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Car Overheating Symptoms – Watch Out For A Vacuum Leak! 

car overheats when the AC is on

Eventually, your car, truck, sedan, or pickup might start running rough and overheating. You might be wondering – what is the cause, and how can you fix it? Even if you are not the most tech-savvy person when it comes to mechanics and cars, you should know the signs and car overheating symptoms to identify the causes of your car overheating while idling. 

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Rough idle when cold is a common problem and can be pretty difficult to diagnose and pinpoint the exact problem. Several factors and systems come into play when trying to diagnose the cause of your car overheating when idling and recognize the car overheating symptoms. 


The way in which your engine is idling is a good indicator of its overall health and how long it is going to last. If you address the rough idling and car overheating when idling issues quickly, you can save potential expensive repairs or replacements in the long run.

10 Most Common Car Overheating Symptoms 

To get a good idea of how much you could spend on fixing the car overheating, we have identified some of the most common car overheating symptoms. By being aware of the warning signs to look out for, and the most susceptible parts that can become damaged and lead to a lack of longevity, you can help keep the repair or replacement costs as low as possible. 

Vacuum Leak

    • Most vehicles have numerous hoses throughout their internal engine system, creating a vacuum for both the car’s fuel and the outside air. In engines that are older models and have a carburetor, this vacuum is in charge of directing the fuel into the engine and powering the car. Most newer cars and modern vehicles have a throttle designed to regulate engine speed and airflow through the system.
      • This modern car mechanism creates a vacuum within the intake manifold. However, over time, these hoses can succumb to wear and tear and become damaged over time and develop a leak
      • If too much air gets mixed in with the fuel, this can cause the engine to misfire, resulting in the car overheating when idling, a high RPM, and one of the most common car overheating symptoms. 
  • Replacing the vacuum hose in your car can range from around $150 to as much as $1,000, depending on the extent and the location of the damage. 

Spark Plug Concerns

    • An engine that causes your car overheating symptoms can also be the result of faulty spark plugs or damaged spark plug wires. 
    • Spark plugs use the electrical current produced from the ignition coils to ignite the correct ratio of the air to fuel mixture within the combustion chamber in the engine. A plug that has become damaged over time or is installed incorrectly can result in the fuel being burned at an improper rate, either too fast or too slow for the combustion to take place.
    • If the damage in the engine and the spark plugs is severe enough, you might also notice that your engine has begun running rough while driving, causing the car overheating symptoms. 
      • Make sure when you're driving or sitting in your car that you pay attention to any jerks from your car or shaking while you are driving, especially if you are accelerating up a hill or accelerating while you are pulling a heavy load. 
  • On average, the spark plugs can usually cost around $30 for you to install and buy yourself, while going to a mechanic could add around $100 to $200 in labor costs for them to install the spark plugs in your vehicle. 

Dirty Fuel Injector

  • Sometimes, dirt that has accumulated in the engine of your car can also be the main cause of the car overheating symptoms. 
  • Fuel injectors disperse and transport fuel into your car’s engine to fuel the car, ensuring optimal performance. That being said, if the fuel injectors in your car are dirty, then this can contribute to poor gas mileage, reduced fuel economy, and the car overheating symptoms. 
  • To replace a damaged or dirty fuel injector, the average price is about $800 to $1,450. Depending on the make and model of your car, the average fuel injector replacement cost is between that amount for the full replacement, with parts alone costing between $600 to $1,200, and the labor cost being between just $200 and $250. 

Carburetor Issues

    • Older vehicles that use a carburetor instead of a fuel injector can have various problems that can cause the car overheating symptoms. 
    • Black exhaust smoke is usually a strong indicator of an issue with the carburetor. Using a carburetor cleaner is a simple step to dissolve these deposits and prevent the car from running a rough idling engine in the cold.
  • You can purchase a bottle of carburetor cleaner at almost any auto parts store or mechanic’s office for around $20 per bottle to remedy the car overheating when idling. 

Faulty Thermostat

    • Your car’s thermostat is a key part of the cooling system, ensuring the right amount of coolant can pass through to the radiator. 
    • If the thermostat is stuck in the closed position, the coolant can no longer move through the hoses and reach the radiator. If the radiator is not cooled off while operating, this can cause the car overheating symptoms. 
  • Replacing the engine thermostat is between $150 and $200, with the labor coming to around $125 and the parts cost around $50 on average. 

Engine Coolant Temperature Sensor Malfunctions

  • One of the main causes of the car overheating symptoms is a faulty ECT sensor.
  • This sensor is in charge of signaling the car’s internal computer, the electronic control unit, when the engine's coolant liquid is running too cold or too hot. The ECT communicates with the computer to deliver more or less fuel to the engine.
  • If the ECT sensor is no longer working properly, you can replace this component between $123 and $158 in most vehicles. 

