Most Likely Culprits:
* Ignition System: Moisture can compromise the ignition coil, distributor cap (if equipped), rotor, spark plug wires, or even the spark plugs themselves. Water bridging between components can cause misfires and sputtering. This is a top suspect.
* Mass Airflow Sensor (MAF) Sensor: The MAF sensor measures the amount of air entering the engine. If it gets wet, it can provide inaccurate readings, leading to a rough running engine. It's often located near the air filter housing and relatively exposed to the elements.
* Engine Control Module (ECM) / PCM (Powertrain Control Module): While less common, water intrusion into the ECM (the "computer" of your engine) can cause all sorts of problems, including erratic engine operation and check engine lights. This is a more serious issue requiring professional attention.
* Wiring Harness: Water entering the engine bay can corrode or short-circuit wires, leading to intermittent problems like sputtering and engine misfires. Look for any areas where wires might be exposed or poorly routed.
* Sensors: Various other sensors (crankshaft position sensor, camshaft position sensor, etc.) can be affected by moisture.
Troubleshooting Steps:
1. Check Engine Light Code: Get the diagnostic trouble codes (DTCs) read from the check engine light using an OBD-II scanner. This will provide clues about the specific problem area. Auto parts stores often offer free code reading services.
2. Visual Inspection: Carefully inspect the engine bay, focusing on the areas mentioned above. Look for any signs of water intrusion, corrosion, or damaged wiring. Pay close attention to the area around the distributor (if applicable), ignition coil, MAF sensor, and any exposed wiring.
3. Ignition System Check: If you have some mechanical experience, you can inspect the distributor cap and rotor (if equipped) for cracks or corrosion. Check the spark plug wires for cracks or damage and consider replacing them if necessary. Inspect the spark plugs for fouling or damage.
4. MAF Sensor Cleaning: You can try carefully cleaning the MAF sensor with a MAF sensor cleaner (available at auto parts stores). Follow the instructions carefully, as these sensors are delicate.
5. Water Intrusion Points: Look for any areas where water might be entering the engine bay. Common entry points include seals around the firewall, cracked seals around the engine, and poorly sealed wiring grommets.
Important Considerations:
* Safety First: Disconnect the negative battery terminal before performing any work on the electrical system.
* Professional Help: If you're not comfortable working on your vehicle's electrical system, it's best to take it to a qualified mechanic. Diagnosing electrical problems can be tricky, and incorrect repairs can cause further damage.
* Water Damage to ECM: Water damage to the ECM is a serious problem that often requires professional repair or replacement, which can be expensive.
The fact that the problem occurs only when it rains strongly points to a water-related electrical issue. Getting the check engine light codes read is the crucial first step in narrowing down the cause.
