Car is a 2011 VW diesel.

I was driving through Arkansas and noticed the water tems were running 20-30 degrees most of the time and sometime fluctuated back up to 190 degrees that it normally runs.
I understood that to mean the thermostat may have stuck open. Sure enough, after stopping for fuel it ran up to 190 for awhile then it dropped to a couple notches below that. The thermostat is not easy on this vehicle and I was out of town, driving to Ill and Wisconsin so I thought I'd just put a band aid on it till I got home.

I blocked off the air to the radiator by putting plastic on the outside of the vehicle, taped closed only allowing a small hole on both sides to get to the radiator. Ok, check to see if the temps would climb and climb too high, right?


Here is the odd part. The car went up to 190 degrees and sat there just like the thermostat worked again.
I expected blocking the air to allow it to run warmer but can't explain why it started operaing normally again. Its now two days later and I've and started the car at least half a dozen times now.

Generally the spring can get weak over time or a piece of debris gets caught in the thermostat leaving it partially open.

To test it. Pull it out and stick it in a pan of water. It should open right before the water boils. Then should close after taking it out the water.

That is the hard part and why I'm taking it to my VW mechanic tomorrow.
The whole front end has to be pulled off to get to it. Front bumper, radiator, lower pan, intercooler and fans all have to come out. The instruction video I saw said 30 minutes but not for me.

Here's the deal. The thermostat is step 1 of diagnostic. It eliminates a possible a problem dealing with the issue. Testing it saves you money.

Go by harbor freight. Buy a pressure tester.

It's a hand held pump with a radiator cap adapter. Pressurise it to the rating listed on the radiator cap itself. Nothing more.


The pressure shouldn't drop. If it doesn't, the cooling system is in good condition. There only a few parts in the cooling system. Heater coil bypass is an overlooked part.

I wanna set hands on this thing.


Fan thermostat?

The

Its not even that. It just does not make sense.

The problem surfaced with the engine staying below normal temps. I thought, bad thermostat.
I covered the front of the radiator or 90-95% of the intake.

What happened after that is that the temps came back up to normal and stuck there. One hundred and ninety degrees. Like a rock.
When it was 30 degrees outside, 190. When its been 75 degrees outside 190. As if there was never a problem.
I am going to take the plastic off the front and see if it goes back to running low temps. If it does I'll scratch my head again.

The problem with this thermostat is that its underneath the engine. It takes a lot of part removal to get to it. Also the thermostat is a module, not like a normal thermostat.

Volkswagen should have kept their air cooled motors and in the back. Where they belong.

I don't think it's a thermostat problem. I would look for a sensor issue or bad connection on a sensor. I had/have the same problem on a Ford Explorer, that I may have solved when I replaced the wiring harness connectors. I haven't had any issues since I did the work on it.
The pressure tester won't do you any good for testing a thermostat, there is a by pass around the thermostat for cold starts.

Picked up the car from our VW diesel mechanic Steve.
The thermostat in this car is a module so he didn't test it. It now works properly.