Why Does My Car Overheat While Driving but Not at Idle?

There could be a few reasons why this might be happening. In most cases, however, it’s likely to be an issue with your cooling system.

An Air Bubble in Your Cooling System

Your car’s cooling system consists of an airtight system of tubes that run around your engine bay and help dissipate heat from your engine’s block, cylinder heads, and head gasket. The fact that this system is airtight is particularly important, as this ensures that the flow of coolant is smooth and consistent.

If the system’s airtight seal is broken at any point, however, then air can leak in. The presence of air in your cooling system lowers the boiling point of the coolant, which causes it to vaporize instead of remaining in liquid form. Because vaporized coolant is ineffective at dissipating heat, the engine overheats.

There are a couple of possible ways that an air bubble could get into your cooling system. The culprit is often a faulty pressure cap, but it can also be caused by a blown head gasket, which is a much more serious issue on its own. 

A Blown Head Gasket

For the reasons we just mentioned, a blown head gasket can also cause your car to overheat when driving. There are usually a lot of symptoms of a blown head gasket aside from overheating, however, and it’s usually pretty easy to tell when your head gaskets are going bad.

In addition to overheating, one of the telltale signs of a blown head gasket is the presence of white smoke coming from your tailpipe. You may also see oil or coolant leaking from between the block and the cylinder head where the gasket is located, and the engine may also lose power when driving due to the decreased compression in the cylinders.

If you suspect your car has a blown head gasket, you should have it repaired immediately, as continuing to drive with a faulty gasket can cause irreparable damage to your engine.

A Failing Thermostat

Your engine’s thermostat is responsible for regulating your engine’s temperature and is one of the most critical parts of your cooling system. When your car has been running for long enough and the internal temperature reaches a certain point, the thermostat automatically opens and the coolant starts circulating through the system.

Occasionally, the thermostat can get stuck in either the open or closed position, and while both of these issues can cause their share of problems, it’s only when the thermostat is stuck closed that overheating becomes an issue.

Other than overheating, a thermostat that is stuck closed may show other symptoms. Be on the lookout for any coolant leaking from your vehicle, particularly if it’s leaking from your thermostat’s housing.

A Bad Water Pump

In your car’s cooling system, the water pump is what’s responsible for actually moving the coolant through the system (even though the car coolant is a synthetic liquid and isn’t technically water). If the water pump breaks down, the coolant has no way to circulate through the engine, and thus the engine will overheat.

There are a few ways you can tell if your overheating issues are being caused by a bad water pump. Look for coolant dripping near the front of your engine bay; it might be coming from your water pump if its gaskets are old and corroded.

You should also inspect the water pump itself to see what kind of shape it’s in. If the water pump looks visibly corroded, or if you notice a buildup of minerals around the pump, these are signs that the water pump is not working correctly and should be replaced.

Also, listen for any strange whining sounds coming from your engine compartment as you accelerate; these could be caused by a loose water pump belt, which is usually a sign that certain components within the water pump are starting to get worn down.

A Faulty Radiator

Your radiator is also one of the most important parts of your car’s cooling system, as it’s responsible for keeping the coolant in your cooling system cold enough for it to work.

A radiator can stop working for several reasons; there might be a coolant leak or an otherwise insufficient amount of coolant in the system, your radiator may be damaged or too clogged with bugs and debris to be functional, or there may be a buildup of sludge in your radiator that is affecting its performance.


While overheating is usually caused by a defect in an engine’s cooling system, that still leaves a lot of ground to cover. There are a lot of different components in modern engine cooling systems, and any one of these components has a chance of failing.

Hopefully, you now have the info you need to correctly diagnose the cause of your overheating issues, but unless you’re a skilled mechanic yourself, we’d suggest taking your car to a repair shop to get it fixed. In any case, you don’t want to wait too long before fixing an overheating issue if you want to avoid more serious damage to your engine.