Hi i had the thermostat changed a year ago and all was fine the needle was in the middle of the guage until 2 weeks ago i had a prang and the rad was cracked. I had to drive it about a mile home with some of the coolant left in the system. Took about a week to scource all the parts at this time it was kept in my garage. Put it all back together bled the system and now the needle on the guage likes to wonder when coming up to rounderbouts the needle goes to 80 or just below on the temp. also when i put the heater on it drops below 80 and comes up gradually. before it never did this even when it was -2. I have noticed it acting diffrently. Checked oil etc and when i took the oil filler cap off it had some of the white gunk not a lot but more than it did have, the car had been used on short journeys through the snow. What should i check the temp sensor and if so how do i check the gauge is working or the sensor is working properly. any help is appriciated.