Why Is My Car Overheating When Idle?


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

Is your car overheating when idling or when driving in heavy traffic? Some specific faults can cause this problem, for example:

  • Low coolant level
  • Bad thermostat
  • Cooling fan
  • Water pump problems

Often, car owners can diagnose the problem and, depending on the case, fix it themselves. However, it is important to have your car’s repair manual to locate components and have the specs for your particular model. If you don’t already have the manual, you can buy a relatively inexpensive copy on Amazon.

Haynes manuals include:

  • Step-by-step procedures
  • System descriptions
  • Photographs
  • Component locations
  • Diagnostics
  • Instructions for installing parts
  • Maintenance schedule
  • Electrical diagrams

Even if you wanted to use the manual just for maintenance projects, you will significantly reduce car breakdowns in the future, and save a lot of money. So it is worth having the manual.

The following sections describe the most common problems that can cause your car to overheat at idle or when driving in heavy traffic.

Start with a visual inspection of the cooling system, as described in the next section. Often, a careful inspection can reveal the problem.

In This Article

1. Signs of Cooling System Problems
2. Faulty Radiator Cap
3. Radiator Air Flow Restrictions
4. Cooling Fan Does Not Turn On
5. Bad Thermostat
6. Leaking Head Gasket
7. Rusted Water Pump Drive
8. Restricted Exhaust
VIDEO: Getting Home Safe When Your Car is Overheating
9. Resources

1. Signs of Cooling System Problems

To perform a preliminary inspection:

  • Check for debris blocking the front of the radiator.
  • Make sure you have enough coolant in the system.
  • If the coolant level is low, check for a possible leak. Inspect around the cooling hoses, the radiator, the engine cylinder head gasket area, and the heater core hoses for signs of leaks.
  • Make sure the engine is really overheating. Use a temperature probe or infrared thermometer to measure the temperature of the inlet radiator tank. The temperature should be between 195°F-220°F (91°C-104°C).

Warning!

Never remove the radiator cap from a hot engine. Boiling coolant will squirt out and seriously burn your skin.

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2. Faulty Radiator Cap

A faulty radiator cap can cause the system to leak coolant. You can do a quick inspection of your radiator cap.

  • With the engine completely cold, remove the radiator cap.
  • Inspect the top and bottom cap seals. If the seals are hard, deteriorated or brittle, replace the cap with another one suitable for your application.
  • Check the seals surface on the radiator neck.
  • Verify the radiator cap pressure is still within the specific range for your car.

Over time, the spring in the cap weakens, lowering the coolant’s boiling point level. This will cause fluid to leak.

Use a cooling system pressure tester to check the radiator cap. You can rent the tool at an auto parts store in your area. Check the pressure range of the radiator cap in your repair manual.

3. Radiator Airflow Restrictions

Take a look at the front of the radiator:

  • Inspect for possible restrictions and signs of leaks.
  • Remove insects, dirt, stones, dry leaves, and other objects that may block airflow.
  • Also, check for possible signs of coolant leaks.
  • Check the condition of the radiator cooling fins. Salt and road debris tend to loosen fins over time. This reduces the capacity of the radiator tubes to transfer heat to the outside.

4. Cooling Fan Does Not Turn On

Make sure the cooling fan is working properly:

  • First, check that the cooling fan shroud or guard is properly mounted to the radiator and that there is no damage.
  • Also, check that the fan blades are in good condition.
  • Make sure the fan comes on at idle when the engine is warm.
  • Most car engine-run fans have a clutch. On these models, clutch failure causes the fan to stop working and the engine to overheat.

Consult your repair manual for more information. You can also refer to the heading on Cooling Fan Problems in the Resources section at the end of this article.

5. Bad Thermostat

A good thermostat should close when the engine is cold or the engine temperature is below the thermostat’s calibrated range. Once the engine temperature reaches the range of the thermostat, the thermostat opens and allows coolant to flow to the radiator.

A faulty thermostat can be the reason for irregular overheating.

If you need to test your thermostat’s performance, the guide listed in the Resources section at the end of this article can help. Also, check your car’s repair manual.

6. Leaking Head Gasket

One of the most common causes of engine overheating is coolant leaks.

Leaks can occur in many parts of the system, including the head gasket.

If you suspect a head gasket leak, inspect around the engine head. Leak points are normally characterized by a wet surface or light gray areas. If necessary, use a radiator pressure tester to locate hard-to-find leaks in the cooling system.

You can rent a pressure tester at your local auto parts store.

For more help, see the Coolant Leak Troubleshooting article under the Resources section at the end of this article. Also, check your repair manual.

7. Rusted Water Pump Drive

Depending on your vehicle model, the water pump is driven by a belt, an electric motor, the crankshaft, or the camshaft.

The water pump transfers coolant from the radiator to the engine. Normally, when the water pump fails it begins to leak, but also the impeller can become rusty over time, and the bearing or shaft can wear out, affecting the pump’s performance.

In either case, the result is engine overheating.

Two of the most common symptoms of a bad pump are:

  • Coolant leaking through the pump
  • Noisy operation

Examine the pump closely for wet areas. Also, you can use a mechanic’s stethoscope (or a piece of hose) to listen for noises during engine operation.

The Water Pump Diagnostics article listed in the Resources section will help you when checking the water pump. Also, consult your car’s repair manual.

8. Restricted Exhaust

Commonly, an exhaust system inspection is limited to searching for leaks or noise. But the exhaust system can also become blocked and contribute to engine overheating.

Collapsed pipes, a clogged catalytic converter or muffler will block the exhaust system.

You can install a pressure gauge in the oxygen sensor cavity to measure system pressure. At idle, the gauge should read less than 1.5 psi (10 kPa); at a speed of 2500 rpm, the pressure should be less than 2.5 psi (17 kPa).

You can also do this diagnosis using a vacuum gauge.

If necessary, rent a pressure or vacuum gauge at your local auto parts store.

For more information, consult your repair manual and the Resources section at the end of this article.

VIDEO: Getting Home Safe When Your Car is Overheating

The following video gives you some tips to get safely home or to a nearby auto shop, if your car’s engine’s temperature is dangerously rising.

9. Resources

  • Do I Need to Replace My Car Thermostat?
    Having problems with your car’s thermostat? This article shows you how to check your thermostat in a few minutes without removing it from your car.
  • How to Check Your Cooling System
    Do you suspect your coolant is not circulating through your engine? Here’s how to do a cooling system check to find the source of leaks, overheating, and overcooling problems.
  • Coolant Leak Troubleshooting
    A low coolant level can mean a coolant leak. These methods will help you locate hard-to-find leaks.
  • Troubleshooting Electric Cooling Fan Problems in Cars
    Check for cooling fan problems using this simple guide and tips.
  • Radiator Check: Diagnostic Tips and Troubleshooting Guide
    Eight ways your car radiator can cause your engine to overheat, plus troubleshooting tips.
  • Diagnosing a Water Pump Failure
    Diagnose a water pump failure faster with these three simple step-by-step methods.
  • Use a Vacuum Gauge to Troubleshoot Your Car’s Mechanical Problems
    Use a vacuum gauge to find car mechanical problems faster at a low cost.
  • Diagnose Exhaust System Problems Using Your Ear and Nose
    Noises and smells can give you clues to diagnose common exhaust system problems, if you know where to look.

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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