Gen 5 – Low Throttle – Stumble/Hesitation

2015 Subaru Outback Limited

The car is hesitating around 1200 – 1500 rpm under low throttle at around 40 – 45 mph. It’s especially evident when there is a slight load on the engine (going up a hill). The car will buck/stumble and the only way to stop it is to either take your foot off the gas pedal or give it more gas (increase RPM to 2000 rpm). On the highway, there is no noticed stumble/hesitation. When parked with the engine running, there is a slight hesitation every 60 – 90 seconds. There are no CEL or codes stored in the engine (checked with Acron CP9125).

Attempted fixes:
We thought it may be bad gas. We were on vacation 200 miles away from home. We added ISO-HEET Water Remover And Premium Fuel Line Antifreeze + Injector Cleaner to the fuel tank (it was 1/4 filled) and filled the tank. We ran through that entire tank (to empty) and added another bottle of ISO-HEET along with a full tank of gas when we returned home. The car is still hesitating/stumbling under low throttle with slight load on engine.

When I returned home, I bought a can of CRC Mass Airflow Sensor cleaner, unhooked the battery for 30 min, removed and cleaned the MAF, reinstalled the MAF and hooked the battery back up. The car is still hesitating/stumbling under low throttle with slight load on engine.

Recent Maintenance:
Spark plugs changed at 70K miles in March 2021. We’ve had no issues up until now including a 400 mile vacation in October. The engine air filter was changed at 60K miles around September 2019.

What else can I check? I’m assuming at this point it may be a bad coil, but on other vehicles that would generally trigger a CEL that would tell me which cylinder the bad coil is located.

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