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2020 Forester Rough Idle and Stalling at Stops

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Nicole W. · rep 0 · May 22, 3:13 AM · 1 replies · solved · Question

Vehicle: 2020 Subaru Forester Premium

My 2020 Subaru Forester Premium has started acting up recently. When I come to a stop, especially after driving for a bit, the idle gets really rough and sometimes it even stalls out completely. It's quite unsettling, particularly in traffic.

Oddly, there's no check engine light illuminated, which makes diagnosis harder. The car restarts fine immediately after stalling, but the problem is becoming more frequent. I've checked the air filter, and it looks clean. Any ideas what could be causing this rough idle and stalling without throwing a code?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:13 AM

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ClearTheCode admin answer: Rough idle and stalling without a check engine light can be tricky, but it often points to issues that affect air-fuel mixture or ignition, but haven't yet reached the threshold to trigger an OBD-II code. For your 2020 Subaru Forester Premium, here are some common culprits:

  1. Dirty Throttle Body: Carbon buildup on the throttle plate can restrict airflow at idle, leading to rough idling and stalling. Cleaning it with a specialized throttle body cleaner can often resolve this.
  2. Mass Airflow Sensor (MAF): A dirty or failing MAF sensor can send incorrect air intake readings to the engine control unit (ECU), leading to an improper air-fuel mixture. Try cleaning it with MAF sensor cleaner.
  3. Vacuum Leaks: Leaks in vacuum lines or intake manifold gaskets can introduce unmetered air, disrupting the air-fuel ratio, especially at idle. Listen for hissing sounds under the hood.
  4. Spark Plugs/Ignition Coils: Worn spark plugs or failing ignition coils can cause misfires, leading to rough idle. While severe misfires usually trigger a CEL, minor ones might not immediately.
  5. Fuel System Issues: A weak fuel pump or clogged fuel filter could cause inconsistent fuel delivery, leading to stalling. However, these often present with other symptoms like hesitation under acceleration.

When to see a shop: If cleaning the throttle body and MAF sensor doesn't resolve the issue, or if you're uncomfortable performing these checks, it's best to have a professional diagnose your vehicle. Even without a CEL, a shop can use advanced diagnostic tools to monitor live engine data, check fuel pressure, and perform smoke tests for vacuum leaks to pinpoint the exact cause of the rough idle and stalling.

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