The brief
The 320i was due for its ignition coils and spark plugs, and the owner wanted them done to BMW's recommended interval rather than waiting for a misfire. That's the right instinct, fresh coils and plugs at the right interval keep the engine firing cleanly, sharpen the response and help the fuel economy. Each cylinder has a coil that delivers the high-voltage spark and a plug that fires it. Both are wear items: the coils heat-cycle and weaken, and the plugs wear, the gap opening and the electrode rounding off, until the spark gets weak and the burn incomplete, which costs you smoothness, response and economy. The interval exists so you renew them before that happens, and you do the full sets, not one at a time, so every cylinder fires the same.
The diagnosis
A check confirmed the coils and plugs had reached their interval, the plugs worn with the gaps opening up and the coils tired from the mileage. The engine was running fine, this was preventive maintenance, getting the ignition refreshed before it became a misfire. So it was a full ignition coil set and a full spark plug set together, every cylinder getting fresh hardware.
The work
The old ignition coils and spark plugs were removed, and a full set of new genuine BMW-spec coils and a full set of new plugs fitted, the plugs gapped and torqued, the coils seated properly. The engine's adaptations were reset so it could relearn on a fresh ignition system. A road test confirmed a steady idle, crisp throttle response, and smooth power through the rev range.
The outcome
Steady idle, sharper throttle response, smooth power, and the fuel economy back where it should be, with the ignition reset for the next interval. The 320i went home running properly. Coils and plugs renewed at the right interval keep the engine doing what it's meant to and head off the misfire before it starts, exactly the kind of maintenance that pays for itself at the pump.
