Car Won’t Start Until It Cools Down? Here’s Why!

After an enjoyable drive, you try to restart the car only to find that the engine won’t start until it cools down. It’s so frustrating, especially when you’re in a hurry!

To make matters worse, there’s not just one easy, obvious cause of this problem; it can be one of many. Some of the most common ones are engine overheating, vapor lock, and a failed sensor, all covered in depth in this article. One thing I can tell you is ignoring this problem can lead to further damage. Therefore, it is essential to understand the underlying issue and address it quickly. 

Reasons Why Your Car Won’t Start Until It Cools Down

When looking at the reasons below for your car failing to restart until it’s cold, pay attention to other symptoms; for example, if the engine is also stalling when running, you may have a faulty fuel pump. I’ve included the other symptoms to look out for with each cause. 

Engine overheating

When the engine overheats, it can cause the metal components to expand beyond their normal tolerance. This can result in a loss of compression, which is essential for the engine to start. In such cases, the engine must cool to its normal operating temperature before it can start again. 

Various faults can cause an engine to overheat; they include: 

There are also driving conditions which can cause an engine to overheat, such as: 

  • Driving in hot weather 
  • Towing heavy loads 
  • Driving at high speeds for extended periods. 

Every combustion engine car has a water temperature gauge and a warning light when temperatures rise to dangerous levels. Pull over if temperatures increase too high or a warning light comes on. This is when you may struggle to restart the car until the engine cools back to normal levels, providing no permanent engine damage has been done. You may also see problems, like the battery warning light, check engine light, or the engine entering limp home mode with an overheated engine, but this depends on the cause. 

Vapor lock

Vapor lock is a condition that can occur when the fuel in the fuel lines becomes too hot and evaporates before it reaches the engine. This can happen when the engine runs in hot weather or is shut off after running for a long period. When the fuel line is filled with vapor instead of liquid fuel, the engine cannot start because there is no fuel to ignite in the combustion chamber. 

The car will need to cool down for some time before the fuel in the lines condenses into liquid form. This cooling-down period can vary depending on the severity of the vapor lock and the outside temperature. Sometimes, it may take only a few minutes but may take up to an hour. 

This is a very common occurrence on older carburetor-engined cars, where instead of fuel evaporating in the fuel lines, it evaporates in the carb. There is little you can do to solve this problem; this is just using older cars in modern times, modern fuels, etc. 

Faulty starter motor

The starter motor turns the engine over. When you turn the key in the ignition, an electrical signal is sent to the starter motor. It then engages a small gear to the engine’s flywheel, which starts the engine. Once the engine runs, the starter motor disengages and is no longer needed.

When a starter motor is faulty, you may hear a click or screeching noise or hear the starter motor running, but the engine won’t fire up. This can happen if the motor is worn out, damaged, or has an electrical problem. When the starter motor is overheated, it may stop working until it cools down because it is failing. This is because when the motor gets too hot, the electrical resistance of its wiring can increase, which can cause the motor to stop working. Once it cools down, the resistance decreases, and the motor will work again.

Replacing the starter motor is the only course of action to solve this problem correctly. Temporarily, you can tap the starter motor with a hammer; this can sometimes help force the starter motor to work again by releasing any stuck internal parts. 

Ignition coil problems

When you turn the key, the starter motor cranks the engine over, but it won’t run without the ignition system. The ignition system is responsible for providing the high-voltage spark that ignites the fuel in the engine. The ignition coil is the vital component that converts the low voltage from the battery into the high voltage required to ignite the fuel. 

However, if the ignition coil is faulty or overheating, it can cause the engine to not start or stall after running for a short time. This happens because the ignition coil cannot provide a strong enough spark. In some cases, the coil may work initially when the engine is cold, but as it heats, the resistance in the coil increases, making it harder for it to produce the necessary voltage. Once the car has cooled down, the coil may work again, allowing the engine to start. 

It’s worth noting that a faulty ignition coil can cause other problems, such as misfires, rough idling, struggling to rev past 3000rpm, check engine light on, and poor fuel economy. Therefore, it’s essential to have your ignition coil checked and replaced if necessary to ensure your car runs smoothly and efficiently.

