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Why Is My Engine Running Cold?

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

An engine running cold may be a symptom of a faulty cooling system.

An engine running cold may be a symptom of a faulty cooling system.

When we say an engine runs cold, we mean it’s not reaching full operating temperature.

The most common reasons for the temperature to remain low include:

  • A thermostat stuck open
  • A bad engine coolant temperature sensor
  • A locked fan clutch

You may notice one or more of the following symptoms:

  • Heater blows barely warm or cool air
  • Poor fuel economy
  • Sluggish engine performance
  • Temperature gauge reads low in the scale

The following sections will help you diagnose the most common faults behind an engine running cold or taking longer to warm up.

Get a Manual

Having the repair manual for your particular vehicle model not only helps you diagnose but repair your cooling system faster. The manual gives you strategies and lists measurement specifications for your specific model.

If you don’t have this manual yet, you can buy one through Amazon. Haynes manuals come with step-by-step procedures for many maintenance, component replacement, and troubleshooting tasks you can do at home. So you’ll be using your manual often and saving money on common maintenance tasks and simple repairs, and complex repairs as well, as you become familiar with it.

What You Can Check to Find Out Why Your Engine Is Running Cool

Here are four checks you can do, described below.

  1. How to Check Your Coolant Level and Temperature Gauge
  2. How to Check If Your Car Thermostat Is Stuck Open
  3. How To Check the Engine Coolant Temperature Sensor and Cooling Fan Relay
  4. How to Check for a Locked Fan Clutch
Make sure your temperature gauge is working correctly.

Make sure your temperature gauge is working correctly.

1. How Check Your Cooling System and Temperature Gauge

Before you begin looking for a faulty engine component causing your engine to run cold, it’s important to do a couple of preliminary checks.

  • First, make sure the cooling system has enough coolant. If necessary, top off the system with a 50/50 mixture of antifreeze and distilled water. Consult your car owner’s manual or repair manual for the recommended antifreeze for your particular model. If the system is low in coolant, check the cooling system for possible leaks.
  • Second, make sure your temperature gauge is working correctly. A faulty gauge may stay in the low range or give the same reading all the time. If necessary, troubleshoot the temperature gauge.

Problems that may cause a temperature gauge to read low may include:

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  • A gauge disconnected from the sending unit.
  • A faulty gauge.
  • Problems with the gauge circuit.
  • A faulty temperature sending unit.
Check upper-radiator hose after a few minutes of engine operation to help diagnose for a stuck-open thermostat.

Check upper-radiator hose after a few minutes of engine operation to help diagnose for a stuck-open thermostat.

2. How to Check If Your Car Thermostat Is Stuck Open

A car thermostat stuck open is often the cause behind an engine running below normal operating temperature.

A thermostat is a small device designed to sense engine temperature and control coolant flow from the engine to the radiator. It basically reduces coolant flow when the engine is below operating temperature and increases flow when the engine is above operating temperature.

You can normally find the thermostat housing by following the upper radiator hose to where it connects to the engine. If necessary, consult your vehicle repair manual.

Thermostats are rated for different temperature ranges, depending on the specific vehicle model. A typical thermostat operates between 180º F and 195º F (82º C and 91º C).

Checking Thermostat Operation Without Removing the Thermostat

On some vehicle models, it’s easy to diagnose a thermostat.

  1. Make sure the engine is cool.
  2. Engage the parking brake.
  3. Set the transmission to Park (automatic) or Neutral (manual).
  4. Start the engine and let it idle.
  5. Pop the hood open.
  6. Remove the radiator cap.
  7. Take a look through the radiator opening to see any motion in the coolant.

    • Coolant should not flow at this point until the engine begins to reach operating temperature, usually in about 15 to 20 minutes.
    • If coolant is flowing with the engine cool, the thermostat is stuck open and needs to be replaced.

On some vehicle models, checking thermostat operation is easier with a digital or infrared thermometer.

