A Guide to Spark Plugs for the 2004-2014 Subaru Forester
Keep your Forester's unique boxer engine running strong by choosing the right spark plugs.
- Stick with OEM-equivalent brands like NGK or Denso Iridium plugs for the best performance and longevity.
- Replacement is a difficult job due to tight clearances; be prepared with wobble extensions and a thin-wall spark plug socket.
- Ignoring misfire symptoms can lead to expensive catalytic converter damage down the road.
- Always replace spark plugs as a full set to ensure even engine performance.
Symptoms of Failing Spark Plugs
When spark plugs wear out, they can't create a strong enough spark. This can cause several noticeable problems with your Forester. You might feel the engine shaking or vibrating when you're stopped at a light, which is called a rough idle. Your car may feel slow or sluggish when you try to accelerate, and your fuel mileage might get worse. In some cases, the engine might misfire, which feels like a stutter or hesitation while driving. This can also cause the "Check Engine" light to turn on. Difficulty starting the engine is another common sign of worn spark plugs.
Don't Ignore Misfires
Continuously driving with a misfiring engine can cause unburned fuel to enter the exhaust system. This can damage the catalytic converter, which is a very expensive part to replace.
Choosing the Right Aftermarket Spark Plugs
For the 2004-2014 Subaru Forester, not all spark plugs are created equal. Using the right type is essential for performance and engine health. The original factory plugs are typically made by NGK or Denso, and sticking with these brands is a safe bet.
Brand Quality Tiers
- Top Tier (OEM-Equivalent): NGK & Denso
These brands are the original equipment manufacturers for Subaru. Their Iridium and Laser Iridium plugs are designed specifically for the demands of the Forester's engine, offering the best performance and longest life (often up to 60,000 miles or more). While they cost more upfront, their durability makes them a worthwhile investment. - Mid-Tier (Reputable Aftermarket): Bosch & Champion
Brands like Bosch and Champion offer quality Iridium and Platinum plugs that can be a good alternative. It's important to make sure you buy the specific model number that matches your Forester's engine requirements. These can provide good performance but may not have the same longevity as the OEM-equivalent options. - Economy Tier: Autolite & Other Budget Brands
These brands often sell basic copper or standard platinum plugs. While they are much cheaper, they have a significantly shorter service life and may not provide the same level of performance or fuel efficiency. For a Subaru engine, it's generally recommended to avoid these unless it's a temporary fix.
Iridium is Worth the Cost
Iridium spark plugs have a very hard, fine-wire center electrode that provides a consistent, strong spark and resists wear much better than platinum or copper. For the difficult job of changing plugs on a Forester, using long-life iridium plugs means you won't have to do the job again for a long time.
Cost Comparison for Spark Plug Replacement
| Item | Estimated Cost |
|---|---|
| New OEM Spark Plugs (Set of 4) | $50 - $140 |
| New Aftermarket Spark Plugs (Set of 4) | $15 - $125 |
| Shop Labor | $350 - $550+ |
Known Issues, Recalls, and TSBs
For the 2004-2014 Forester, there are no specific recalls or Technical Service Bulletins (TSBs) directly related to the spark plugs themselves. Searches on the NHTSA database did not show any recalls for spark plugs on these models. The most significant "issue" is the difficulty of the replacement job due to the boxer engine design. The spark plugs are located deep on the sides of the engine, with the frame rails leaving very little room to work. Some owners have also reported finding oil on the spark plug threads, which can be a sign of a leaking spark plug tube seal 🎬 See how to replace valve cover gaskets and tube seals., a part of the valve cover gasket.
Frequently Asked Questions (FAQ)
How often should I replace the spark plugs on my Forester?
For models in this generation using the recommended NGK or Denso iridium plugs, the typical replacement interval is around 60,000 miles. However, always check your owner's manual for the specific schedule for your vehicle's engine.
Is it hard to change the spark plugs myself?
Yes, this is considered an intermediate to difficult DIY job. The tight space requires a specific combination of tools, including a thin-wall spark plug socket, various extensions, and a swivel or wobble adapter. To get access, you typically need to remove the air intake box on the passenger side and the battery on the driver's side. 🎬 Watch this step-by-step walkthrough for replacing your Forester's spark plugs.
Do I need to check the gap on new spark plugs?
Most high-quality iridium plugs from brands like NGK and Denso come pre-gapped from the factory. It's a good practice to gently check the gap with a feeler gauge, but you should not attempt to adjust the gap on a fine-wire iridium plug, as you can easily break the delicate electrode.
Should I use anti-seize on the new spark plug threads?
This is a debated topic. Major brands like NGK state that their plugs have a special trivalent metal plating that acts as its own anti-seize, and that adding more can cause you to over-torque the plug. It's best to follow the specific recommendation of the spark plug manufacturer.
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Technical Specifications
OEM Part Numbers: ILFR6B SILZKAR7B11 PFR6G
Engine design requires removing the airbox and battery for access. A 14mm or 5/8" thin-wall spark plug socket is typically required, depending on the specific engine.
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
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