Crankshaft or camshaft position sensor issue

Both the crankshaft and camshaft position sensors do exactly as the name suggests: they detect the position of the camshaft or crankshaft so the power control module can make necessary calculations to the fuel and spark when starting the engine and while it’s running.

When the crankshaft or camshaft position sensor malfunctions, the ECU cannot determine the correct position of the engine’s pistons and valves. This can cause the PCM to send incorrect signals to the engine, preventing it from starting. You may also experience the engine stalling and then failing to restart until it cools down and the check engine light illuminated. 

Depending on the severity of the sensor, replacing it is an ideal fix. However, sometimes you can try to do a force reset by driving at 50-60 mph and then allowing the engine to drop back to about 40 mph naturally. This will allow the engine control unit to relearn the camshaft or crankshaft position and might solve the issue.

Coolant temperature sensor fault

The coolant temperature sensor sends information to the ECU about the engine temperatures, aiding the engine in deciding on the correct quantity of air and fuel for combustion.

A faulty coolant temperature sensor can cause the ECU to receive incorrect information about the engine’s temperature, which will result in the wrong fuel-air mixture being used to try to start the car. If the sensor is reading the engine’s temperature as higher than it is, the ECU may not provide the correct amount of fuel, causing the engine to flood and not start. 

On the other hand, if the sensor is reading the engine’s temperature as lower than it is, the ECU may not provide enough fuel, causing the engine to starve, leading to you struggling to start the engine. Either way, the engine won’t start until the sensor cools down or provides the correct temperature readings. 

With a faulty coolant temperature sensor, one thing to look out for to confirm it is faulty is the coolant temperature gauge going crazy, jumping from one temperature to another. 

Faulty Fuel Pump 

The fuel pump forces fuel from the gas tank through fuel lines into the engine. It uses a small electric motor to create pressure that pushes fuel through the lines.

If the pump is faulty, it may not be able to generate enough pressure to deliver fuel to the engine. This happens with age or contaminants in the fuel affecting the pump motor’s internals. As a result, the engine may not start or stall at any time. When the pump cools down, it may generate enough pressure to deliver fuel to the engine again, allowing the car to start.

Replacing the fuel pump is the only way to solve this correctly; other methods exist to start a car with a faulty fuel pump. You can read more about it here. 

Waiting for the engine to cool before it will restart

How Long Does It Typically Take For A Car Engine To Cool Down? 

The time it takes for a car engine to cool down can vary depending on factors such as the outside temperature, the engine type, and how long the car was driven. However, on average, a car engine can take 30 minutes to an hour to cool down entirely if left to cool naturally. However, there are ways to speed this process up and cut the cooling down time in half to approximately 10 – 15 minutes. 

How to Cool the Car Down Quickly

To speed up cooling the car down, you can do a few things that will help: turn the AC blowers on hot and open the vents fully. This will allow hot air to escape the engine bay, although it will aid in flattening the battery quickly if it is not running, and you will get quite hot quickly. 

Opening the hood is one of the most straightforward ways to cool down a car engine. With the hood closed, the engine bay acts like an oven, with only a few air escape points that depend on the car’s movement and the air passing through the front radiator to dissipate heat and keep the engine cool.

Lastly, it is to make the radiator cooling fan operate. If the engine is hot enough, you should hear the electric cooling fan running regardless of whether the car is started. If it isn’t, you can force it to come on if you have an aftermarket fan switch fitted, or if you know what you are doing, you can apply +12 volts to the fan to make it work (a wire straight from the battery + to the live terminal on the fan plug). With the fan working, it will help remove heat from the engine bay quickly, especially with the hood open as well. 

Can You Drive a Car if It Won’t Start Until It Cools Down?

The issue with driving with this problem is that often, these issues can lead to car stalling, and then you will fail to start the vehicle again. This can put you and other road users in grave danger, especially as it will always happen in the wrong place or when traveling on a fast road. Therefore, I strongly advise you not to drive with this type of issue; instead, seek to have the car repaired or inspected immediately. 

Final Words

To summarise, a car that won’t start until it cools down can be caused by several factors, usually an engine overheating, vapor lock, or crankshaft or camshaft position sensor malfunction. Addressing the issue quickly to prevent further damage is essential. Regular maintenance and timely repairs can ensure your car runs smoothly and efficiently so you don’t get stuck on the road in a hurry.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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