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2003 jetta tdi heat problems

In the cooler weather (below 40 F) the engine does not heat up sufficiently. The temp gauge remains way below the 190 mark. I get very little heat in the cabin. When it’s really cold if I use the heater and turn the heater fan on the temp guage falls. I have put three thermostats in, changed the temp sensor, and flushed the cooling system recently and replaced the coolant with dexcool. The problem remains the same. This didn’t happen when the car was new if I remember right. What do you think is the cause of this problem?

I’d say thermostat, but you’ve already replaced it several times. Have you measured the ACTUAL coolant temp, or are you relying on the dashboard gauge?

Just out of curiosity, what made you decide to use dexcool?

You have a TDI (Turbo DIESEL Injection) Diesels are more efficient that gasoline engines and create far less waste heat. You car is being cooled off by the cold air flowing over the engine. It is common with TDI in cold weather. Mine does the same thing and in the below zero weather we are having I am lucky to get it up to 150? F.

If you want to do something about it, a number of people have used those foam noodles the kids use in swimming pools to block the radiator.  I suggest you might want to stop by and you can find directions there as well as all kinds of information about your TDI you never thought existed.

For a start replacing with Dexcool was dumb . The VW specified G12 coolant is as good as it gets .
The problem may be a loose impeller in the water pump - VW uses plastic impellers that are not the best way to go .
Get the coolant temp checked first .