* Low Coolant Level: The most basic, yet crucial, check. A low coolant level means the system isn't effectively transferring heat, leading to overheating and boiling. Check for leaks around the radiator, hoses, water pump, and the reservoir itself. A leak could be slow and only visible when the system is under pressure.
* Faulty Thermostat: A stuck closed thermostat prevents coolant from circulating properly. The engine overheats as the coolant can't reach the radiator for cooling. A stuck *open* thermostat can also cause issues, but usually leads to slow warm-up and insufficient heat in the cabin, rather than immediate boiling.
* Water Pump Failure: The water pump circulates coolant. If it fails, coolant won't circulate, leading to severe overheating and boiling. You might hear unusual noises (whining, grinding) from the water pump if it's failing.
* Radiator Problems: A clogged radiator (due to corrosion or debris), a damaged radiator (leaks or holes), or a failing radiator fan (or a non-functioning fan clutch) prevents effective heat dissipation. Check for clogs by feeling the upper and lower radiator hoses when the engine is running – they should both be hot.
* Head Gasket Failure: A blown head gasket is a serious problem. It allows combustion gases to enter the cooling system, creating pressure and causing the coolant to boil and possibly forcing coolant out of the reservoir. You might notice white smoke from the exhaust or milky oil (oil contaminated with coolant). This is the most expensive issue to fix.
* Cooling System Leaks: Leaks anywhere in the system (hoses, radiator, water pump, etc.) will reduce coolant levels and cause overheating.
* Faulty Radiator Cap: A faulty radiator cap might not maintain the proper pressure in the cooling system, leading to boiling and loss of coolant.
What you should NOT do:
* Do not open the radiator cap or reservoir cap while the engine is hot or even warm. The pressurized system can release scalding coolant causing serious burns.
What you SHOULD do:
1. Turn off the engine immediately. Continuing to drive could cause catastrophic engine damage.
2. Let the engine cool completely. This might take several hours.
3. Check the coolant level (carefully, after it's cooled).
4. Inspect all hoses and connections for leaks.
5. Check the radiator for clogs and leaks.
6. Have a mechanic diagnose the problem. Attempting repairs without proper knowledge can be dangerous and potentially worsen the issue.
Overheating can cause significant engine damage. Get this addressed by a mechanic as soon as possible.
