← Subaru WRX (GD/GG, 2002-2007) problems

Free-flowing downpipe can run 2004-05 WRX lean on stock tune

Reported · Severity: High · Years: 2004-2005

Symptoms

Lean air/fuel under boost; detonation risk.

Cause

Grassroots warns that the 2004-05 WRX's stock software can let the engine run too lean after a free-flowing exhaust swap (and that on STI the stock turbo's wastegate may let boost run away).

Fix (summary)

Pair exhaust changes with a professional tune; do not run catless/free-flow exhaust on the stock map.

Sources

• Insider guide: Make the most of your 2002-'07 Subaru Impreza WRX — Grassroots Motorsports (Feb 2009 issue) (editorial; read 2026-10-15; full page read)
• The Truth Behind The Subaru EJ-Series Engines - Tech-Knowledge — MotorTrend (editorial; read 2026-10-15; search excerpt only)

Summaries are in our own words. Not repair instructions; consult the factory service manual.