Parts for your 2004 Subaru Impreza-Spark plugs

2004 Subaru Impreza spark plugs: purpose, service intervals and handy tips

Technical sources including the 2004 Subaru Impreza Owner’s Manual, Subaru EJ20/EJ25 service manuals, and NGK/Denso application catalogues all specify spark plugs for every 2004 Impreza petrol variant (2.0 and 2.5, including WRX). Spark plugs are therefore fitted and fully service‑relevant on this model.

Spark plugs ignite the air–fuel mix in the boxer engine’s cylinders, giving clean starts, a smooth idle, tidy fuel use and crisp throttle response. Fresh plugs help the 2004 Impreza pull cleanly through the revs and keep emissions under control. Platinum or iridium plugs handle heat and last longer, which is handy on turbo models, while copper‑core plugs cost less but need more frequent changes.

For most AU/NZ vehicles, owners should plan replacements about every 60,000–100,000 kilometres depending on plug type, tune and driving conditions, with WRX/turbo examples usually at the earlier end. If the service history’s a mystery, it’s smart to start fresh and then track intervals in the logbook.

• Tell‑tale signs the plugs are tired: rough idle, sluggish acceleration, harder cold starts, higher fuel use and misfire codes (often P0300–P0304 under the check‑engine light).
• Maintenance tips:
  • Use OEM‑equivalent quality plugs with the correct heat range (stock engines are typically range 6, heavily worked turbo setups may need one step colder).
  • Replace the full set at once and inspect coil boots, a light smear of dielectric grease helps prevent arcing.
  • Work on a cold engine. Blow debris from plug wells, use a proper plug socket and thread by hand first to avoid cross‑threading.
  • Avoid anti‑seize on modern nickel‑plated plugs, it alters torque and can lead to over‑tightening.

Fitment notes for the 2004 Impreza: plug gap is typically 0.7–0.8 mm unless the manufacturer specifies otherwise. Many iridium/platinum plugs arrive pre‑gapped, so avoid forcing the fine centre electrode. Final torque is about 25 N·m into the aluminium heads. Access on the flat‑four is a bit tight, the easiest approach is through the front wheel wells—turn the steering, ease back the splash liners and mind the coil connectors. Allow 60–90 minutes on the driveway with basic tools.

If the engine still misfires after new plugs, check coils and leads (where fitted), look for intake leaks and inspect the plug tubes for oil, rocker cover gaskets can seep with age and should be sorted to protect the new plugs.

Popular questions about 2004 Subaru Impreza spark plugs

What spark plug type suits a 2004 Subaru Impreza?
For stock 2.0/2.5‑litre petrol engines, an OEM‑equivalent platinum or iridium plug with the factory heat range (typically 6) is ideal. Turbo/WRX models benefit most from platinum or iridium due to higher temperatures. Modified or track‑driven cars may step one heat range colder, but only when tuning and usage justify it.

How often should the spark plugs be changed?
Expect about 60,000–100,000 kilometres depending on plug material and driving. Turbo engines, short‑trip use and spirited driving push intervals to the earlier end. If unsure on history, fit a fresh set and then follow the service schedule.

What’s the correct gap and torque?
The typical gap is 0.7–0.8 mm for 2004 Impreza petrol engines. Many premium plugs are pre‑gapped—avoid bending the fine wire on iridium types. Torque to about 25 N·m on a cold engine with clean threads. Skip anti‑seize on modern plated plugs to maintain accurate clamping force.