A Guide to the Transmission Filters on a 1998-2002 Subaru Impreza
Your automatic Impreza has two different transmission filters, and choosing the right one is critical for performance.
- Your Impreza has two filters: an external spin-on filter and an internal mesh strainer; know which one you are replacing.
- For the external spin-on filter, always use a genuine Subaru OEM part to avoid potential transmission damage.
- Aftermarket 'kits' replace the internal strainer; ATP is a good choice due to its high-quality rubber gasket.
- The internal strainer generally does not need to be replaced unless the transmission pan is already removed for other service.
Understanding Your Impreza's Two Transmission Filters
When shopping for a transmission filter for your 1998-2002 Impreza, it's crucial to know which part you're replacing. These cars came with the reliable 4-speed automatic transmission (known as the 4EAT), which uses two distinct components for filtration.
External Filter vs. Internal Strainer
The most common source of confusion is the difference between the external filter and the internal strainer. The external filter is a spin-on canister, much like an engine oil filter, and does the real work of cleaning the fluid. The internal strainer is a simple mesh screen inside the oil pan that is only meant to stop large, catastrophic debris. Most aftermarket "kits" are for the internal strainer and pan gasket.
Symptoms of a Clogged Transmission Filter
A clogged filter restricts the flow of transmission fluid, which can starve the transmission of the pressure it needs to operate correctly. Symptoms often start small but can lead to serious damage if ignored.
- Delayed or Harsh Shifting: The car may hesitate or jerk when shifting between gears.
- Transmission Slipping: You might notice the engine RPMs flare up during a shift without a corresponding increase in speed.
- Grinding or Whining Noises: Unusual sounds from the transmission area can indicate fluid starvation.
- Burning Smell: Overheated or broken-down transmission fluid can create a distinct burning plastic odor.
- Overheating Light: The "AT OIL TEMP" light on your dashboard may illuminate if the fluid gets too hot, a common result of poor flow.
Choosing the Right Aftermarket Part
Part quality matters, especially with transmissions. For this specific Impreza, the advice is different for the external filter and the internal kit.
External Spin-On Filter: Stick with OEM
For the external spin-on filter, it is highly recommended to use the genuine Subaru OEM part. Forum discussions among Subaru technicians and experienced owners show that aftermarket filters, which are often just re-branded engine oil filters, can have the wrong internal bypass valve. The Subaru filter is specifically designed to ensure constant fluid flow to the transmission cooler, even if the filter media gets clogged. Using the wrong filter could risk starving the transmission of fluid, leading to failure.
Internal Strainer Kit: Aftermarket Tiers
For the internal strainer and gasket kit, aftermarket options are generally acceptable because the strainer itself is a simple, non-critical part. The real difference between brands comes down to the quality of the pan gasket.
| Brand Tier | Brands | Assessment |
|---|---|---|
| Good | ATP, Wix | ATP is well-regarded for its high-quality, durable rubber gaskets that prevent leaks. Wix is a known filter brand, but their kits sometimes use cork gaskets, which are more prone to leaking than rubber. |
| Economy / Basic | Premium Guard, Pioneer Cable | These brands are typically budget-friendly. While likely functional, there is less specific owner feedback on the gasket quality and longevity for this Subaru application. |
| OEM Equivalent | Vaico | Vaico often produces parts that meet OEM standards, but they are more common for European vehicles and have limited reviews from Subaru owners. |
Frequently Asked Questions (FAQ)
Do I really need to replace the internal strainer?
Not usually. Subaru considers the internal part an "oil strainer," not a filter, and it is not a standard maintenance item. It only needs to be cleaned or replaced if you are already removing the transmission pan for another service (like fixing a leak) or if the transmission has suffered a major internal failure.
Which filter is more important to change?
The external spin-on filter is the more important serviceable part. It contains the media that actually filters fine contaminants from the fluid. While Subaru once called it a "lifetime" part, most mechanics recommend changing it periodically to prolong the life of the transmission.
Can I use a regular engine oil filter instead of the Subaru transmission filter?
No. While they may look identical and have the same thread pitch (like the Wix 51356), the internal construction is different. The OEM transmission filter does not have a traditional bypass valve and is designed to guarantee fluid flow. Using an engine oil filter can lead to fluid pressure issues.
How often should I change the transmission fluid and filter?
For a 1998-2002 Impreza under normal driving conditions, performing a transmission fluid drain-and-fill every 30,000 miles is good practice. If you decide to change the external spin-on filter, doing it at the same time is convenient. The internal strainer does not need to be part of this regular interval.
Technical Specifications
OEM Part Numbers: 38325AA032 (External Spin-On Filter) 31728AA050 (Internal Strainer/Filter)
Transmission Type: 4-Speed Automatic (4EAT)
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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.
- Subaru Impreza:
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- Understanding Your Impreza's Two Transmission Filters
- External Filter vs. Internal Strainer
- Symptoms of a Clogged Transmission Filter
- Choosing the Right Aftermarket Part
- External Spin-On Filter: Stick with OEM
- Internal Strainer Kit: Aftermarket Tiers
- Frequently Asked Questions (FAQ)
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