98 sunfire 2.2 wont start below certain tempature

I have a 98 sunfire 2.2 and it starts fine all spring and summer but when winter hits it wont start if it sits longer then 5 -6 hours and the tempature is below 30 degrees. it cranks but just wont start. it did this the last 2 winters. If I put a small heater under the hood it will start right up after about 5 - 10 minutes.

well it sounds like antifreeze it can't handle the cold i never had a problem with mine doesn't sound like a thermastate and the block heater wouldn't be the issue either it still should starting after sitting for a few hours. Get a rad flush that's the only thing i could think of.