98 Subaru Impreza RS 2.5: Intermittent P0301, P0302, and P0304 Codes After Exhaust and Intake Modifications
Hello, I recently purchased a 1998 Subaru Impreza RS 2.5 with pre-existing issues including a faulty knock sensor and a broken heater wire in the #2 oxygen sensor. These were repaired, and an initial retune was performed along with replacement of worn spark plugs and oil. I then modified the vehicle by replacing the exhaust system with a 2.25" pipe (keeping the stock manifold and header), adding a high-flow catalytic converter, resonator, and canister-type muffler. The intake system was also upgraded with an aftermarket cold air intake (CAI) while reusing the original MAF sensor. The MAF was cleaned, and the ECU was reset. These modifications improved fuel efficiency from approximately 23 MPG to about 27 MPG on highway driving—within expected gains for such upgrades. However, I’ve now encountered intermittent engine misfire codes. On my first road trip (approximately 600 miles), after a near-full-throttle acceleration through gears, the vehicle triggered a P0301 code. After clearing it, no further issues occurred for several weeks of normal daily driving (about 300 miles per week). Today, while cruising at 75 MPH on the highway about 5 miles from work, the Check Engine Light illuminated and displayed two instances each of P0302 and P0304—totaling four codes. I cleared them immediately with my OBD-II reader. About a mile later, the CEL flashed again, showing the same codes once more. I’ve noted that when a misfire code appears, it often persists only after engine restart (full clear requires a restart). However, during the initial P0301 event, I cleared the code while driving and it did not return afterward. Given this history, I’m wondering if the spark coil pack—possibly previously replaced—is contributing to these intermittent misfires. I’ve observed that many aftermarket suppliers offer only 3-month warranties on coil packs, which raises concerns about long-term reliability. The car runs smoothly under normal conditions and has no noticeable engine misses or performance issues beyond a rare, low-pace idle (400–500 RPM) that occasionally lopes. This issue resolves when the engine is revved and allowed to settle back into a stable idle at 750–800 RPM. Before investing $100 in a new coil pack set, I’d like to understand whether this misfire pattern is likely related to a failing coil pack or if other components—such as fuel pressure, injectors, or wiring—are more probable causes. Additionally, how long do spark coil packs typically last under normal driving conditions? Thanks for any insights! Les
The coil pack is a plausible suspect—replacing it could resolve the issue. However, it's best to confirm that the misfire is actually originating from the coil before replacing it. Before your exhaust and intake modifications, were you experiencing any misfires? Did they appear immediately after the changes? Start by inspecting all wiring harnesses connected to the coil packs for loose connections or damage. Also check the injector wires—specifically injectors #1, #2, and #4—for any signs of looseness or physical damage. Ensure that the intake manifold is properly attached and tightly secured. A leak could introduce unmetered air into the system, potentially causing misfires. Please update with your findings—I'm interested in hearing how things progress!