Car Overheating: What to Do First, and What Not to Do

What to do in the first minute when your car overheats

Turn the A/C off and crank the heat to full, fan on high, windows down. It feels backwards on a July afternoon, but the heater core is a small radiator. Running it pulls engine heat into the cabin and buys you a little time to find a safe spot.

Then pull over properly. A parking lot, a side street, a wide shoulder. Shut the engine off once you're stopped. If you want to know whether it's cooling down, turn the key to on without starting it and watch the gauge.

Picture a Saturday in July: Route 15 backed up toward Shediac, everyone heading for Parlee Beach, 30 degrees out, A/C blasting, crawling along at walking pace. That's about the hardest work a cooling system does all year. Low speed means almost no air through the radiator, so the electric fan is doing everything, and the A/C condenser is dumping its own heat in front of it. If something in the system is weak, this is where you find out.

• A/C off, heat and fan on full
• Pull over somewhere safe and shut the engine off
• Hood stays closed until the steam stops
• Radiator cap stays on until the engine is cold
• No cold water on a hot engine

Why you never open a hot radiator cap

A running cooling system is under pressure. That pressure raises the coolant's boiling point, which is the whole reason it can run hotter than 100 °C without boiling. Pop the cap on a hot engine and the pressure drops all at once, the coolant flashes to steam, and it comes straight out at your face and hands. People get properly burned this way every summer.

Wait. Firestone says to give it at least 15 to 30 minutes before you even open the hood, and longer before you touch the cap. If you need to check the level sooner, look at the plastic overflow bottle, not the radiator. Once it's fully cold, you can top up with the coolant type in your owner's manual. Plain water will get you home in a pinch, but that's a summer-only fix around here. Water left in the system at -25 °C can freeze and crack things, so get the mix sorted before the fall.

And don't pour cold water over a hot engine to speed things up. Hot aluminum and a sudden cold splash is how cylinder heads crack.

What usually causes a car to overheat

Most overheating comes back to one of a handful of things. When it happens tells you a lot about which one.

Overheats in traffic but fine on the highway? Suspect the radiator fan, its relay or fuse, or the fan motor itself. At highway speed the air coming through the grille does the cooling, so a dead fan only shows up when you slow down. Overheats on the highway or climbing hills but fine around town? That points more toward a radiator that's plugged up, a thermostat that isn't opening all the way, or a water pump that's tired.

Low coolant is the most common cause of all, and coolant doesn't get used up. If it's low, it went somewhere. Look for a sweet syrupy smell, green, orange or pink crust around hose clamps, a wet spot under the front of the car, or a damp radiator. Road salt and our freeze-thaw winters are hard on hoses and clamps, so leaks often show up in spring and get blamed on the summer heat.

• Low coolant from a leak: hoses, clamps, radiator, water pump weep hole, heater core
• Thermostat stuck closed or only partly opening
• Water pump failing: worn bearing, leaking seal, or a slipping belt on belt-driven pumps
• Radiator fan not coming on: motor, relay, fuse, wiring or temperature sensor
• Radiator blocked: inside with scale, or outside with bugs, dirt and leaves
• Bad radiator cap that won't hold pressure
• Head gasket leaking combustion gas into the coolant

Signs it might be the head gasket

This is the one everyone's afraid of, and it's less common than people think. But a few clues are worth taking seriously: white exhaust smoke with a sweet smell that doesn't clear once the engine warms up, coolant disappearing with no leak you can find, oil on the dipstick that looks like chocolate milk, or bubbles rising in the overflow bottle while the engine runs.

A head gasket can also be the result, not the cause. Firestone lists a blown head gasket and a warped cylinder head as two of the things that happen when people keep driving an overheating engine. That's the real reason not to push on for "just a few more kilometres".

One thing that catches people out: when coolant gets very low, the temperature sensor can end up sitting in steam instead of liquid. The gauge can read normal, or even drop, while the engine is cooking. If you've had steam from the hood, or the heater suddenly blows cold air while the engine is hot, don't trust a normal-looking gauge.

Is it safe to drive a car that overheated?

Not while the needle is in the red. Every minute you drive it hot raises the odds of turning a hose or thermostat problem into an engine problem.

After it's fully cooled and topped up, a short, careful trip can be reasonable if the cause was obvious, like a split hose you can see or a level that was just low. Keep your eyes on the gauge the whole way, and if it climbs, stop again. If you saw steam pouring out, noticed white smoke from the tailpipe, found milky oil, or the coolant was nearly empty with no idea where it went, don't drive it. That's a tow, or a mechanic coming to the car.

What a proper overheating diagnosis looks like

Topping up the coolant and calling it fixed is how people end up overheating again on the Causeway a week later. A real diagnosis finds where the heat or the coolant is going.

It usually starts with a pressure test: pump the cold system up to the pressure on its cap and watch for drips and a dropping gauge. Then the cap gets tested on its own. With the engine running, the mechanic checks that the fan comes on when it should, that the thermostat opens and the upper hose gets hot, and that the pump is moving coolant. A scan tool reads the engine's own coolant temperature and any stored codes. If a head gasket is suspected, a combustion leak test checks the coolant for exhaust gas before anyone talks about pulling the engine apart.

That's the part we do in your driveway. If you're in Dieppe, Moncton, Riverview or Shediac, we can come to the car instead of you nursing it across town with the heater on full. If the fix can be done where the car sits, we'll tell you. If it turns out to need a shop, we take it in and manage the repair through to the end. Send us the vehicle, what happened and your postal code, and we'll call to confirm what's going on and the price before anyone drives out.