If you own both a Ford Focus and a Kia Soul, or you're switching between vehicles, you might assume spark plugs are all the same. They're not. The gap the space between the center and ground electrode is tuned for each engine. Using the wrong gap can cause misfires, poor fuel economy, and rough idling. Here is why the gap matters and how it differs between the Ford Focus and the Soul.

What is spark plug gap and why doesn't it stay the same across all cars?

Spark plug gap controls the spark voltage required to ignite the air-fuel mixture. Higher compression engines, like many modern Foci, need a precise, often tighter gap to prevent misfires under high cylinder pressure. Engines using older ignition systems or different combustion chamber designs, like some Soul engines, might use a wider gap for better idle quality and low-speed torque.

How different are the spark plug gaps between a Ford Focus and a Kia Soul?

Let's look at a common example. A 2014 Ford Focus with the 2.0L GDI engine usually specifies a gap of 0.028 - 0.030 inches. A same-year Kia Soul with the 2.0L MPI engine specifies a gap of 0.043 - 0.051 inches. That is a significant difference. The Focus uses direct injection and a high-energy ignition coil, allowing it to run a tight gap reliably. The Soul uses a standard coil system and needs a wider gap to create a sufficient flame kernel. If you want to see how these specs change across different Ford Focus generations, you can check out this Ford Focus spark plug gap comparison across models.

What if I mix them up? Can I use Soul spark plugs in a Focus?

Physically fitting the threads is one thing, but the gap tolerances are very different.

Using a Soul's wider gap on a Focus: The high cylinder pressure in the Focus can blow out the spark. You will get misfires, especially when accelerating hard or going uphill. The engine will feel sluggish.

Using a Focus's tighter gap on a Soul: The spark will be weak. The Soul's engine will struggle to burn the fuel completely. You might notice rough idling and a check engine light for cylinder misfires.

Every Ford model has its own specific gap requirements. For example, the Ford Ranger's spark plug gap differs from the Soul's as well due to its turbocharged engine and specific ignition mapping.

What tools do I need to set the gap correctly on my Focus or Soul?

Do not use a standard feeler gauge for spark plugs. A wire-loop gap tool is better because it measures the space where the spark actually jumps. For fine-wire iridium plugs, which are common in both cars today, you should not physically adjust the gap with a tool that pries on the center electrode. Instead, gently bend the ground electrode. Always check the gap before installing, even if the box says "pre-gapped."

What is the easiest way to find the exact gap for my specific year and engine?

Look at the emission label under the hood first. It sometimes lists the gap directly. If not, check the owner's manual or look up the OEM spark plug part number. The gap requirements change depending on whether you have a naturally aspirated engine or a turbo. The Ford Mustang, for instance, runs very tight gaps from the factory to handle boost. If you're curious about that, see the Ford Mustang spark plug gap versus 2005 Ford Soul discussion.

Common mistakes to avoid

  • Assuming all 2.0L engines are the same. The Focus 2.0L and Soul 2.0L have completely different gaps.
  • Using a cheap feeler gauge for fine-wire plugs. You can damage the delicate electrode.
  • Not tightening to the correct torque. A loose plug ruins the coil. A cross-threaded plug ruins the head.
  • Installing plugs without anti-seize on the threads. Check the plug manufacturer's instructions for your specific application.

Quick checklist before you buy spark plugs for your Focus or Soul

  1. Identify the engine code, usually an 8-character code under the hood.
  2. Buy the recommended OEM plug type for your specific engine.
  3. Have a wire gap tool ready to check the gap.
  4. Record your vehicle's specs using a legible format. You can print the specs using a standard font like Roboto for your garage maintenance binder.
  5. Gap the plugs carefully before installation, handling fine-wire plugs gently.
  6. Torque them to spec, usually between 15-20 ft-lbs for most aluminum heads.

The spark plug gap is a small measurement that makes a big difference. Taking a few minutes to look up the right spec for your specific Ford Focus or Kia Soul will save you hours of troubleshooting a rough-running engine later.