Car Overheating Causes & How to Fix | YourMechanic Advice

Reasons for a Car Overheating

A car Overheating is one of the easiest ways to do some serious damage to your engine. Any time you experience an overheating condition, it is important to immediately stop driving the vehicle and begin to troubleshoot and repair the problem. If you continue to drive an overheating car, the result can be catastrophic.

To troubleshoot a cooling system, you must first understand how the coolant system works. Coolant flows through your engine block and head, propelled by the water pump. The circulation and temperature is regulated by your thermostat and radiator fans, and the fans are triggered by relays that turn the fans on at a predetermined temperature. The temperature is monitored by a temperature sensor, which relays that information to the vehicle’s engine control unit (ECU).

Checking & Fixing the Overheating Car

Materials Needed

Step 1: Check the coolant level and radiator cap. With the vehicle completely cool and parked on a flat, level surface, remove the radiator cap by pressing down while turning counter-clockwise. It’s crucial to do this while the vehicle is cold because you could seriously hurt yourself as the contents are under pressure. You could potentially get boiling hot steam in your face and hands even if your car is showing a low temperature.

Most newer cars have clear coolant expansion tanks where you can see the coolant level without needing to remove the radiator cap. They have minimum and full lines on the side of the plastic bottle. If your car doesn’t have a clear tank, you’ll need to remove the radiator cap and you should see coolant filled all the way to the top of the radiator. If it is low, top off the coolant. If the coolant is low, the radiator will dramatically lose its ability to cool sufficiently.

While you have the cap off, check to make sure the cap isn’t leaking. This is a common issue as the rubber seal cracks and leaks over time. Also make sure to inspect for corrosion, rust, or any visible sign of damage. Replace the cap, if necessary.

  • Tip: If at any point during this walkthrough you find a problem or worn out part, fix the issue and re-test to see if the overheating has stopped.

If you do not test after a repair, and you repair more than one item, you will not know what the original cause of the overheating was.

  • Note: Keep in mind that this walkthrough is written as accurately as possible in order of least expensive solution to most expensive solution.

Step 2: Make sure there is no air in the system. This next procedure is called “burping.” If there are air bubbles in the coolant system, it can cause the coolant to not circulate properly.

To perform this procedure, you will need a spill-free funnel and some coolant. You connect the funnel and run the vehicle to get the air bubbles to rise up and out of the radiator.

There will be a variety of attachments that come with the funnel to attach it to your radiator. Follow the instructions that come with the funnel for use on your specific vehicle.

  • Note: Some cars may require a more advanced solution where you need to have a special tool that does a pressurized evacuation and fill.

Step 3: Verify that the radiator fans are spinning. Turn the vehicle on and wait for it to begin approaching operating temperature. Make sure that all climate control is turned off. Leaving the heat on, or climate control settings on defrost will sometimes bypass the relay trigger, which is what this procedure is testing for.

Pop the hood and listen and visually watch for the fans to kick on. If the car begins to overheat and the fans never kick on, the issue is either the cooling fan motor itself, or the relay that controls it.

  • Tip: If the fans are spinning normally, skip Step 4.

Step 4: Test the fan motor. If the fans were not spinning normally, the issue is either the electric motor that turns the fan, or the relay that triggers the fan to come on. Testing the fan motor will narrow this down to one or the other.

Unplug the electrical connector and expose the two metal pins inside. While wearing gloves, take two lengths of wire, and run one from the positive battery terminal to one of these electrical pins. Take the other length of wire and run it from the negative battery terminal to the other pin. The fan should spin.

  • Tip: Because the vehicle runs on 12v battery power, this test is nearly fool-proof. You cannot connect the wires backwards because in 12v systems, all that this does is reverse the direction the fan spins. If the fan spins, it passes this test.

Step 5: Check the serpentine belt. Make sure that the serpentine belt is still installed. Sometimes these belts break while driving and leave little to no trace that the belt was ever there.

Pop the hood of the vehicle and look for pulleys without a belt on them. The serpentine belt is responsible for turning the water pump and other engine accessories like your alternator. If this belt breaks, the water pump will not turn, which causes the coolant to not circulate. This will cause an overheating condition extremely quickly.

If the belt is there, double check that it is tight. If the belt feels too loose, tighten it with the tension adjustment screw. Many times this screw is found on the alternator or power steering bracket.

  • Tip: Tighten the belt until it has a deep resonance when plucked. The sound is very much like the sound of plucking a bass guitar string. Do not continue tightening past this point. If the belt is too tight, it can snap under load.

Step 6: Check for leaks. Chances are high that if you had a major leak, that would be one of the first things you noticed. However, small coolant leaks can be just as problematic and do not always leave a puddle on the ground.

Leaks not only let coolant out, but they let air in. Sometimes the leak can be so small that it only leaks when the engine is at operating temperature as heat causes expansion. Look around your hoses for coolant.

Sometimes with small leaks, the heat from the engine causes the coolant to dry up and leave a greenish residue around the hoses, rather than a wet spot. If you find a leak, the hose or hoses will need to be replaced.

Step 7: Check the thermostat. Usually, a stuck thermostat will not make the car overheat quickly. When a thermostat goes bad, it will either stick open or stick closed. If it gets stuck closed, the coolant will not circulate properly.

In order to check a thermostat, you will need to drain the coolant and remove the water neck that the radiator hose attaches to.

To test a thermostat, you will need boiling water. Remove the thermostat and boil a pot of water. Place the thermostat in the boiling water for a few moments and then remove it. It should be open when removed from the water and slowly close as it cools.

At this point, if you have not found a problem, the cause of the overheating condition is likely a more serious issue. The water pump may be bad or you may need to replace a head gasket. On the extreme end of this, the engine/head could be cracked or warped. If you end up with any of these problems, there will be a significant amount of work that would be necessary to fix.

Replacing the water pump is the best case scenario if you end up this far. A water pump replacement is not the hardest procedure to do on your own, but if the head gasket is blown or the engine block or head have warped or cracked, then the vehicle will likely need a new engine or major repiars as those jobs are beyond the scope of this particular walkthrough.

If you are unable to diagnose this overheating condition on your own, YourMechanic has mobile technicians who can come to your home or office to inspect your overheating issue for you.

Next Step

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