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Why Is My Car Overheating?

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Why is my car overheating?

Why is my car overheating?

A car can overheat in various ways and under different driving conditions:

  • In heavy traffic
  • Driving at speed
  • Soon after car started
  • All the time

These conditions point to different sets of potential sources of trouble.

The next sections describe these and other conditions and what might be causing the problem. Jump to the section that best describes your current symptoms, and check the list of components likely to cause the engine to overheat.

Also, have your vehicle repair manual on hand while diagnosing your vehicle. It'll help you to locate components, how to diagnose a specific system or component, if necessary. If you don't have this manual yet, you can buy an inexpensive copy through Amazon.

Haynes manuals include:

  • Step-by-step procedures
  • Photographs
  • Parts locations
  • Diagnostics
  • Electrical diagrams
  • Systems descriptions
  • Maintenance schedule

Plus other important sections you'll find useful when facing electrical or mechanical issues with your vehicle. Even if you just want to service your vehicle using the maintenance schedule included, you'll keep breakdowns to a minimum and increase your vehicle's service life.

In This Article

1. My Engine Overheats Soon After I Start the Car
2. My Engine Overheats at Any Time or Erratically
3. My Engine Overheats When Driving at Speed
4. My Engine Overheats in Heavy Traffic
5. My Engine Seems to Run a Bit Too Hot All the Time
6. Engine Overheats at Idle
7. I Can See Bubbles in the Coolant Expansion Tank
VIDEO: Blown Head Gasket Causing the Engine to Overheat
8. There's Air in My Radiator, But the Expansion Tank is Full
9. Resources

1. My Engine Overheats Soon After I Start the Car

First, make sure the engine is actually overheating. You can use a digital or infrared thermometer to check engine temperature near the thermostat housing and the upper radiator hose that connects to the thermostat housing. If necessary, consult your manual for this and the list of titles listed under the Resources section at the bottom of this post.

If the engine temperature is fine, check the temperature sender circuit. The sender unit operates the temperature indicator light or temperature gauge in your vehicle.

Consult your vehicle repair manual to locate the sending unit.

  1. Unplug the wire that connects to the sending unit.
  2. If necessary, use a jumper wire to ground the wire you unplugged from the sending unit.
  3. Turn the ignition key to the On position, but don't start the engine.
  4. The indicator light should glow or the gauge should read hot.
    • Otherwise, the circuit to the sending unit is bad.
    • If the indicator light glows or the gauge reads hot, then the sending unit needs to be replaced.
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2. My Engine Overheats at Any Time or Erratically

An engine that overheats erratically without a set pattern can point to one or more potential problems. Check the following.

  • Make sure coolant level is correct
  • Check thermostat operation
  • Bad radiator cap
  • Faulty temperature sender unit or circuit
  • Check cooling fan operation

Your vehicle repair manual can help you check these components or systems. Also, you'll find some help in the Resources section at the bottom of this post.

3. My Engine Overheats When Driving at Speed

It can be tricky to check an engine that only overheats when driving at speed. Several potential faults can cause this problem.

  • Make sure the front of the radiator is free of leaves and debris that might block proper airflow.
  • Check for a blocked or restricted radiator
  • Check the radiator cap
  • Verify proper thermostat operation
  • Water pump not working properly
  • Collapsed lower radiator hose
  • Check for dragging brakes

Your vehicle repair manual will help you check these components. Also, you'll find some help at the Resources section at the bottom of this post.

4. My Engine Overheats in Heavy Traffic

You are driving in heavy traffic for a few minutes before you notice the engine begins to overheat.

An engine that overheats only in heavy traffic can point to problems in the cooling system or exhaust system. Check the following components.

  • First, make sure there's enough coolant in the system.
  • If necessary, check for a system leak or a leaking head gasket.
  • Check for a faulty thermostat.
  • Make sure the cooling fan is turning on.
  • Inspect the front of the radiator and make sure it is not restricted with debris.
  • Make sure the water pump is working properly.
  • Check for a restricted exhaust system.

5. My Engine Seems to Run a Bit Too Hot All the Time

This condition is similar to an engine running hot when driving at speed. And you should check the same components that affect an overheating engine in such condition.

  • Check the thermostat.
  • Check the radiator cap for proper operation.
  • Verify air can flow freely through the radiator front. If necessary, remove debris.
  • Make sure the lower radiator hose is not damaged or collapsed.
  • Inspect the cooling fan for proper operation.
  • Make sure the radiator is not restricted with rust, scale or gel.

Your vehicle repair manual can help you check these components. Also, check the Resources section at the bottom of this post.

6. Engine Overheats at Idle

Another common condition under which overheating may manifest is at idle. You'll see the engine temperature in the gauge rising beyond its normal range. However, once you start moving, the temperature goes back to normal.

  • Usually, the most likely cause of an engine overheating is a failure of the cooling fan to come on, possibly a faulty relay.
  • Another cause for this condition is debris clogging the radiator core, preventing airflow to lower flowing coolant temperature when the cooling fan comes on.
    • If necessary, clean the front of the radiator using low-pressure water and compressed air.
    • Also, you may want to flush the radiator to remove rust from inside the core that might prevent free coolant flow.

If you need help, consult your vehicle repair manual and the Resources section at the bottom of this post.

7. I Can See Bubbles in the Coolant Expansion Tank

Bubbles in the expansion tank can indicate a minor or more serious cooling system issue.

  • First, make sure the radiator cap is working correctly and the hose that connects to the expansion tank is not damaged.
  • Check for a possible head gasket leak.

A blown head gasket may cause bubbles in the coolant expansion tank and the engine to overheat, if the cylinder compression escapes into the coolant jackets.

However, a head gasket may fail in different ways and cause different symptoms. The following video shows a head gasket causing an engine to overheat.

If you need more help with a possible head gasket failure, take a look at the Resources section at the end of this post and your vehicle repair manual as well.

Blown Head Gasket Causing the Engine to Overheat

8. There's Air in My Radiator But the Expansion Tank is Full

If you know there's air in the radiator, check the following items:

  • Check for a coolant leak.
  • Check for a coolant leak between the radiator cap and the expansion tank.
  • Bleed the cooling system.
  • Test the radiator cap.
  • Check for a bad head gasket.

Your vehicle repair manual can help check the cooling system and the rest of the components. Also, look for more help in the titles listed in the Resources section at the bottom of this post.

9. Resources

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2022 Dan Ferrell

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