My Car Has a Rough Idle When Cold

Updated on May 18, 2020
Dan Ferrell profile image

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

If your car has a rough idle when cold, check common sources of trouble.
If your car has a rough idle when cold, check common sources of trouble. | Source

Have you noticed your car has a rough idle when cold? You can start your diagnostic by taking a look at those components known to be a common source of trouble. For example:

  • Engine coolant temperature (ECT) sensor
  • Intake air temperature (IAT) sensor
  • Cold-start injector
  • Ignition coil(s)
  • Internal coolant and oil leaks

You may experience your car's rough idle as an engine vibration. And, if you pay attention to the exhaust pipe at this time, you may even hear a popping sound. This is the uneven burn of the fuel mixture in the cylinders.

But once the engine warms up, this vibration and popping sounds go away. Your engine is idling smoothly.

The following sections will help you diagnose the most common sources of trouble causing an engine's rough idle when cold. You'll find some simple tests you can do at home to help you locate the problem so you can make the necessary repairs yourself, if possible.

Having the repair manual for your particular vehicle make and model can help you not only locate components but troubleshoot those particular to your application. If you don't have this manual yet, you can buy a relatively cheap, aftermarket copy from Amazon. Haynes manuals come with step-by-step procedures for many troubleshooting, replacement and maintenance projects you can do at home. So you can recoup your small investment soon.

Index
1. Engine Coolant Temperature (ECT) Sensor
2. Bad Intake Air Temperature (IAT) Sensor
VIDEO: Dirty MAF Sensor Causes Rough Idle
3. Cold-Start Injector Fault
4. Ignition Coils
5. Internal Coolant Leaks
6. O-ring Gaskets Gone Bad
7. Dealing with a Rough Idle on a Cold Engine
A faulty ECT sensor is one of the most common sources of rough idle on a cold engine.
A faulty ECT sensor is one of the most common sources of rough idle on a cold engine. | Source

1. Engine Coolant Temperature (ECT) Sensor

One of the most common causes of rough idle on a cold engine is a faulty ECT sensor.

This sensor signals the car's computer when the engine's coolant is cold or hot, so the computer delivers more or less fuel to the engine. A cold engine requires more fuel than a warmed engine.

An ECT sensor is a variable resistor that responds to changes in temperature. An ECT sensor with an electrical open circuit will affect engine idle when cold. On the other hand, a sensor with an electrical short will affect engine driveability when warmed.

Testing the ECT sensor using an Ohmmeter:

  1. Locate the ECT sensor. Most sensors are located around the cylinder head, intake manifold or close the thermostat housing.

  2. Unplug the sensor's electrical connector.

  3. Using your ohmmeter, connect the leads across the sensor connector terminals. On an old sensor with a single wire, connect one lead to the terminal and the other to the sensor's body.

    • If you get an infinite resistance reading, the sensor has an electrical open and needs to be replaced.
    • Depending on your particular application, you may get a reading of 40,500 ohms or higher at a temperature of about 65F (18.3C). If your reading is significantly lower, your sensor may not be responding to low temperatures, but continue with the rest of the test.
  4. Set the transmission to Park (automatic) or neutral (manual).

  5. Engage the emergency brakes.

  6. Start the engine and let it idle.

  7. After two minutes, take another resistance reading of your ECT sensor and make a note of your reading.

  8. After two minutes, take another reading and make a note of your results.

  9. After two minutes, take another reading and make a note of it.

  10. You should see a gradual decrease in your resistance readings as the engine's temperature increases; otherwise, the sensor is not working appropriately and should be replaced.

At engine operating temperature, between 180 to 220 F (82.2 to 104.4 C), you may get a reading between 3800 and 1840 ohms.

If the sensor seems OK, check the connector and wiring between the computer and the ECT sensor. Make sure the connector is in good condition and free of corrosion; the wires should be properly connected and free of damage.

Consult your vehicle repair manual to locate the sensor, the correct resistance values for your particular application, and how to test the circuit, if necessary.

The intake air temperature (IAT) sensor is an integral part of the MAF sensor on many applications.
The intake air temperature (IAT) sensor is an integral part of the MAF sensor on many applications. | Source

2. Bad Intake Air Temperature (IAT) Sensor

Just like a faulty ECT sensor, a bad IAT can have an adverse effect on the air-fuel mixture and ignition timing. You can check an IAT sensor the same way you test an ECT sensor using your Ohmmeter. The IAT sensor is a variable resistor. The sensor's resistance decreases as incoming air temperature increases, and resistance increases as incoming air temperature decreases.

On vehicle models with a manifold absolute pressure (MAP) sensor, the IAT sensor is usually located in the intake manifold. On newer models equipped with a mass air flow (MAF) sensor, the IAT is usually part of the MAF component.

