I have a 2001 Jetta IV with only 70k miles on it, but was unfortunate enough to have the heat go out on one of the coldest days of the year. It also was over-heating. After a little sleuthing on the car repair forums, I came to the conclusion the water pump was toast.
You see for reasons I don't think anyone can explain this car was shipped with a plastic (ppe plastic) impeller on the water pump. Perhaps it was to save a few pennies or maybe to lighten the car.. who knows? However, you can imagine the hostile environment the water pump has do endure with all the heating, cooling and constant forces. Actually, I'm surprised it lasted as long as it did.
It's not an easy job, but not impossible either for the adventurous DIY'er. The book time on this job is 4.5 hours and will cost between $1000 and $1400 dollars at a repair shop. You can save some serious money considering the parts cost less than $200.
The water pump is on the timing belt so naturally you'll replace the timing belt, main belt, water pump and timing belt tensioner (need special tool for this).
There's a really awesome post on VWVortex with detailed replacement instructions that has everything you need to know. Make sure you buy the right timing belt kit for your engine and get the special tool to adjust the tensioner.
Ultimate Timing Belt DIY | VWVortex
ECS Tuning Ultimate Timing Belt Kit | ECS Tuning
Timing Belt Tensioner Tool | ECS Tuning
