Coil packs have replaced the standard distributor cap in many engines, since they improve the efficiency of the ignition system. Rather than use a distributor cap and rotor to send a spark through the wires, the coil pack times the spark via an electronic pulse from the ignition system. A coil pack consists of a small plastic case containing an electronic module that connects to four, six or eight plug wire outlet nipples (depending on the size of the engine).
Engine Light and Trouble Codes
An illuminated "Check Engine" light or a trouble code will provide your first indication of a faulty coil pack. If your "Check Engine" light turns on while the engine runs, it may indicate one of many possible problems---including a faulty coil pack. If necessary, visit your dealership and have a mechanic hook up a code scanner (a small handheld computer) to your vehicle to determine the exact source of the problem. The code scanner lists a trouble code number; the mechanic then refers to a trouble code book to decipher the problem component. To troubleshoot your own system problem, rent a code scanner and book from your local auto supply store and follow the directions.
Cracked Coil Packs
Cracked coil packs show symptoms particularly during cold and wet weather when moisture has entered the engine compartment. Cracked coil packs can cause a rough-running engine or a misfire at idle. Moisture that sits in the crevice of a crack on the coil pack will allow a spark (arc) to travel the length of the crack until it reaches a ground source, where it will cause a direct short. Electrical sparks jumping from a crack in the coil pack case accompanied by a loud electrical clicking or snapping sound indicate a cracked coil pack.
Poor Fuel Economy
Any one of the coil terminals can fail, robbing the ignition system of the spark required to burn the fuel. Unburned fuel allowed to pass through the exhaust will cause a noticeable reduction in fuel economy, since the engine has to work harder to maintain the same speed and power. Sluggish engine performance will accompany faulty coil pack performance.
Any black smoke exiting the tailpipe will indicate, raw, unburned fuel. A too-rich mixture appears as cloudy, black smoke exhaust that exits the pipe during all engine operations, whether idle or high-speed revolutions per minute (rpm). Revving the engine several times can show this condition. A back-draft of unburned fuel can enter the vehicle through open windows while you drive; you'll notice the smell of raw gas, which can cause nausea.
A smell of sulphur or burnt rotten eggs coming from the undercarriage of the vehicle will indicate a fuel-saturated catalytic converter. Catalytic converters can become wet with fuel when the coil pack does not produce a spark strong or consistent enough to ignite the fuel mixture. This failure can clog the catalytic converter and cause back pressure in the exhaust. Extreme back pressure will cause the engine to run roughly, die during idle or not start at all.
Miss Under Load
If the vehicle engine misses or hesitates while under a load, as in driving up an incline while hauling a trailer, you may either have a faulty coil pack or loose plug wires at the coil pack nipple connections.