A weak coil pack can cause uneven idle, engine run ability to be poor, lack of power, and poor fuel mileage. Often times a vehicle that runs well on start up and first few minutes of driving then starts to stutter or misfire can be warning signs of a failing coil pack. Newer vehicles with OBD I or II will often times throw a trouble code leading towards a bad coil pack. i.e. cylinder misfire. If you experience these symptoms yet when vehicle cools completely down then starts and runs fine a coil pack could fix these issues. A bad coil can cause failure of other tune up components.