, 00:24:07, 120, 2823, 106.0, 91.2, 101.3, 12.9
Peak Start,00:24:37, 101, 2385, 108.0, 92.3, 104.0, 41.1
+0.5 Min, 00:25:07, 88, 2997, 108.0, 93.0, 105.1, 54.0
+1.0 Min, 00:25:37, 0, 715, 108.0, 93.5, 106.2, 56.8
+1.5 Min, 00:26:07, 68, 1588, 104.0, 93.2, 106.6, 76.6
+2.0 Min, 00:26:37, 66, 1859, 102.0, 91.9, 106.6, 86.0
+2.5 Min, 00:27:07, 38, 1112, 102.0, 90.5, 106.6, 67.0
+3.0 Min, 00:27:37, 22, 1551, 100.0, 89.8, 106.6, 74.4

Here's more data, i still feel something is wrong at cruising speeds

[ciclo]

I completely get where you're coming from. It honestly feels wrong and defies common sense that a car should run hotter when you're moving fast in cold weather. Anyone with driving experience would feel exactly the same way you do.

The real culprit here isn't your mechanical sympathy, it's modern emissions engineering, these cars are built to fight against our natural logic, running hotter than we'd like just to satisfy a computer map.

Since those numbers are giving you anxiety, definitely hand the logs over to your workshop.
You paid for a repair, so you deserve 100% peace of mind. Keep us posted on what they say!

[toadJo]

Car is at my mechanic and from what i saw from my airtag he is driving it so i assume he is trying to replicate the problem so idk we will see