Ignition System:
* Worn spark plug wires: Moisture can cause arcing and misfires in deteriorated spark plug wires. Rain exacerbates this.
* Bad spark plugs: Worn or fouled plugs can misfire, and moisture can worsen the problem.
* Distributor cap and rotor (if equipped): Cracks or corrosion in these parts can lead to ignition problems, significantly worsened by rain. The Windstar likely uses coil packs, rather than a distributor, but it's worth checking for corroded connections.
* Ignition coil(s): A failing coil can cause misfires, and moisture can contribute to the issue. Windstars use individual coil packs for each cylinder, so one failing coil will affect only one cylinder, while others may cause rougher running.
Sensors:
* Mass airflow sensor (MAF): A dirty or faulty MAF sensor can cause a rough idle and poor performance. Rain could affect a sensor with exposed elements or poor sealing.
* Crankshaft position sensor (CKP): This sensor tells the computer the engine's rotational position. A faulty CKP, especially one exposed to moisture, can cause misfires and a rough running condition.
* Cam position sensor (CMP): Similar to the CKP, a faulty CMP can cause significant running problems and is vulnerable to water intrusion.
Electrical System:
* Wiring harness: Corrosion in the engine compartment wiring harness due to water ingress can cause intermittent electrical problems, leading to misfires and rough running. Check for corrosion around connectors and wiring near the firewall and under the hood.
* Poor ground connections: Corrosion on ground connections can disrupt the electrical flow, causing similar issues as a faulty wiring harness.
Fuel System:
* Fuel injector(s): A clogged or faulty fuel injector can lead to rough running. While less directly related to rain, the problem might be exacerbated if moisture somehow gets into the fuel system.
* Fuel pump: Though less likely to be directly affected by rain, a failing fuel pump can lead to inconsistent fuel delivery, leading to a rough idle and performance issues.
Other Possibilities:
* Vacuum leaks: A vacuum leak can cause a rough idle. Rain might not directly cause the leak but could reveal it by changing air pressure.
* PCV valve: A clogged PCV valve can affect engine performance.
Troubleshooting Steps:
1. Inspect the engine compartment: Look for obvious signs of water intrusion, corrosion, or damaged wiring.
2. Check spark plugs and wires: Examine them for wear, fouling, or cracks.
3. Check the coil packs and their connections: Inspect for cracks, corrosion, or loose connections.
4. Have the computer scanned for diagnostic trouble codes (DTCs): This will provide valuable clues about the problem. An OBD-II scanner is needed.
5. Test the MAF, CKP, and CMP sensors: These can be tested with a multimeter or a professional scan tool.
It's crucial to diagnose the problem systematically. Since the problem worsens in the rain, focus on components susceptible to moisture damage. Don't hesitate to consult a mechanic if you're not comfortable performing these checks yourself. The longer you wait, the more damage could occur.