Other potential symptoms of a bad IAT sensor may include:

  • Engine stalling
  • Engine surging
  • Hard starting
  • Excessive fuel consumption
  • Stumble on acceleration

Consult your vehicle repair manual to locate the IAT sensor, resistance values for your particular application, and appropriate checks.

The following video shows you how a dirty MAF sensor was causing rough idle on a cold start.

Dirty MAF Sensor Causes Rough Idle

A bad cold-start injector will cause a rough idle on a cold engine.
A bad cold-start injector will cause a rough idle on a cold engine. | Source

3. Cold-Start Injector Fault

Some vehicle models come equipped with a cold-start fuel injector. The sole purpose of this special injector is to add extra fuel to the engine during cold engine starting.

Usually, the injector is operated through a thermo-time switch or through the car's computer. And it works like a regular solenoid type injector.

The cold-start injector helps to smooth idle when the engine is below operating temperature. Once the engine reaches proper temperature, the cold-start injector shuts off.

It the injector is dirty or your are dealing with a bad fuel injector, it won't supply the extra fuel needed. The problem can also be with the switch, circuit or control system operating the injector. Either way, your cold engine will experience a rough idle until it reaches operating temperature.

If necessary, consult your vehicle repair manual to check the injector, circuit or control system.

Check ignition coils for signs of contamination and damage.
Check ignition coils for signs of contamination and damage. | Source

4. Ignition Coils

Manufacturers like to change the way components are set up around the engine to accommodate new components or a specific configuration, leaving some parts vulnerable to unforeseen events.

Take for example the ignition coils. With new coil-on-plug (COP) models, some coils are subject to oil, coolant or even moisture accumulation if a leak happens to occur near the component.

A bit of oil or water around a coil from a small leak may evaporate once the engine reaches operating temperature. In the meantime, though, you'll notice a rough idle.

Remove the coils and make a close inspection of the components, including spark plugs, to find clues of possible leaks, corrosion or damage that might be interfering with an ignition component. You can also test an ignition coil at home using a digital multimeter.

Consult your vehicle repair manual if you need to remove a coil and plugs, and check for appropriate tests for your application, if necessary.

A faulty head gasket can leak coolant into one or more cylinders.
A faulty head gasket can leak coolant into one or more cylinders. | Source

5. Internal Coolant Leaks

This problem is an expansion of the previous one. Internal leaks can develop into all kinds of driveability problems. A rough idle on a cold engine is just one of them.

A small coolant leak that creeps into a combustion chamber can prevent a cylinder from firing the air/fuel mixture properly. But once the engine reaches operating temperature, the coolant in the cylinder evaporates and proper combustion resumes.

This type of fault is hard to diagnose because internal and external leaks can produce different symptoms, depending on where they occur and where coolant or oil ends up.

You can test your engine for internal coolant leaks as well. But it's important to keep in mind and check spark plugs, wires and boots, ignition coils, and fuel injectors as well when trying to diagnose hard to solve rough idles, specially intermittent or temperature related issues.

A faulty O-ring gasket on some vehicle models can cause an intake manifold to leak over time.
A faulty O-ring gasket on some vehicle models can cause an intake manifold to leak over time. | Source

6. O-ring Gaskets Gone Bad

On some models, manufacturers have been using O-rings gaskets to seal plastic intake manifolds. Over time, the O-ring may become brittle due to the high engine temperatures under which it operates.

Eventually, the O-ring may fail to make a perfect seal when cold, producing a rough idle. But as the engine warms up, heat expands the seal and the vacuum leak disappears.

This other post can help you troubleshoot for vacuum leaks.

If you find a leaking gasket, check with your local dealer for a possible technical service bulletin first before replacing the O-ring. A better gasket may have been developed and ready for installation.

7. Dealing with a Rough Idle on a Cold Engine

If your car experiences a rough idle when cold, try to diagnose and fix the problem as soon as possible. When one or more cylinders fail to completely burn the air fuel mixture, raw fuel enters the exhaust system and can seriously damage the catalytic converter.

Diagnosing and replacing a relatively cheap, faulty component now can save you from an expensive repair later on. Use this guide as a reference and consult your vehicle repair manual as well.

Often, most rough-at-idle conditions, especially those temperature related, are easier to diagnose and repair. And, usually, you only need a few common tool for the job.

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

Comments

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    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      5 weeks ago

      Glad it helped.

    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      5 weeks ago

      Glad it helped.

    • profile image

      tony gochi 

      5 weeks ago

      Thank you very much, I changed the intake air temperature sensor and the intake camshaft sensor, problem solved car travelling and idling like new

    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      5 weeks ago

      The most common suspects are the ECT, IAT sensors, and cold start injector. You can start here, and download trouble codes form the computer memory. You may not see the check engine light on, but there could be a pending code that can help you. Hope this helps.

    • profile image

      tony gochi 

      5 weeks ago

      I have a BMW E46 Coupe it idles rough bouncing up and down when cold then shuts off, on second start it runs fine and even better when it warms up completely, any ideas please?

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