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My Car Temperature Gauge Fluctuates

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

When your car temperature gauge fluctuates, look behind these potential problems:

  • A gauge or electric circuit in need of service
  • Low coolant level or coolant leak
  • Bad water pump or related problem
  • Cylinder head gasket leaking

For the most part, the average car owner can diagnose a fluctuating temperature gauge at home using a few simple tools.

Likely, you'll need the vehicle repair manual for your particular vehicle make and model. If you don't have this manual yet, you can buy a relatively inexpensive copy through Amazon.

Haynes manuals include:

  • Step-by-step procedures
  • Location of parts
  • Troubleshooting guides
  • Description of the different systems
  • Photographs and images
  • Electrical diagrams
  • Maintenance schedule

Furthermore, the manual will help you keep up with your car maintenance to avoid unnecessary breakdowns and save a lot of money in the process.

The next sections will guide you to diagnose your gauge temperature problem.

In This Article

1. Coolant Level Low
2. Worn Drive Belt
3. Faulty Temperature Gauge or Sensor
3.1 Testing an ECT Sensor Using a Digital Multimeter
4. VIDEO: A Fluctuating Temperature Gauge in a 1999 Ford Ranger
5. Water Pump Issues
5.1 Water Pump Is Sucking Air
6. Cylinder Head Gasket Leak
7. Resources

Warning!

Never remove the radiator cap on a hot engine. Hot coolant may squirt forcefully and cause serious skin burns.

Check coolant level first.

Check coolant level first.

1. Coolant Level Low

Insufficient coolant in the system can also lead to temperature gauge fluctuations.

As air builds up in the cooling system, thermostat operation varies as well, causing the valve to open late.

With the engine cool, remove the radiator cap and, if necessary, top off the system with fresh coolant.

If you suspect a possible leak, check the cooling system:

  • Radiator tanks
  • Upper and lower radiator hoses
  • Thermostat housing
  • Heater core and hoses
  • Cylinder head gasket area

2. Worn Drive Belt

This problem is similar to a faulty water pump.

On some vehicle models, the timing belt runs the water pump; on other models, a drive belt or serpentine belt operates several accessories, including the water pump.

A belt may slip and keep the water pump from working properly, causing erratic coolant flow and inaccurate temperature gauge readings.

Several reasons may cause a drive belt to slip:

  • The belt is too stretched out due to wear.
  • The belt's outer surface has become glazed, or the inner surface is damaged.
  • The belt is contaminated with grease or oil.
  • The belt is loose.
  • The idler pulley has failed.

Check the drive belt or serpentine belt in your vehicle. Consult your vehicle repair manual and the Resources section at the bottom of this article.

Test the engine coolant temperature sensor.

Test the engine coolant temperature sensor.

3. Faulty Temperature Gauge or Sensor

Most newer vehicle models use a single sensor for the temperature gauge and the car's computer. Older vehicle models use a sensor for each of these devices.

The connection to this sensor can help you make a quick diagnostic on the temperature gauge and the sensor itself. If necessary, consult your repair manual to locate the sensor.

  1. Disconnect the electrical connector from the sending unit (engine coolant temperature [ECT] sensor). On most models, the sensor is located on the engine.
  2. Connect one end of a jump wire to the electrical connector you just unplugged, and the other end to engine ground. You can use any metal bracket or screw on the engine itself, or connect to the battery's negative post.
  3. Set the transmission to Park (automatic) or Neutral (manual).
  4. Turn the ignition key to the On position, but don't start the engine.
  5. Confirm temperature gauge operation. A good, operating temperature gauge will read Hot. Unplug the jumper wire from the ground connection. The temperature gauge should read cold again.
  6. If the gauge seems to be operating properly, replace the sending unit or ECT.
  7. If the gauge doesn't appear to operate properly:
    • Check the electrical connector for corrosion, damage or loose wire.
    • Check the temperature gauge electrical circuit.
    • Check the temperature gauge.

For help on this diagnostics, consult your vehicle repair manual. Also, check the titles listed under the Resources section at the bottom of this post.

3.1 Testing an ECT Sensor Using a Digital Multimeter

Most car shops use a capable scanner tool to test the ECT sensor. If you own a car computer scanner, follow the instructions that came with your tool to test this sensor.

Once you get the ECT sensor temperature using your scanner, read engine temperature using an infrared pyrometer or a contact-type temperature probe.

Compare both temperature readings. If the difference in temperature is greater than 10F or 5C, check the electrical connector and wiring for corrosion or damage.

Using a Digital Multimeter

You can also use a digital multimeter to test the ECT sensor at home. Consult your vehicle repair manual, and the article Coolant Temperature Sensor Test listed in the Resources section at the bottom of this post.

4. VIDEO: A Fluctuating Temperature Gauge in a 1999 Ford Ranger

The following video shows a 1999 Ford Ranger that required a new ECT sensor to fix a fluctuating temperature gauge.

The vehicle owner shows where this sensor is generally located.

Before changing your ECT sensor, though, test it as I described earlier.

5. Water Pump Issues

As the mechanism wears out, problems with the water pump will eventually appear.

The most common water pump failure is a coolant leak. However, corrosion buildup can also cause the pump bearing to fail or the pump's impeller to become loose.

For more help on diagnosing a possible faulty water pump, consult your vehicle repair manual, and the article on Water Pump Failure under the Resources section at the end of this post.

5.1 Water Pump is Sucking Air

Another way to introduce air into the cooling system is through a leak on the suction side of a water pump.

The leak may appear around the lower radiator hose that connects to the water pump, causing the thermostat to open late.

Carefully examine the water pump and lower radiator hose for leaks or damage. The hose clamp should be tight, and the hose properly connected to the water pump.

6. Cylinder Head Gasket Leak

A blown cylinder head gasket not only can cause a thermostat to open late, but can contaminate the cooling system with exhaust gases.

  • Check for contamination of the engine oil.
  • Check the exhaust system for the presence of white steam coming out of the tail pipe.

Use a block leak tester to diagnose the cooling system yourself. Most auto parts stores will loan you the tool to use at home.

Consult your vehicle repair manual, if necessary, for more help on checking a possible cylinder head gasket leak. Also, take a look at the titles listed under the Resources section at the end of this post.

7. 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