Likely Culprits Exacerbated by Rain:
* Ignition System (as you suspected): While the easy starting suggests the ignition system isn't *completely* failed, rain could have caused corrosion on connections (distributor cap, rotor, wires, coil pack). Moisture can lead to arcing and poor spark delivery under load when the engine demands more energy.
* Distributor Cap and Rotor: These are particularly vulnerable to moisture damage. Inspect for cracks, corrosion, or carbon tracking. Replace them if necessary.
* Ignition Wires: Check for cracks, fraying, or corrosion on the spark plug wires. Replace any damaged wires.
* Sensors: Various sensors (like the crankshaft position sensor (CKP), camshaft position sensor (CMP), or oxygen sensor) could be affected by moisture. These sensors influence fuel delivery and ignition timing. A bad sensor might show no issue at idle but fail under load.
* Wiring Connections: Water intrusion can corrode connectors throughout the engine bay. Inspect all electrical connectors for corrosion and clean or replace as needed. Pay particular attention to ground connections, which are crucial for proper electrical flow.
* Intake Manifold Gasket: Water could have found its way into the intake manifold, causing erratic running under load. This is less likely to only be a problem under load, but is worth checking if other issues are ruled out.
* Fuel System: Although less likely given the easy starting, rain could have affected fuel delivery. Look for any signs of leaks in fuel lines or moisture in the fuel filter.
Troubleshooting Steps:
1. Visual Inspection: Start with a thorough visual inspection of the entire engine bay, focusing on the areas mentioned above. Look for any signs of water damage, corrosion, or loose connections.
2. Ignition System Check:
* Distributor Cap and Rotor: Inspect and replace if necessary.
* Ignition Wires: Inspect and replace if necessary.
* Spark Plugs: Inspect and replace if necessary (worn, fouled, or damaged plugs are common). Gap should be correct.
* Coil Pack (if applicable): Inspect for cracks or damage.
3. Sensor Check: This may require a multimeter or a scan tool to check sensor readings. A bad sensor will often throw a diagnostic trouble code (DTC).
4. Wiring and Connections: Carefully inspect all wiring and connectors for corrosion and water damage. Clean any corroded connections with a wire brush and dielectric grease.
5. Fuel System Check: Inspect fuel lines and filter for leaks or moisture.
6. Scan Tool: A scan tool can read diagnostic trouble codes (DTCs) stored by the engine control unit (ECU). These codes can pinpoint the problem area more precisely.
Important Note: If you're not comfortable working on your Jeep's electrical system, it's best to take it to a qualified mechanic. Improperly diagnosing or repairing electrical issues can lead to further damage. Electrical problems, especially those involving the ignition system, can be dangerous.
