50/50 works best unless you're in Alaska. Then it's 70/30 coolant and distilled water.
Cars operate in a wide variety of temperatures, from well below freezing to we­ll over 38C. So whatever fluid is used to cool the engine has to have a very low freezing point, a high boiling point, and it has to have the capacity to hold a lot of heat.
Water is one of the most effective fluids for holding heat, but water freezes at too high a temperature to be used in car engines. The fluid that most cars use is a mixture of water and ethylene glycol (C2H6O2), also known as antifreeze. By adding ethylene glycol to water, the boiling and freezing points are improved significantly.
Fluid - Freezing Point - Boiling Point
Pure Water: 0C - 100C
50/50 mix of C2H6O2/Water: Minus -37C to 106C
70/30 mix of C2H6O2/Water: Minus -55C to 113C
The temperature of the coolant can sometimes reach 121C to 135C. Even with ethylene glycol added, these temperatures would boil the coolant, so something additional must be done to raise its boiling point.
The cooling system uses pressure to further raise the boiling point of the coolant. Just as the boiling temperature of water is higher in a pressure cooker, the boiling temperature of coolant is higher if you pressurize the system. Most cars have a pressure limit of 14 to 15 pounds per square inch (psi), which raises the boiling point another 25C so the coolant can withstand the high temperatures.
Antifreeze also contains additives to resist corrosion.
Don't forget that 'coolant' is 'anti-boil' as well as 'anti-freeze'. Drain a glass full from your expansion tank and stick it in the freezer overnight. If it freezes, it's too weak. Household freezers are usually around the -21C mark.
Worth a try, as it might be something as simple as your coolant being too weak that is causing it to boil or bubble.
