Rough idle is back even though I just did plugs 20k ago

[yvonnemakepeace]

I'm kinda scratching my head here. I've got a 2018 330i and about 20,000 km ago I replaced the spark plugs myself. It ran great for a while but now the rough idle is creeping back, especially when the AC is on at a stoplight. I used NGK and gapped them correctly from what I remember, but maybe I rushed it or didn't torque them properly? I'm thinking of just taking it somewhere that does a lot of these engines because I don't want to chase my tail buying parts that aren't the issue. I've been looking at a Meticulous Spark Plugs Replacement service where they actually check coil condition at the same time and use a torque wrench instead of the "good and tight" method. Does anyone know if these engines are picky about plug brand? I've seen some people swear by Bosch only for the B48. Also could old coils cause the new plugs to foul faster? Trying to figure out if I should just redo everything properly or if something else like injectors might be the real problem. Really don't wanna spend a fortune on guesswork.

[gracefulwebb]

Rough idle returning at 20k km after a DIY plug change that felt rushed is worth redoing properly before assuming injectors or coils are the cause. Incorrect torque or improper seating on the B48 causes exactly this kind of gradual return of idle issues as the plug works slightly loose.

[graciaweekley]

Old coils absolutely can cause new plugs to foul faster by not firing cleanly at the correct energy level. Coil condition testing alongside a plug replacement is not optional on a B48 showing rough idle symptoms, it is the complete job done correctly.

[gracielaweightman]

B48 is not particularly brand sensitive between NGK and Bosch as long as the spec matches exactly. The correct part number matters far more than the brand, and gapping needs to match BMW specification rather than whatever the box states as the default.

[gratianaweir]

AC load causing rough idle at a stoplight specifically suggests the engine is struggling to compensate for the compressor demand. This points toward either a weak coil on one cylinder or a slightly misfiring plug that becomes more noticeable when the engine has less margin to absorb it.

[haganwelsford]

A specialist who checks live misfire counts per cylinder while the rough idle is occurring will tell you immediately which cylinder is the problem before touching anything. That single piece of data makes the difference between targeted repair and replacing parts that were never the issue.