I had my coolant replaced in my MK3 by a garage 2 months ago because there was insufficient anti-freeze in it. I was there when the garage tested it, the engine temp gauge rose up and the fan kicked in and cooled it down.
Anyway, I had a water leak, and kept topping it up with anti-freeze due to the time of year. I eventually replaced the split hose, lost a lot of fluid and topped up with half water/half anti-freeze.
Anyway I got stuck in traffic the other day, the temp guage rose and went up to 100C and I'm sure the fan wasn't kicking in.
Have I possibly got the balance of coolant and anti-freeze incorrect such that the thermostat isn't telling the fan to kick in ?