Broken Radiator

  • The car radiator has an important purpose in the inner workings of your vehicle – it operates to reduce the temperature of the hot liquid coming from the engine. By reducing the liquid temperature and returning it cooled to the engine, it can prevent car overheating symptoms. 
  • If the radiator is not working correctly, you will find this is one of the most common reasons for the car overheating when idling. 
  • Replacing the car radiator will cost between $300 and $900, with the labor involving the mechanic checking for leaks, removing the radiator, flushing the system, and installing a new radiator. 

Faulty Air Intake Temperature Sensor

    • Similar to that of the ECT sensor, the IAT sensor can have a detrimental effect on the air and fuel ratio, the ignition timing, and the car overheating symptoms. 
    • The same way you can check the ECT sensor, you can check the IAT sensor using an ohmmeter. The IAT sensor is supposed to decrease in resistance as the incoming air temperature increases, and the resistance increases as the temperature outside declines. 

Internal Coolant Leaks

    • The problem with internal leaks is that they can quickly develop into further driveability problems, like the car overheating symptoms. 
    • A small coolant leak can harm an important mechanism, like the combustion chamber or the engine, and prevent a cylinder from firing. Once the engine reaches the correct operating temperature to run smoothly, the coolant evaporates, and the combustion within the engine can return as normal. 
  • The average cost for a coolant change is between $94 and $118 based on the make, model, and year of your car. 

Malfunctioning Radiator Fan

    • The radiator fan works to pull air across the radiator to reduce the coolant’s temperature before it is sent to the engine.
    • If you have a broken clutch or fan motor, this is one of the main car overheating symptoms, since it can’t help reduce the temperature. 
  • Replacing the car radiator is between $292 and $1193 at most automotive shops, with the average cost for the radiator replacement coming to $671. 

Damaged O-Ring Gasket 

  • On some car models, manufacturers use O-ring gaskets to seal plastic intake manifolds. Eventually, the O-ring gaskets might become brittle due to the high engine temperatures, causing damage and the car overheating symptoms. 
  • Once the damage has been done, the O-ring might fail to make the proper seal when the temperatures are cold. As the engine warms up, however, the heat can expand the seal, and the vacuum leak can disappear. 
  • If the problem is not solved, owners may need to replace the O-ring gasket in their car. Replacing this part is typically between $900 and $1,800 depending on the severity of the damage and any further issues with the valve seals and the valve spring

How to Minimize Car Overheating Symptoms and Damage 

If you find yourself in a situation where you are certain your car is overheating, there are some steps you can take to reduce the permanent damage to your vehicle and the car overheating symptoms. 

  1. Turn off your air conditioning in your car immediately. As soon as you smell overheating or burning form under the hood, switch off all AC controls.
  2. Turn your defroster on the control panel to HOT, with the fan on the highest setting. Although this may sound counterintuitive, it can help prevent excess overheating. 
  3. Along with this setting, roll down your windows so that the car heater will suck out as much heat as possible from the engine. 
  4. If you are stopped in traffic when you notice the overheating, put your car into the ‘neutral’ or ‘park’ position and increase the engine speed and RPM level. This step helps move cool liquid throughout the internal system of your car to help remedy some of the car overheating causes. 
    1. Don’t excessively press on the pedal, as increasing the engine past 2,000 RPM can exacerbate the car overheating causes.
  5. If you are slowly moving your car, put the car in the lowest possible gear to maintain at a safe speed. 
  6. Once it is safe, get your car off the road as soon as possible to prevent any further engine damage. 

If you keep an eye and ear – and nose – out for the car overheating symptoms, you can help remedy some of the most common car overheating causes and prevent irreversible engine damage for your vehicle. 

Troubleshooting Car Overheating Symptoms 

  • Check the Coolant Level

  • The first step is to turn the vehicle off, park the car on a level surface, and remove the radiator cap.
  • The coolant level should be filled to the radiator’s top. if the level is too low, top-off the coolant with the specific kind for your vehicle. 
  • If needed, replace the radiator cap to prevent any future car overheating symptoms.
  • Make sure there is no air in the system

  • Next, check for any air bubbles within the system that can cause the coolant not to circulate properly.
  • If there is excess air present, this could be one of the main car overheating symptoms in your case.
  • Check the Radiator Fans Are Spinning

  • Turn the car on, pop the hood, and listen to see if the radiator fans will turn on. 
  • If the car begins to overheat and you do not hear the radiator fan, this is a sign the cooling fan motor is one of the main car overheating symptoms. 
  • Test the Fan Motor

  • If the fans do not spin, the issue could be with the electric fan motor or the fan relay.
  • Check the Serpentine Belt

  • The final diagnostic step is to check the serpentine belt for any looseness or cracks. 
  • Pop the hood and look for any pulleys without a belt that can be one of the main car overheating causes, since the water pump will not run and the coolant will no longer circulate.

The Bottom Line

Car owners have to keep an eye out for the most common car overheating symptoms to prevent any unwanted side effects and expensive replacements in their vehicle. 


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