Cooling guide: why your car boils in traffic and how to avoid it

Engine overheating, popularly known as “boiling car”, is one of the most worrying mechanical failures for any driver, especially in heavy traffic situations and at high temperatures. This problem not only causes immediate inconvenience, but can also lead to severe and expensive engine damage, such as head warping. Understanding why the car boils in traffic and how to check the cooling system preventively is essential to guarantee the longevity and reliability of the vehicle. This guide details how the system works, the most common causes of overheating and presents a practical maintenance checklist.

Components and operation of the cooling system

The cooling system is a closed circuit responsible for controlling the engine’s working temperature, keeping it in an ideal range for efficiency and durability. It operates through the circulation of a specific fluid (coolant) that absorbs the heat generated by combustion and dissipates it into the environment. Its main components are:

• Radiator: Honeycomb-shaped structure that promotes heat exchange between the hot liquid coming from the engine and the external air.
• Coolant: Composed of demineralized water and an ethylene glycol-based additive. The additive increases the boiling point, reduces the freezing point and protects metal components against corrosion and oxidation.
• Water pump: It boosts the circulation of liquid throughout the system, ensuring a continuous flow.
• Thermostatic valve: Controls the flow of liquid to the radiator. It remains closed when the engine is cold to accelerate heating and opens when the ideal temperature is reached, allowing cooling.
• Fan (electrofan): It forces air to pass through the radiator when the vehicle is stationary or at low speed, a common situation in traffic, where the natural air flow is insufficient.
• Expansion reservoir: It stores excess liquid that expands with heat and replaces it in the system as it cools, keeping the circuit pressurized and free of air bubbles.
• Hoses and clamps: They connect all components, allowing fluid to pass through.

Diagnosis: why does the car stall in traffic?

In slow traffic, the engine generates continuous heat with little or no natural ventilation. In this scenario, the cooling system is pushed to its limit, and any failure becomes evident. The most common causes for overheating in this condition include:

• Fan failure: The electric fan is crucial in traffic. If your electric motor, relay, fuse or temperature sensor (known as “onion”) fails, the fan will not turn on, and the temperature will rise quickly.
• Thermostatic valve stuck (closed): If the valve does not open at the correct temperature, it prevents the hot liquid from passing into the radiator. The fluid is restricted to the engine block, causing localized and rapid overheating.
• Clogged radiator: The obstruction can be external (dirt, leaves, insects) or internal (corrosion deposits, use of tap water). Both conditions drastically reduce heat dissipation capacity.
• Low coolant level or quality: Leaks or evaporation can reduce the volume of the liquid, compromising heat exchange. The use of an inappropriate or expired additive loses its properties, lowering the boiling point and allowing the system to “boil” earlier than expected.
• Damaged reservoir cap: The cap is responsible for maintaining the correct pressure in the system. A faulty lid does not seal properly, which reduces the boiling point of the liquid and can lead to overheating.

Checklist: how to check the cooling system to avoid problems

Preventive maintenance is the most effective and economical way to ensure that your cooling system works properly. Carrying out periodic checks is a simple procedure that can be done by the driver himself.

1. Check the fluid level (cold engine): With the engine completely cold, observe the liquid level in the expansion tank. It must be between the “MIN” and “MAX” markings. Never open the cover when the engine is hot, as the system is pressurized and can cause serious burns.
2. Inspect the quality of the liquid: The liquid must have the color specified by the manufacturer (generally green, pink or blue) and be translucent. A cloudy, rusty or particulate color indicates contamination and the need for complete replacement and cleaning of the system.
3. Look for leaks: Visually inspect the hoses, clamps, water pump and radiator for signs of moisture, stains or drips. Dry, swollen or cracked hoses must be replaced immediately.
4. Test the fan drive: With the car stopped and the engine running, observe whether the fan turns on automatically when the temperature needle approaches the middle of the scale. If the air conditioning is on, in many models the fan operates continuously at low speed.
5. Follow the owner’s manual: The manual contains crucial information about the correct type of additive for your vehicle and the recommended interval for a complete coolant change, which generally varies between 30,000 km and 60,000 km, or every two years.

Keeping the cooling system in perfect condition is one of the most important actions for engine health. Overheating problems, especially in severe conditions such as urban traffic, are almost always the result of neglected maintenance. A simple visual check routine and strict adherence to coolant change deadlines, as specified by the manufacturer, are sufficient to ensure reliable operation and avoid costly repairs. Attention to components such as the thermostatic valve and fan is essential for a heat-proof system.