What exactly causes radiator overflow? I turned on my heater this morning and when I got home noticed medium smoke from my hood gaps. Opened my hood and coolant was boiling over. I was gonna flush it when I get home but could it really fix this dillema or is there something bigger going on? Any advice would be grateful. Peace.

 

Where was the coolant boiling over from? The radiator or the overflow tank on the side?

Boiling coolant overflow indicates a problem. I would start by replacing the radiator cap, then thermostat. Also, your radiator may be clogged. To test, empty the coolant and blow through it....see if you pickup any stiffness.

Replace the thermostat first....thats the easy fix, and make sure to bleed your system properly afterwards.

 

If your coolant was at normal level and one day just started boiling you do have a problem.

 

basically, you have the radiator and the overflow tank. when the car is cold, the radiator is full of coolant and the reservoir has a little bit of fluid in it. when the car gets warm, the coolant in the radiator expands. when it expands from warmth, the coolant has to go somewhere. a valve in the radiator cap opens and lets a little bit of coolant into the overflow tank. when you shut the car off and it cools down, the coolant de-expands. there is another valve in the radiator cap that uses vacuum to take coolant back from the reservoir to the radiator

the system was created to make up for any small leaks, for the environment, and to keep the radiator full at all times. years ago coolant overflow tanks werent used, when you filled up the radiator and the coolant expanded, the coolant would get dumped onto the ground. when the car cooled off, there would be a inch or so of air on the top of the rad to allow for expansion. if the car never overheated after that, you wouldnt loose coolant

if this morning your coolant reservoir was full to the brim with coolant, either the valve in your radiator cap went bad and kept letting coolant into your overflow excessively, the thermostat failed, or its the headgasket failing.if its a bad headgasket, what happens with a bad headgasket is combustion gas gets into the radiator and causes excess pressure so the rad cap opens to relieve the pressure and you lose all your coolant that way

if you want to take a shot at it, replace your rad cap and thermostat. both are cheap, if it overheats again something bigger is up