Coolant temp taking ages to rise

Hi guys I have just replaced the thermostat on my 2001 aufi a6 c5 1.9tdi awx engine. I replaced the top coolant sensor a few weeks ago and 5 he engine is still taking ages to reach 90°. I took the car for a journey on the by pass and it did reach 90 and stayed but this morning I came out temp was about 3° outside and took my normal 11 mile drive to work about 25 mins averaging about 40 mph and it only got upto 70°. The heater are working brilliantly only took about 10 mins to get really hot if that. Can anyone tell me if this is normal or if something is wrong. Also don't know if it's related but white smoke was coming out exhaust for about 10 mins into journey also for a wee while fuel seems to drop about 20 mpg for about 10 mins. Any help would be appreciated.