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So you may be enjoying the nice weather, but suddenly your car won’t start. Is it a coincidence, or can the sun stop your car from starting?

In this article, we’ll explore possible reasons your car won’t start, what you can do to get it started, and how to prevent it in the future.

Briefly, before we get into detail.

The possible reasons your car won’t start when parked in the sun are vaporization of fuel, a weak or bad relay or starter motor, a weak battery, and low coolant level. 

Let’s discuss each cause one by one.

1. Bad Starter Relay 

Sometimes, if your car won’t start in hot conditions, there’s a problem with the electrical connections. The most likely connection is the starter relay, which might not turn on when it gets hot. 

Now, most relays are certified up to 125 o C, but for relays that are starting to fail, a temperature below this might be the final act that makes it fail completely.

Relays are found inside the fuse box. Most cars have a least five of them located there. Their job is to open a circuit when a switch within the car is turned on or off. Most relays are interchangeable, so you might find when you look inside the fuse box, many look identical to the others. 

If this is the case, look at the legend – which is normally on the box’s lid – and see if the starter relay can be swapped out for an identical one in the box. If so, swap them out and try and start the car. If the original relay is at fault, this test will confirm it. 

If your starter relay differs from all others within the fuse box, you’ll need to test it. 

Relays are a cheap fix and will cost between $20 and $40 depending on the make and model of your car. It’s always best to go for OEM replacement parts. You may pay a few dollars more but the increase in reliability is worth the added cost.

2. Bad Starter Motor

The problem might be the starter motor if the relay is good. 

A higher temperature significantly affects the resistance of the electrical components; the higher the temperature, the higher the resistance. 

So, if the car is parked under the sun for hours, the heated starter motor won’t draw as much current as is needed to start the car. The copper windings within the starter can get extremely hot and increase the resistance. This is called heat soak.

In that case, when you turn the key, you hear the struggling sound of the starter motor. This may indicate that the starter is not functioning properly. 

The starter motor has likely shown signs of failing before. Have you turned the key, and nothing has happened for a second before the car decides to crank?

3. Weak Battery

Another reason why the car won’t start is a weak battery. You may be familiar with the function of the battery. The battery provides the power to turn the starter.

If you already have a weak battery and have parked your car in the sun, the heat drains the battery very quickly. 

Car acid batteries contain an electrolyte, and when the temperature rises, this liquid inside the battery evaporates. 

It’s easy to check the battery without equipment. Turn your car key once, and then turn your headlights on. Are they bright? Turn on the A/C and see if that is weaker than you would expect. 

A more thorough test can be done with a multimeter.

This is a sure sign that your battery has been drained. A car with a bad battery in the heat will struggle even more in the winter, so now might be the time to invest in a new one.

Many visitors also read this article: 10 Dead Car Battery Tricks To Get You Back On The Road Fast

4. Fuel Vaporization

Fuel is a volatile compound that can change into vapors at a high temperature. When the car is kept under the sun for quite a long time, there is a possibility that the fuel inside your car gets vaporized. 

When that happens, the vapors act as a hindrance for the fuel to circulate. Due to lack of circulation, the car becomes difficult to start. It only takes a temperature within the fuel system of 60 celsius or 140 Fahrenheit for fuel to start turning to vapor. This temperature has never been recorded in the U.S.  outside in the open air but can easily be achieved inside cars and fuel systems where the heat can’t dissipate.

This problem is common in old cars that don’t have fuel injectors and use carburetors. You won’t face this problem in modern cars with Electronic Fuel Injection (EFI) systems. Since in these newer cars, the fuel is injected by the injector at higher pressure. 

To fix this problem, you’ll have to cool down your car with water – not always possible – but you’ll need a good supply to bring the temperature down or wait until the sun sets and the car temperature drops.

Now, if your car has an EFI system, the fuel might not be a reason for the car not starting. You need to look for other reasons that are causing this to happen, which we’ll examine below

5. Low Coolant Level

Another reason your car cannot start is due to low coolant. As mentioned, it does get very hot under the hood, and if the liquid in your coolant reservoir is mostly water, it will evaporate.

Many visitors also read this article: Low Coolant Symptoms – Full List of 10

If the level of coolant in the engine is low, then the car’s ECU won’t allow the car to start since it would damage the engine. 

On average, you should change your coolant after 30,000 miles as it will start losing its heat and cold resistance effectiveness after this period.

6. Coolant Temperature Sensor

When you try to start your car, the coolant temperature sensor sends the message to ECU that the engine is too hot to start. 

Under such circumstances, the car won’t start until the engine returns to normal temperature. You can wait a few hours till the engine regains its normal temperature. Then try to start it again.

What might be the other causes for the car not starting, apart from hot weather?

If you are experiencing problems starting your car in the sun but also on cloudy days. That may be due to faulty spark plugs, blocked air filters, dead battery, corroded battery terminals, or a faulty coolant sensor. 

Briefly, we’ll examine those here.

Spark Plug

If a spark plug is at fault, the engine will crank but won’t start. Your car’s increasing fuel consumption, reduced acceleration, and engine misfires might have already shown themselves. 

Air Filter 

Apart from fuel, the other thing that is very crucial for combustion is air. The car contains an air filter that ensures clean air enters the combustion chamber. 

If this filter gets clogged with dirt, the airflow is affected, resulting in incomplete combustion and the production of soot, which damages the spark plug. Also, you might notice black smoke in the exhaust. This blocked air filter also makes it hard for the car to start.

Corroded Battery Terminals

The terminals of a car’s battery get corroded over time. As a result, you may have difficulty starting your car. This may be because the alternator is overcharging the battery or the cells in the battery are cooked. This is called sulfation.

Faulty Coolant Sensor 

This sensor sends information to the ECU about the engine’s temperature. If the sensor is disconnected or faulty, the ECU may interpret the temperature as too high, and the car won’t start. 

In conclusion

We’ve outlined the five main reasons the sun can stop your car from starting. 

To Recap, they are:

  • Bad Starter Relay
  • Weak Battery
  • Low Coolant
  • Fuel Vaporizing
  • Coolant Temperature Sensor.

Most fixes require simply waiting for the temperature to cool. You could lift the hood and let out the excess heat.

A longer-term fix is to replace weak parts. Relays are cheap, a battery will cost between $75 and $200, and fresh coolant will only cost about £20 if you do the job yourself.

Of course, you can always try and park your car in a shaded area, perhaps in an underground car park or under a tree. I realize this may not be possible and isn’t a long-term solution, though!

Often OBD2 readers are used to check fault codes in a car but can leave you unable to start your car after using it. If this happens to you, be prepared by reading our recent article: Car Won’t Start After Using OBD Reader – 3 Quick Checks

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