Ignition System Issues (Beyond the basics):
* Ignition Coil: A failing coil can't provide sufficient voltage to the spark plugs, leading to a weak or absent spark. This is a common culprit, even if your cap and rotor are new.
* Ignition Control Module (ICM): This module controls the ignition timing and spark delivery. A faulty ICM can cause misfires, especially under load or at certain RPMs.
* Distributor: Problems within the distributor itself, beyond the cap and rotor, such as a worn shaft or faulty pickup sensor, could be the issue.
* Worn Distributor Gear: This gear meshes with the camshaft, and wear can lead to poor timing and misfires.
* Wiring Issues: Even with new wires, there might be a break or short somewhere in the harness leading to or from the ignition system components. Look for chafing or damage.
Fuel System Problems:
* Fuel Injectors: A clogged or failing fuel injector won't deliver the correct amount of fuel, resulting in a misfire. This would usually be more apparent as a rough running condition, or loss of power, not just a misfire in one specific cylinder.
* Fuel Pressure Regulator: A faulty regulator could deliver incorrect fuel pressure.
* Fuel Pump: A weak fuel pump might not supply enough fuel pressure, especially under load.
Other Potential Causes:
* Vacuum Leaks: Leaks in the intake manifold or vacuum lines can disrupt the air/fuel mixture, causing a misfire.
* Compression Issues: Low compression in one cylinder due to worn rings, valves, or head gasket problems will result in a misfire. A compression test is necessary to rule this out.
* Camshaft Position Sensor (CMP sensor): If the engine is not getting the correct signal about the position of the camshaft, misfires can occur, especially on the specific cylinder. This is less likely if the ignition system components are new, but could still cause a problem.
Troubleshooting Steps:
1. Check for Spark: Use an inline spark tester to verify a strong spark at each plug wire.
2. Compression Test: This is crucial to rule out mechanical issues in the cylinders.
3. Fuel Pressure Test: Measure fuel pressure to ensure it's within specifications.
4. Vacuum Leak Test: Listen for hissing sounds around the intake manifold and vacuum lines.
5. Inspect Wiring: Carefully examine all wiring for breaks, shorts, or chafing.
Since you've already replaced the common ignition components, focusing on the coil, ICM, distributor, fuel system, and compression is the next logical step. A mechanic with experience in older vehicles will likely be able to diagnose the problem more efficiently using diagnostic tools. A simple code reader might not be enough for this older system.
