Thermostat. It's worn out. Get a new one.

Can you do "basic hand tools?" If you can, and your fingers are fairly nimble, there's no reason to spend $400 to replace it. I've done an AWM thermostat without removing anything (except the belly panel - and, that's only to be able to catch the coolant that you'll dump) from the vehicle - just need a long extension, a swivel socket (10 or 11mm, I forget), a good light, and a steady hand.

Damage possible? To your MPGs, definitely. Engine, possibility of some increased wear, because the oil won't get up to temp (and, therefore, can't burn condensed water out of itself.)

 

Yes, it can hurt the car, due to the sludge which is more likely to form in the oil due to water not being boiled off, as cuppie alluded to. The 1.8 is already prone to trouble with sludge/coking, so why encourage it? Cuppie also mentioned the MPG, which will suffer because the ECU will always be running the fuel injectors in warm-up mode, which will over time contaminate the oil with un-burned fuel, plus the catalytic converter will be running hot dealing with the extra gas. A Behr 87* thermostat is just over $13 at autohausaz as an example, and not a big deal to change. Now if you had a 2.8 V6, that would be another story!

 

Good point about the water not boiling off, thank you!

 

Does the 1.8T models have tabs present in the thermostat housing like the TDI models? I know when I replaced the thermostat on our TDI the tabs had been broken off making the seating of the thermostat/gasket a task. A new thermostat housing fixed that issue.

 

When did you last replace it? I would still be skeptical because your car warms up OK and you still get hot air. I would probably replace the CTS first before I dove into the thermostat job. You can also validate temps with an IR thermometer.

 

I second Hirnbeiss

I have read many times that the temperature gauge isn't all that accurate in our Passat's. Might want to make sure the actual temp reading is correct before jumping into a thermostat replacement. Could be the thermostat or the new CTS could be faulty.

I was just in Ill this past weekend. Nice city :thumbup:

 

I also say temp sensor. Have you put your hand around the engine to see if it is Hot at all? How about the coolant hoses, mainly the lower radiator hose. If the thermostat is working it should be warm. Just does not seem right that you are getting heat in the cabin with the temp reading so low.
TomK

 

I'm in the CTS theory. The engine is going to get hot, that's a given, is it getting cooler because of the colder weather, yes but marginally.

IIRC the CTS does two things, sends temp readings to the computer but also sends a signal to the temp gauge in the dash. They are one but separate. The CTS isn't a totally accurate reading, meaning the dash says 190 but the actual temp can be more or less but it is within range of the sensor. The part sending to the gauge on the dash can go nuts. The CTS part that communicates with the computer is working correctly otherwise you would have starting issues. It is very possible the CTS you put in was bad, happens all the time especially with replacement parts (and even parts straight from the factory, had Jeep in the 1990''s that did the same with right off the showroom floor).

Is it going to "hurt the car"? I personally don't believe so, the thermostat and CTS to computer are working properly just he sending unit to the gauge isn't quite right. If you were overheating then yes that would be an issue. I would obviously still replace it because the anxiety of seeing that gauge fluctuate can be unnerving. I bought a 1984 Ford F150 off CL for $500 4 years ago and the temp gauge read 0* for 2 years before I fixed it. Still runs today.