Water temperature randomly drops while car is in motion (VW Golf Mk6)

Question

I have a 2011 VW Golf Mk6 Variant (European spec) with a 2.0 TDI Engine (CFHC engine code: with DPF but no AWD and no AdBlue), automatic gearbox (6-speed DSG), and 200.000 km on the clock (around 125.000 miles).

I have been facing a strange issue in the last year or so: the engine warms up normally (up to 90°C), and then the water temperature (as indicated by the temperature gauge on the dashboard) will randomly start to (slowly) drop until it reaches around 60°C (this happens while the car is in motion). It stays at 60°C for a short while, and then gradually starts to go back up to 90°C. It happens completely at random: sometimes it doesen't happen at all for hours, sometimes it happens multiple times during a single hour. The car drives normally otherwise (no power loss or anything like that).

Pulling over and restarting the engine does not help. The coolant level is fine (close to max). Moreover, there seems to be little to no connection with the outside temperature (the issue appears both in -10°C wheather during the winter as well as 30°C during summer) or engine RPM (it happens in both S gear - when the gearbox shifts at ~3000 RPM, and D gear - when it shifts at ~2000 RPM). However, the issue does tend to happen a bit more often when it's cold outside and/or when I'm driving at lower RPM (in D gear).

Futhermore, the Oil Temperature (as indicated by the on-board computer) also drops whenever the water temperature drops, but not as much (when the water is at 90°C, the oil is in the high 80s, and when the water temp drops to the mid 60s, the oil drops to the mid 70s, and comes back up once the water temp increases).

I'd also like to point out that issue happened roughly at the same time when I replaced the car's battery. I completely forgot to check the battery, and realized that it was due for a replacement only when it was too late: when it was unable to start the car and I was left stranded. The mechanic which replaced my battery with a new one assured me that my that car does not need any kind of coding after putting in a new battery (which is in line with what I've read on the Internet: VW only added mandatory coding after replacing the battery starting with the Golf Mk7, but mine is a Mk6).

I also have a VAG-COM (VCDS) clone which I've used to run an OBD scan, and there were no error codes in the Engine or Automatic Gearbox modules (the only modules which had errors were infotainment-related).

Any idea on what might be the cause of this strange temperature issue? I've read on forums and such that it may (or may not) be a bad thermostat or temperature sensor. I plan to take the car in for servicing at a VAG specialist soon, but I could use an opinion on what to expect (unfortunately, in my country it's not unheard of to be ripped off by mechanics unnecessarily replacing working parts).

Answer 1

It can be:

• a sticky thermostat

Hard to diagnose properly without an independent thermometer, because a lot of modern cars show 90C at the dashboard while the temperature varies between 75C and 125C.

• a failing temperature sensor or a loose sensor wiring

Since the oil temp goes down together with the water temp, the sensors are probably OK (it is not much probable for both of them to fail in the same pattern).

It is easy to get an electronic thermometer with a ~2m wire to the sensor and use it to double-check the real temperature of the engine.

• a buggy software in ECU

The car is old enough and the problem should be known, if exists.

• as well as pretty much a normal behavior in some driving patterns.

All of my cars cool down to 60C and even lower on a prolonged downhill. Diesels should be even worse in this regard.

• Contaminated (aged) coolant.

Does your coolant (after running the car for a while) appear transparent and feature pure, bright color? (It can be blue, red, pink, violet or maybe green depending on the coolant type and brand).

Discoloration (no color, but still transparent) = problems soon.

Brown (rusty color), opaque coolant = problems now.

• While you are at it, check the coolant level.

Low level = problems.

High level (over the "high" mark) = expensive problems.

Answer 2

I think you can probably boil it down to (no pun intended, lol) a bad thermostat. It sounds as though its just not working as intended. You can pretty much eliminate the ECU as part of the problem, because nothing changes when you've restarted the car. If the thermostat is sticking or slow to react, it can go in/out of temperature like you're describing. About the only other thing to check is that the coolant level is up to snuff. If it is and there's not been any work to the cooling system (ie: water pump change; coolant maintenance), then there'd most likely be no air pockets, which can also cause this issue. If there had been cooling system maintenance for any reason, you might try and see if the system needed burped (however, I believe your VW has a self-purging system on it, so probably not the issue).