Why Is My Engine Running Cold?

Updated on June 27, 2020
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

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. | Source

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.

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.

1. Coolant and Temperature Gauge Checks
2. Car Thermostat Stuck Open
Video: Stuck-Open Thermostat Replacement
3. Bad Engine Coolant Temperature Sensor or Relay
4. Locked Fan Clutch
5. Is it Safe to Run an Engine Below Operating Temperature?
Make sure your temperature gauge is working correctly.
Make sure your temperature gauge is working correctly. | Source

1. Coolant and Temperature Gauge Checks

Before you begin looking for the faulty 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 on the low range or give the same read out all the time. If necessary, troubleshoot the temperature gauge.

Problems behind a temperature gauge reading low may include:

  • A disconnected gauge 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. | Source

2. Car Thermostat Stuck Open

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

You can normally find the thermostat housing connected at the engine end of the upper radiator hose. 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 180 F and 195 F (82 C and 91 C).

Checking thermostat operation:

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 and idle the engine.
  5. Pop the hood open.
  6. Remove the radiator cap.
  7. Take a look though the radiator opening.

    • 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.

An alternative method:

Still, another option 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 under ambient temperature. If the unit is open, replace it.

Test the thermostat using boiling water:

If the thermostat doesn’t respond adequately when submerged in boiling water (it should open) and outside the water (it should close), replace the unit.

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

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

Stuck-Open Thermostat Replacement

A bad ECT sensor may cause several performance issues.
A bad ECT sensor may cause several performance issues. | Source

3. Bad Engine Coolant Temperature Sensor or 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 back in the sensor’s electrical connector.
  3. Start and let the engine 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.

Relays are another potential cooling fan 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. | Source

4. Locked Fan Clutch

Some vehicles use an engine-powered fan to help remove heat from the engine. The fan mounts to 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 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. | Source

5. 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 accelerates
  • This is because components fail to expand as they do with 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 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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