Car Overheating: What to Do and Why It Happens

What to Do RIGHT NOW If Your Car Is Overheating

I'm Anthony Calhoun — 25-year ASE Master Technician. If your temperature gauge is climbing into the red or your temperature warning light just came on, do these things in order:

1. Turn off the AC immediately. The AC compressor adds load to the engine and generates additional heat.
2. Turn the heater to MAX heat, fan on HIGH. Yes, I know — it is miserable. But the heater core acts as a mini radiator. Running the heater pulls heat away from the engine. It can buy you enough time to get somewhere safe.
3. If the temperature keeps climbing, pull over. Find a safe spot — parking lot, shoulder, side street. Turn on hazard lights.
4. Shut off the engine. Let it sit. Do not touch anything under the hood for at least 30 minutes.

Why Cars Overheat — The Simple Version

Your engine is basically a series of controlled explosions happening thousands of times per minute. That generates enormous heat. The cooling system's job is to absorb that heat and dump it into the air. Here is how it works:

1. Coolant (a 50/50 mix of antifreeze and water) circulates through passages in the engine block and cylinder head, absorbing heat.
2. The water pump pushes coolant through the system.
3. The thermostat is a valve that opens when coolant reaches operating temperature, allowing it to flow to the radiator.
4. The radiator is a heat exchanger — hot coolant flows through thin tubes while air passes over them, cooling the fluid.
5. Cooling fans pull air through the radiator when the vehicle is moving slowly or stopped.
6. The cooled coolant returns to the engine and the cycle repeats.

If any part of this system fails, heat builds up faster than it can be removed, and the temperature climbs.

Common Causes of Overheating

1. Low Coolant (Most Common — About 35% of Cases)

If the system does not have enough coolant, it cannot transfer heat effectively. Coolant does not evaporate or get used up — if it is low, it is leaking somewhere. Common leak points: radiator, radiator hoses, water pump, heater core, and head gasket.

2. Stuck Thermostat

The thermostat is a valve that opens at around 195-210 degrees F. If it sticks closed, coolant cannot flow from the engine to the radiator. The engine overheats rapidly, often within minutes of starting. Thermostats are cheap ($15-$40 for the part) but are a very common cause of overheating.

3. Failed Cooling Fan

Electric cooling fans turn on when coolant temperature reaches a set threshold (usually 200-230 degrees F depending on the vehicle). If the fan motor burns out, the relay fails, or the temperature sensor that triggers the fan is bad, the fans never turn on. You will overheat in traffic or at idle but be fine on the highway because airflow from driving substitutes for the fan.

4. Leaking or Cracked Radiator

Radiators are made of aluminum and plastic. The plastic end tanks crack over time (especially on GM vehicles), and aluminum cores can develop pinhole leaks from internal corrosion. A leaking radiator loses coolant over time, and a clogged radiator cannot transfer heat efficiently.

5. Failed Water Pump

The water pump circulates coolant through the entire system. When the impeller wears down, cracks, or the shaft bearing fails, flow decreases or stops. Signs of a failing water pump include a coolant leak from the front of the engine, a whining noise, and overheating that gets worse at higher RPMs.

6. Blown Head Gasket

The head gasket seals the combustion chamber from the cooling passages. When it fails, exhaust gases can push into the cooling system, displacing coolant and creating air pockets that prevent proper circulation. Signs include white smoke from the exhaust, bubbling in the coolant reservoir, milky oil on the dipstick, and persistent overheating after other causes have been ruled out.

Can Overheating Damage My Engine?

Yes, and it happens faster than most people think.

• Mild overheating (gauge just above normal, caught quickly): Usually no damage. Cool it down, fix the cause, and you are fine.
• Moderate overheating (gauge in the red for a few minutes): Possible warped cylinder head, damaged head gasket, or damaged seals. Repair costs: $1,200-$3,000.
• Severe overheating (gauge pegged, steam from engine, engine dies): Likely warped or cracked head, blown head gasket, possibly cracked block or seized engine. Repair costs: $2,000-$8,000+ or engine replacement.

Aluminum cylinder heads (which most modern engines have) warp at much lower temperatures than the old cast iron heads. I have seen engines destroyed by overheating for just 5 minutes because the driver kept going.

Repair Costs

How to Prevent Overheating

• Check coolant level monthly. Takes 30 seconds. Look at the overflow reservoir — is it between MIN and MAX?
• Replace coolant on schedule. Most manufacturers recommend coolant replacement every 5 years or 100,000 miles (some long-life coolants go further). Old coolant loses its corrosion protection and can clog the system.
• Watch for leaks. Green, orange, or pink puddles under your car = coolant leak. Fix it before it causes overheating.
• Replace the thermostat if it is ever questionable. It is a $150-$350 repair that can prevent a $3,000 head gasket failure.
• Keep an eye on the temperature gauge. Know where it normally sits and notice if it starts creeping higher. Catching overheating early is the difference between a minor repair and a major one.

For a deeper dive into cooling system diagnosis — including pressure testing, thermostat testing, and head gasket diagnostics — the resources at APEX Tech Nation cover it from a working technician's perspective.