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What Makes My Engine Skip or Misfire When Cold?

I have owned some real clunkers in my day. I remember my first car was a 1983 Plymouth Horizon. The engine never ran great and seemed to always misfire. I’m lucky I was able to get even a couple of years out of that car. But what causes those skips and stutters?

An automobile’s engine will skip or misfire when it’s cold due to a less-than-optimal air-to-fuel mixture preventing proper combustion from taking place. Cold temperatures aggravate the problem because the lower the temperature, the narrower the combustible range of the air-to-fuel mixture becomes.

In this article, I will discuss why cold weather tends to increase engine misfires. Also, I will cover other contributing factors to engine skipping and when it can be symptomatic of something more serious.


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Why Does My Car Misfire on Cold Start?

To understand why a car misfires, you need to understand the basic concept of the internal combustion engine.

Your car misfires on cold start because combustion occurs when air and fuel vapor are exposed to a spark that creates the energy in your car’s engine power. This combustion cycle involves the fuel you put in your vehicle, air, and the spark from a spark plug in most cars.

When dealing with misfires, it is the air-to-fuel ratio that plays the most significant role.

The Air-To-Fuel Ratio’s Role in Misfires

Ideally, this air-to-fuel ratio consists of 14.7 parts of air to one part of fuel vapor, which can be affected by many factors, including: 

  • The type of gasoline in the vehicle. 
  • Engine performance 
  • Temperature

For the most part, automobile engines will function well without skipping or misfiring when the air-to-fuel ratio is between 8:1 and 18.5:1.

The Temperature’s Role in Misfires

Temperature affects the flammable range of all combustible fuels such as gasoline. The colder the temperature, the narrower the range, so for this reason, your engine may experience a misfire when you attempt a cold start. 

It is also the reason why as soon as your car warms up, the misfires stop, and your engine runs smoothly.

Why Does My Car Run Rough When Cold?

Angry young woman sitting in car
A car not working properly can be a frustrating experience.

Your car runs rough when cold due to the air-to-fuel ratio and how the chemical properties of gasoline cause its flammable range to narrow in cold temperatures. During the first few minutes of running, the air-to-fuel mixture will be either too lean or too rich to deliver optimal performance. 

This situation can translate into sluggish performance on acceleration, unusual or uneven engine sounds, vibrations, and a rough idle.

Modern vehicles with advanced sensors will compensate for these imbalances by sending more air or fuel to the cylinders. Older cars that do not have such sensors will usually rely on the engine warming up to bring a healthy balance back to the combustion cycle, thus eliminating the engine roughness.

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How Do You Fix Rough Idle When Cold?

Your vehicle’s engine management sensors will usually do a proper job restoring the correct air-to-fuel ratio required for optimum combustion, which eliminates rough idles when cold. If, however, you are operating an older vehicle or one that has compromised engine sensors, there are two ways to go around this.

To fix or minimize a rough idle when cold, you need to warm up your car engine before driving. However, mechanical factors can cause rough idles, such as faulty oxygen sensors or cylinder compression. If your car has these issues, you need to get it serviced to fix the rough idle and misfires.

I’ll discuss further several instances when the rough idles, and misfires are attributable to a mechanical failure down below:

  • Faulty oxygen sensors: The sensor is delivering the wrong information, and the result is an air-to-fuel mixture that is too lean or too rich.
  • Spark plugs that need to be replaced. A faulty spark plug would affect the “spark” component of the combustion cycle.
  • Fuel injectors that require attention. If a fuel injector is not functioning correctly, the air-to-fuel mixture will be compromised.
  • Cylinder compression problems: If the compression within a cylinder in your engine is compromised, it will affect your engine performance and create misfires and rough idles.

What Would Cause a Diesel to Misfire?

The mechanics of a diesel engine versus a gasoline engine are very similar. Therefore, the part that the air-to-fuel ratio plays in misfires is the same. However, diesel engines have special attributes and unique components compared to gasoline engines that need special attention.

Diesel engines do not have spark plugs but instead, rely on the heat created in the cylinder chamber to trigger the combustion. There is no spark as in gasoline engines.

A diesel can misfire when a diesel engine is cold, causing the cylinders not to have sufficient heat to create the ignition process for mixing the air and fuel. For that reason, diesel engines have glow plugs. 

These devices generate heat in the chamber to “spark” the initial combustion cycles until the thermal energy created by these ignitions is sufficient to keep the sequence going.

As such, spark plug issues cannot cause a misfire in a diesel engine, but the other causes outlined above for gasoline engines can be a cause. Likewise, a failing glow plug can cause a misfiring cylinder, especially when first starting the vehicle.

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When To See a Mechanic If Your Engine Is Misfiring When Cold

Smiling mechanic writing on a clipboard with a car with an open hood in the background

Below are three factors for you to consider in determining whether or not you need to take your car to a mechanic due to misfires.

1. How Often the Misfires Occur

If your car is experiencing misfires regularly, a visit to your mechanic is in order. Otherwise, a single misfire would most likely be an outlier and not indicative of a severe issue requiring attention from a mechanic.

2. The Age and Model of Your Vehicle

If you have a newer vehicle, misfires, rough idle, and engine skipping are more prone to indicate a more significant problem than in older vehicles. The reason is that most newer vehicles are equipped with sensors and engine management computer systems that will rapidly and effectively minimize the conditions for misfires—even in cold weather.

Misfires in newer vehicles can indicate failures in the systems or components designed to prevent them from happening.

If your car is older, the margin of tolerance for misfires would be more significant before a visit to a mechanic would be in order. That said, if the misfires start happening on a warm engine, then I would suggest a trip to your mechanic.

3. The Weather Condition When the Misfires Occur 

As I explained at the beginning of this article, cold temperatures reduce the range of effective combustion of the fuel and air mixture. The most evident symptom of this is the form of an engine misfire. 

Therefore, if your car misfires when it is first being started on a cold day, that fact alone would not be enough to merit a visit to the auto shop. If, however, they occur during warm weather or on a warm engine, getting the opinion of a service technician would be recommended.

Bottom Line

An engine is more prone to misfire, experience rough idles, or skip in cold weather. The leading cause for this is how cold temperatures reduce the range required for the effective combustion of the fuel and air in the cylinder chamber.

An occasional misfire is usually not a cause for concern. 

However, if the problem increases in frequency or starts happening even after the engine has warmed up, attention from an automotive technician would be in order. Likewise, you should reduce your tolerance window for misfires if your car is equipped with advanced engine management systems.

Final Thoughts

It’s funny how I still miss my first car. It was certified junk, but it was mine, free and clear. Of course, I’m lucky I survived the thing since I drove it until the wheels fell off, literally. I remember driving 70 down the highway, pulling into my drive, putting it in park, and having the left front wheel fall inward as the suspension crashed into the ground. Good times.

Anyway, I digress. I hope this article was informative and helpful. Thanks for reading!

For more, check out Choosing the Best Cylinder Engine for Your Driving Needs.