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2020 WRX STI rough idle, misfires, CEL flashing

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Chris P. · rep 0 · May 22, 3:14 AM · 1 replies · solved · Question

Vehicle: 2020 Subaru WRX STI

Hey performance enthusiasts,

My 2020 Subaru WRX STI is giving me some headaches. Lately, it's developed a really rough idle, almost like it's going to stall. I've also noticed occasional misfires, especially under light acceleration, and a definite loss of power. The Check Engine Light has been flashing intermittently, which I know is a bad sign.

The car has about 40,000 miles on it, and I keep up with all the maintenance. I haven't done any major mods recently. What could be causing this? I'm worried about damaging the engine.

Any insights or similar experiences would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:14 AM

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ClearTheCode admin answer:

A flashing Check Engine Light (CEL) on a performance vehicle like your 2020 WRX STI indicates a severe misfire that could potentially damage the catalytic converter due to unburnt fuel entering the exhaust. This requires immediate attention.

Common Causes & Diagnostics:

  1. OBD-II Codes: You're likely to have codes such as P0300 (Random/Multiple Cylinder Misfire Detected) and specific cylinder misfire codes like P0301, P0302, P0303, P0304 (Cylinder 1, 2, 3, 4 Misfire Detected).
  2. Ignition System:
    • Spark Plugs: Worn, fouled, or incorrectly gapped spark plugs are a very common cause of misfires. Given your mileage, it might be time for a check or replacement.
    • Ignition Coils: A failing ignition coil pack for one or more cylinders can lead to intermittent or constant misfires.
  3. Fuel System:
    • Fuel Injectors: Clogged or faulty fuel injectors can cause a cylinder to run lean or rich, leading to misfires. Carbon buildup on the injector tips is also possible.
    • Fuel Pressure: Low fuel pressure from a failing fuel pump or clogged fuel filter can affect overall engine performance.
  4. Air/Vacuum Leaks: Unmetered air entering the engine due to a vacuum leak (e.g., cracked hose, faulty PCV valve, intake manifold gasket) can throw off the air-fuel mixture, causing rough idle and misfires.
  5. MAF Sensor: A dirty or faulty Mass Airflow (MAF) sensor can send incorrect air readings to the ECU, leading to an improper air-fuel mixture.
  6. Carbon Buildup: Direct injection engines, like those in the WRX STI, are prone to carbon buildup on the intake valves, which can restrict airflow and cause misfires.

When to See a Shop:

  • Immediately: Do not continue to drive with a flashing CEL. Get your vehicle to a qualified Subaru performance shop or dealership as soon as possible. Driving with severe misfires can lead to costly damage to your catalytic converter and potentially other engine components.
  • Professional Diagnosis: A comprehensive diagnostic scan is needed to retrieve all stored codes and live data. A technician can then perform further tests (e.g., compression test, leak-down test, fuel pressure test, smoke test for vacuum leaks) to accurately pinpoint the root cause.
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