My car just started to randomly overheat out of nowhere. It doesn't do it everytime I drive but mostly on the interstate but it also does it when I idle. Its fine when I turn the heat on but now the heat doesn't even come on everytime I just have to get lucky and hope it does..any suggestions on what to check?

 

Coolant level.

 

i just filled the coolant and the radiator fan does come on at higher temps..

 

Lower radiator hose cold when it overheats?

 

i have never checked that hose i know the upper hose is expanding and gets very hot so im planning on changing that today

 

Thermostat

 

I was thinking the thermostat but wouldn't that cause it to overheat every time i drive it?

 

and what would cause my heater to work randomly?

 

If the thermostat were stuck closed, then yes, it'd overheat all the time. When thermostats go bad, sometimes they get stuck in one position (totally open, closed and everything in between) and sometimes they don't open all the way or open in a linear fashion. And when they are stuck closed, they don't allow coolant to circulate through the engine or the heater.

I typically replace the thermostat as part of routine maintenance when doing a timing belt (every 100k miles) so as to avoid this problem.

 

ok well i will be replacing the thermostat but why would my heater just randomly not come on?

 

You get heat because hot coolant is flowing through a small radiator with a fan blowing air across it into your cabin. If hot coolant isn't flowing, you don't get heat.