Topped up coolant, now temp guage fluctuates when heater on full blast

[Jasikasisback]

I recently topped up my coolant and this morning the temp guage went down to 80 when I was driving with the heater on full blast. The car doesn't overheat at all, but I was just wondering if this is normal.
It did not do this in the past. I topped up with water, so is it the lack of antifreeze itself that could  cause this? Just wondering if I should worry about it...The air coming out of the heater is warm and does not get cooler as the temp guage goes down.

I was able to take the cap right off the coolant reservoir after driving 50 miles...no release of air whatsoever.....?

I think I am being a bit paranoid as the cold weather is coming and I don't want to crack the block.

[spudboy6]

You need a thermostat and coolant change mate , I had exactly the same

[tweed]

If the temp drops then the thermostat is not doing its job 

[Jasikasisback]

the car doesn't overheat....and it was alright on the drive home.....i had the fan turn on when i finished driving and this time when I unscrewed the coolant reservoir a lot of coolant came out...so I have left it and will refill when the car cools down...I hope that will do the trick! If not I will have to change the themostat and coolant......

[spudboy6]

the car doesn't overheat....and it was alright on the drive home.....i had the fan turn on when i finished driving and this time when I unscrewed the coolant reservoir a lot of coolant came out...so I have left it and will refill when the car cools down...I hope that will do the trick! If not I will have to change the themostat and coolant......

it wont over heat because the thermostat is stuck open hence the long warm up . the temp drops because the car cant hold heat as it doesnt fully warm up . and the heaters in the car is an extension of your cooling system . so when you have hot air in the car your taking hot ait directly out of the engine coolant hence temp drop .

forget about this air in the system . theres no way it would have snuck in over night for a sleep over unless you have a leak .

thermostat is a pain to fit . but there is a guide on here showing how .