* Ignition Control Module (ICM): The ICM is a common culprit in older Ford Explorers. It's essentially a switch that controls the power to the coil pack. A faulty ICM can prevent spark to certain cylinders. This is a very likely suspect.
* Worn or Damaged Spark Plug Wires: Even if you've replaced the coil pack, inspect the spark plug wires very carefully. Look for:
* Cracks or breaks in the insulation: These can cause shorts or prevent proper spark delivery.
* Corrosion or damage at the terminals: Poor connections at either the coil pack or spark plug ends can disrupt the spark.
* Incorrect routing: Wires routed too close to hot components can cause interference.
* Spark Plugs: While you're checking the wires, inspect the spark plugs. Worn, fouled, or damaged plugs can hinder or prevent sparking, even if the wire and coil are good. Try replacing them – it's a relatively inexpensive test.
* Distributor Cap and Rotor (if applicable): Some 1991 Explorers might have a distributor. If so, inspect the cap for cracks, carbon tracking, or worn contact points. The rotor should spin freely and make good contact with the cap.
* Wiring Harness: Check the wiring harness for any breaks, shorts, or corrosion, particularly around the ICM and coil pack connections.
* PCM (Powertrain Control Module): While less likely, a faulty PCM could be sending incorrect signals to the ICM, affecting spark delivery. This is more difficult to diagnose and usually requires professional tools.
Troubleshooting Steps:
1. Visual Inspection: Thoroughly inspect all the components listed above. Look for any obvious signs of damage.
2. Continuity Test: Use a multimeter to check for continuity in the spark plug wires and the wiring harness.
3. Spark Test: With a spare spark plug (grounded to the engine block) held near each wire going to the coil pack, crank the engine and observe if a spark jumps. This will pinpoint exactly which cylinders lack spark.
4. ICM Replacement (if necessary): Given its common failure rate, this is often the best first guess after ruling out damaged wires and spark plugs.
Important Note: Working on the ignition system involves high voltage that can be dangerous. Disconnect the negative battery terminal before starting any work. If you are not comfortable working with automotive electrical systems, it's best to take your Explorer to a qualified mechanic.
