What to do if your car overheats

What happens if your car overheats and you keep driving?

How long can a car overheat before damage occurs? Not long unfortunately. Overheating is really bad news and can quickly cause serious and costly damage to your motor. You’ll need to decide for yourself what you want to do. Should you risk driving it to a shop? Or call for a tow? It’s your car and your money, but we’ll try to help you make an informed decision.

What causes a car to overheat?

Overheating is usually caused by five types of issues:

  • Low coolant
  • Poor coolant flow
  • Lack of air flow over the radiator
  • Lack of cooling system capacity
A Toyota thermostat sitting on a counter.

Car overheats when ac is on

Turning on the air conditioner does a couple of things. First, it puts and extra load on the engine. The A/C compressor makes the engine work harder and when the engine works hard it creates more heat. If you’re making the engine work hard to climb a hill and cool the inside of the car at the same time, the car may overheat. This is why you’ll often see “turn off you air conditioning signs before a long grade.

The other thing using the air conditioning does is to put all the heat it removes from inside the cab and put it outside the car. But guess where it goes. Right in front of the radiator! The radiator needs to have cool air flowing over it to cool the engine coolant. When the A/C is on, the air is warmed by the A/C condenser before it reaches the radiator.

If your car overheats when idling with AC system on, then you may have an issue with the condenser fan, which comes on whenever the air conditioning is turned on. This fan pulls air over the condenser and radiator so that when the car isn’t moving, there is still enough air flow to keep the AC working and the engine running cool.

Car overheats when idling but not while driving

If your car overheats when idle but cools when driving, then you likely have a cooling fan issue. This is actually one of the more common reasons why cars overheat. Just like the condenser fan, the radiator fan pulls cool air over the radiator while you’re idling.

If you’re wondering, “Why does my car overheat when I stop?”, a problem with the radiator fan is a likely issue.

Can you drive the car in, or do you need roadside assistance? Well, calling for a tow is always a safe bet, even if not necessarily the cheapest or most expedient. However, if you can drive like a shark, meaning not stopping and sitting in traffic, then maybe you can make it to an auto repair shop. Just be sure to keep an eye on the engine temperature gauge!

Car overheats when driving but not when idling

What if you have the opposite problem? What if the car is fine while idling or stuck in traffic, but engine overheating begins as soon as you start driving on the highway. When you drive aggressively, or on the freeway, or up a hill, the engine needs to work harder.

If your temperature gauge is fine until you make the car work, the most likely issue is coolant flow. A coolant flow issue can be caused by a faulty thermostat (not opening fully), a water pump impeller or loose drive belt, a bad electric water pump, or a restriction in the radiator.

The other possibility is that cooling system capacity is too low. Cars are designed with excess capacity, that is they more ability to cool than they should need. However, over time some cooling system capacity is lost. Just like human arteries slowly fill with cholesterol deposits, radiator passages get coated over time and lose both flow and thermal transfer.

Car overheats unless heater is on

Turning the heater on can solve a lot of problems. When the heater is on, your car’s cooling system capacity is increased. The heating system uses a mini radiator under the dash to heat the car. It’s called a heater core. When the heater is on the blower blows air over the heater core which will decrease the coolant temperature. Of course, it will also make the cabin hotter, which might be unpleasant on a hot day. But it’s better than buying new engine.

Low coolant

Low coolant is by far the most common cause of overheating. If you can’t drive 15 minutes under any conditions without the car overheating, suspect coolant level.

Where did the coolant go? Unless you can see it spraying out, you may need to hire a mechanic to pressure test the cooling system. If that doesn’t reveal the leak, UV dye testing or checking for head gasket leaks with a 5-gas analyzer may be needed.

You should add coolant before continuing to drive, but NEVER REMOVE THE RADIATOR CAP WHILE THE ENGINE IS HOT. Hot coolant will spray out and remove skin. It can be bad! Check for pressure by squeezing a radiator hose before removing the cap.

Low coolant can prevent the gauge or light from warning you when the engine starts to overheat which can be really dangerous.

Imagine holding a thermometer over a pot of boiling water. You won’t read the temperature of the water because air doesn’t transmit head as well as water because it’s less dense.

Cars almost always have the temperature sender (like a thermometer) at a high point in the cooling system. Heat rises, and the engineers want to know what the temperature is at the hottest point in the system. That’s all great until the coolant level drops. When that happens, your gauge may say everything’s OK when it absolutely isn’t. You won’t see the gauge rise until the engine is extremely hot and the coolant is boiling.

Can low oil make your car overheat?

Not really. Well, yes sort of. Oil does help cool, but you’ll have other issues long before overheating if the oil level is too low. So, check your oil level regardless. It’s very important.

What to do when your car overheats and shuts off

First, try to avoid this. Frequently it signals the end of a car engine. However, if it does happen, don’t give up yet. Tow the car to an auto mechanic and have it evaluated. Sometimes an engine will survive, even when it has overheated so badly that it stops running.

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