2015 Legacy 3.6 Shudder on Acceleration

Dec 22, 2023
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Been trying to tackle an issue on a 2015 Legacy for quite some time now and hoping someone can point me in the right direction.

Started off with rear end noise where the dealer replaced the rear diff and rear wheel bearing. Then got random shaking at highway speeds which turned into a misfire on cylinder 5. Replaced all plugs and issue continued. Replaced the defective coil pack and now that's fixed.
Now the "shaking" or shuddering only happens on low speed acceleration. Transmission was replaced by the dealer at around 55k (at which I'd assume new fluid was put in too) and we're at a bit below 102k now. No issues up until about 1000 miles after the rear diff and wheel bearing.

No check engine lights, no transmission codes. It will almost always happen on slow speed acceleration. My dad explains it as "between 1st and 2nd gear" to which it sounds like he's talking about between the fake shift points programmed into the car. We'll never see any issues when slowing down and accelerating on main roads.

TL;DR Timeline:
-Rear end noise -> Dealer replaced rear diff and rear wheel bearing (100,500)
-Misfire on Cylinder 5 -> Replaced all spark plugs with OEM (101,200)
-Misfire continues -> Swap coil pack and test (101,400)
-Misfire moved to another cylinder -> Replace coil pack with OEM (101,500)
-Shaking at low speed continues -> about 102k miles now.
Issue resolved. Failing coil pack on Cylinder 3 which did not trigger a check engine light but did trigger an error on a more expensive OBD2 scan tool.

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