The brief
The 318i had been losing power and jerking during drives, the classic feel of a misfire, and the car was around the 120,000 km mark, where the plugs and the ignition coils are due on these engines. He brought it in. A misfire is a cylinder that isn't lighting cleanly. When that happens you lose the power that cylinder should give, the engine runs uneven so it jerks, it burns more fuel, and the computer flags it. The parts behind it are the spark plugs, which wear out, and the ignition coils that sit on top of them, which fail with age, and at 120,000 km both are at the end of their life. On a high-mileage engine showing misfires, the proper fix is to refresh the whole ignition side rather than chase one cylinder.
The diagnosis
A diagnostic scan confirmed the misfire and pointed at the ignition side. The plugs came out worn past spec, well into their replacement window, and at this mileage the coils were the same wear item, the same age. So the call was the standard refresh, a fresh set of plugs and a fresh set of coils, which heads off the misfires for the long run rather than fixing one and waiting for the next.
The work
New genuine BMW-spec spark plugs went in, gapped to spec, and new genuine BMW-spec ignition coils went on top of them. The harness clips were reseated, and the stored fault codes cleared. Then the engine was run to confirm it was firing cleanly with no shudder, followed by a road test to confirm the power was back, the jerking was gone, and no misfires returned.
The outcome
Power back, no jerking, a steady idle, no misfires on the road, and the check engine light out after a drive cycle. The 318i went home running cleanly and good for the next interval. Plugs and coils are wear items that age together, so doing the full set in one go means even, reliable ignition rather than another misfire a few thousand kilometres down the road.