  1. Engage the parking brake and start the engine.
  2. Let the engine idle for about 10 minutes.
  3. Measure engine temperature near the thermostat housing.
  4. Measure the upper radiator hose temperature near the thermostat housing.
  5. Make a note of both temperatures.
  6. Repeat steps three and four in about 5 minutes.
  7. If both temperatures in each reading are close to each other, the thermostat is not closing and you need to replace it.

How to Check a Thermostat Using Boiling Water

Another way to check a thermostat is to remove it from the engine and make a visual inspection of it. A thermostat should be at its closed position when at ambient temperature.

Try submerging the thermostat in boiling water; it should open. If you take the thermostat out of the water it should close. If it doesn't act as it should, replace the unit.

Consult your vehicle repair manual to locate the thermostat and best test method for your application, if necessary.

Replacing a Stuck-Open Thermostat

In the following video, the owner of a BMW model explains how he changes a thermostat to fix his engine running too cold.

A bad ECT sensor may cause several performance issues.

A bad ECT sensor may cause several performance issues.

3. How To Check the Engine Coolant Temperature Sensor and Cooling Fan Relay

Vehicles with electric cooling fans use an electric motor and a thermostatic switch to operate the fans. The switch is usually an engine coolant temperature (ECT) sensor.

The ECT sensor is a thermistor that reduces electrical resistance as temperature increases. This variable resistance changes the voltage signal sent to the car’s computer. This is how the computer knows when to turn on the cooling fans.

However, problems with the fan switch or circuit may cause the cooling fans to operate continuously, causing the engine to run below operating temperature.

You can easily check the ECT sensor in your vehicle using an ohmmeter.

  1. With the engine cold, unplug the sensor electrical connector and measure the resistance across the sensor’s terminals.
  2. Plug the sensor’s electrical connector back in.
  3. Start the engine and let it idle for a few minutes.
  4. Turn off the engine, unplug the sensor’s electrical connector and measure its resistance again.

Compare your readings to specifications. You can find ECT sensor resistance values for your car in the vehicle repair manual for your specific model.

Cooling Fan Relay: Another Potential Problem

The cooling fan relay in the circuit may become stuck and keep the cooling fan operating continuously. This may prevent the engine from reaching its ideal running temperature. If necessary, troubleshoot the fan relay.

A faulty fan clutch can keep the cooling fan running.

A faulty fan clutch can keep the cooling fan running.

4. How to Check for a Locked Fan Clutch

Some vehicles use an engine-powered fan to help remove heat from the engine. The fan is mounted on the front of the engine, bolted to the water pump hub and pulley.

The engine-powered fan may operate through a thermostatic fan clutch. This is an oil flow control mechanism with a temperature-sensitive thermostatic spring.

When the engine is cold, the spring allows the clutch to slip, allowing the engine to reach operating temperature. Once the engine is warmed, the spring locks the clutch. This causes the fan to start spinning, forcing air to circulate around the engine.

When it fails, the fan clutch may lock up and cause the fan to run continuously.

To do a quick fan-clutch check:

  1. Pop the hood open.
  2. Make sure the engine is cool.
  3. Start the engine and let it idle.
  4. Engage the parking brake and set the transmission to Park (automatic) or Neutral (manual).
  5. Check the fan.

    • The fan should not run with the engine cool.
    • If the fan runs with the engine cool, it’s locked. Replace the fan assembly.
An engine running cold continuously will suffer from serious performance issues in the future.

An engine running cold continuously will suffer from serious performance issues in the future.

Is it Safe to Run an Engine Below Operating Temperature?

Although your vehicle may seem to be OK otherwise, an engine running at lower than operating temperature can suffer severe internal damage over the long run.

  • Internal engine wear. Components fail to expand as they should without heat, and won’t reach their correct fit with respect to each other.
  • Poor fuel economy. Engine and internal metal components absorb more heat, reducing combustion efficiency, so there’s more fuel consumption.
  • Damage to the catalytic converter. More raw fuel reaches the catalytic converter, eventually causing significant damage to the device.
  • Sluggish performance. Since there’s less heat available for combustion, there’s less expansion of gases in the cylinder and less piston pressure.

As soon as possible, make sure you find out why your engine is running cold. Failure to make a simple fix now can turn into an expensive repair later.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Dan Ferrell

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