1. Verify the obvious (again):
* Spark Plug and Wire Installation: Double-check that cylinder 4's spark plug and wire are correctly installed. A loose connection or incorrect placement is a common oversight. Ensure the plug is properly seated and the wire is securely snapped onto both the plug and distributor cap (if applicable). Inspect for any damage to the plug or wire itself.
* Spark: With the plug wire disconnected, ground the metal part of the spark plug to a grounded engine part. Have someone crank the engine. You should see a strong, consistent spark. A weak or inconsistent spark indicates a problem with the ignition coil for that cylinder, the ignition module, or a wiring issue.
2. Investigate potential causes beyond ignition:
* Ignition Coil Pack: The 3.8L uses a coil-on-plug (COP) system. While you've replaced the wires, the coil itself for cylinder 4 could be faulty. These coils can crack internally or develop resistance, leading to weak sparks even with new wires. Test the coil's resistance using a multimeter (compare to the specs in a repair manual). Visual inspection for cracks or damage is also recommended. A faulty coil is a high-probability culprit.
* Injector: A clogged or malfunctioning fuel injector could starve cylinder 4 of fuel. You'll need to perform a fuel injector pressure test to check fuel delivery pressure and possibly an injector flow test (requires specialized equipment).
* Compression Test: Low compression in cylinder 4 indicates a mechanical issue such as worn piston rings, a damaged valve, or a blown head gasket. This is a more serious problem.
* Vacuum Leaks: A significant vacuum leak can affect engine performance and cause misfires. Inspect all vacuum hoses and connections for cracks, looseness, or damage. Pay close attention to the intake manifold gasket.
* PCM (Powertrain Control Module): In rare cases, a faulty PCM can cause misfire codes. This is less likely if other cylinders are running fine, but it's a possibility. This usually requires professional diagnosis.
* Sensor Issues: While less likely, a faulty crankshaft position sensor (CKP) or camshaft position sensor (CMP) can disrupt the ignition timing and lead to a misfire. These require testing with a scan tool or multimeter.
3. Systematic Troubleshooting:
1. Start with the easiest checks: Re-check the plug and wire installation.
2. Test for spark: As mentioned above.
3. Visual inspection: Carefully inspect all components related to cylinder 4 for visible damage.
4. Resistance testing: Test the ignition coil with a multimeter.
5. Fuel system diagnostics: Perform a fuel pressure and if possible, injector flow test.
6. Compression test: This should be done if other checks are inconclusive.
7. Vacuum leak check: Thoroughly examine vacuum lines.
8. Consider professional help: If you're uncomfortable performing these tests or if the problem persists, take the vehicle to a qualified mechanic. They have the tools and expertise to diagnose more complex issues.
Important Notes:
* Repair Manual: A repair manual specific to your 1996 Ford Windstar 3.8L is invaluable. It will provide detailed diagnostic procedures, wiring diagrams, and component specifications.
* Safety: Always disconnect the negative battery terminal before performing any electrical tests. Be cautious when working around moving engine parts.
By systematically working through these steps, you should be able to pinpoint the cause of the recurring cylinder 4 misfire. Remember that fixing the underlying problem, not just clearing the code, is crucial to resolving this issue permanently.
