Reasons Why Your Car Is Overheating

A man dejectedly staring at his engine while his dog looks out the window.
Noel Hendrickson / Getty Images

An overheating engine is more than an inconvenience, it can be an expensive engine killer. It may even leave you on the side of the road then on to the repair shop for a serious repair bill. 

If your car has been running hot, you know the feeling. You're sitting in traffic, the light turns green, and you hope that traffic breaks fast enough for you to get some air flowing through the radiator so the temperature needle will go down. It's beyond stressful, and there's no reason you should be forced to endure this.

The fact is, there are usually a few culprits to look into when your engine is running hot.

Your Engine Overheats on Short Trips

If your engine is overheating shortly after you leave, or it heats up even on short trips, you should check the following possible causes and repair suggestions.

Symptom: Engine quickly overheats. Engine runs fine but gets very hot shortly after you start it. This problem usually occurs after only five minutes or after traveling about a mile. You may or may not notice steam coming from the hood or smell coolant.

Possible causes:

  1. Engine coolant level may be very low. The Fix: Refill the coolant to the proper level.
  2. Engine's drive belts may be broken or slipping. The Fix: Tighten or replace the belts.
  3. The electric cooling fan may not be coming on. The Fix: Repair or replace the cooling fan. Repair wiring. Replace the cooling fan temp sensor.
  4. The ignition timing may be set wrong. The Fix: Adjust ignition timing.
  5. There may be a vacuum leak. The Fix: Check and replace vacuum lines as required.
  6. The engine may have mechanical problems. The Fix: Check compression to determine the engine's condition.
  7. The engine's thermostat may be stuck closed. The Fix: Replace the thermostat.
  8. There may be a leak in the cooling system. The Fix: Repair the leak and refill coolant.
  9. Cylinder head gasket(s) may be bad. The Fix: Replace any bad gaskets.

Your Engine Overheats After Extended Driving

In some cases, your engine may be running fine and the overheating problem only occurs on extended drives or long waits in traffic. If this is the case with your car or truck, check on the following possible issues.

Symptom: Engine overheats. Engine runs fine but gets very hot while driving. This problem usually occurs after moderate to extended periods of driving. You may or may not notice steam coming from the hood or smell coolant.

Possible causes:

  1. Any of the above causes for overheating on short trips.
  2. The car is overloaded or being driven too hard. The Fix: Lighten the load and back off the gas.
  3. The radiator or block may be clogged. The Fix: Reverse flush the cooling system and fill with fresh coolant.

Fixing the Most Common Overheating Problems

Some of those possible overheating causes apply to both situations and these are among the easiest repairs you can tackle in your own garage.

Low Coolant

By a large margin, the most common cause for engine overheating is simply a low coolant level. Your engine's cooling system relies on coolant to circulate and remove heat from the engine. If you don't have enough coolant in there to do the job, heat will build up and your engine will overheat.

No amount of running the heater in the summer will help if you don't have enough coolant in the radiator to transfer the heat. By far, the first thing you should do if your engine seems to be running hot is check your coolant level.

Electric Cooling Fan Failure

If you have an electric cooling fan that isn't coming on, this can cause your engine to overheat. This fan draws cooler air through your radiator when your car isn't going fast enough to do the job naturally.

You can test this by letting your car idle long enough for the engine to heat up. If you have an overheating problem in traffic, keep an eye on your temperature gauge. When it starts creeping into the danger zone, look under the hood to see if your electric fan is running. If it's not, you'll need to figure out why. Typically, it comes down to one of two problems.

Bad Electric Fan: Sometimes your fan motor will just burn out and your fan won't come on at all. To test this, find your radiator fan switch and disconnect the wiring harness. Get a jumper wire and insert it into both contacts, your fan should come on. Another way to test the fan is to turn on the air conditioning. The cooling fan is activated in most—but not all—cars when you turn the AC to either a medium or high speed.

Bad Radiator Fan Switch: There is a switch that tells your cooling fan to come on when your coolant reaches a certain temperature. The easiest way to test this switch is to disconnect the wiring harness and run a jumper wire across the harness contacts. If the fan comes on, you need to replace the switch.

Thermostat Is Stuck Shut

The most common symptom of a failed thermostat is overheating at highway speeds. Your engine may be able to stay cool at low speeds because it's not working that hard, and therefore not creating as much heat. When you hit highway speeds, however, your engine needs a lot of coolant flowing through to keep it cool.

If the thermostat doesn't open, there isn't enough flow to keep things cool. In this situation, you may find yourself looking more like a steamship than a sedan going down the highway.

Broken Fan Belt

There are still many engines out there that have a fan belt to drive the engine cooling fan. If you see a belt attached to your fan, you're in this club. The good news is your repair will be cheaper than electric-driven fans and you can easily replace the fan belt yourself if it's broken.

Clogged Radiator

If your car has more than 50,000 miles on it, your radiator may begin to get gummed up. You can avoid this and other problems associated with old coolant by flushing your radiator once a year.

Regular Maintenance Can Keep Engines Cool

There's nothing good about an overheating problem. If your engine is running hot you should try to fix the problem as quickly as possible. A hot engine can do damage to itself, even if it isn't fully overheating.

Regular maintenance can help with this issue. Beyond flushing your radiator, check your oil regularly to be sure you are providing adequate lubrication to your engine. Keep up with other maintenance as well because anything you can do to reduce heat buildup helps. 

Remember, it's important to keep an eye on your engine temperature. Many people mention that their engines are "running hot," though they don't seem too concerned. Fixing a cooling problem is usually fairly inexpensive, even if it involves a trip to the repair shop. On the other hand, engine damage due to a neglected cooling system and regular overheating can be expensive. You may even lead you to think about getting rid of the car altogether.