1. Ignition System: This is the most common culprit.
* Worn or faulty spark plugs: This is the first thing to check. Inspect the plugs for fouling (black, wet, or oily), excessive wear, or breakage. Replace them if necessary. Pay close attention to the gap – it should be within factory specifications.
* Worn or damaged spark plug wires: Check for cracks, fraying, or corrosion in the wires. A simple visual inspection is often sufficient. Consider replacing the entire set as a preventative measure, especially if they're old. Use a spark tester to confirm each wire is delivering spark.
* Distributor cap and rotor: Inspect for cracks, arcing, or excessive wear on the cap and rotor. Replace if necessary. Pay attention to the rotor's contact point – even minor damage can cause misfires.
* Ignition coil: A failing coil can cause misfires in multiple cylinders. Testing with a multimeter is recommended. A bad coil will usually show either an open or short circuit.
* Ignition module (electronic ignition): This is less likely, but a faulty module can disrupt the spark to multiple cylinders. These are generally tested with a specialized tool or replaced if suspected.
* Ignition control module (ECM/PCM): In some 1986 models, the computer might be implicated. Diagnosis requires a scan tool to check for trouble codes.
2. Fuel System:
* Fuel injectors: A clogged or faulty injector will prevent fuel from reaching the combustion chamber. Testing involves checking fuel pressure and injector pulse width with specialized tools. You may need to use a fuel pressure gauge to ensure adequate pressure.
* Fuel filter: A clogged fuel filter will restrict fuel flow to the engine. A simple replacement is a good preventative measure.
* Fuel pump: A weak fuel pump might not deliver sufficient pressure. This usually results in a problem across multiple cylinders, not just two, but it's worth checking.
3. Engine Mechanical Issues:
* Valves: A burnt, bent, or sticking valve will prevent proper combustion. This requires more in-depth diagnosis involving a compression test and potentially a leak-down test.
* Camshaft: A problem with the camshaft, such as a lobe wear, can prevent a valve from opening correctly. Similar diagnosis as valves.
* Piston rings: Worn piston rings can lead to low compression and misfires. Again, compression and leak-down tests are necessary.
Troubleshooting Steps:
1. Visual Inspection: Start with a thorough visual inspection of the spark plugs, wires, distributor cap, and rotor.
2. Spark Test: Use a spark tester to verify spark at each plug wire.
3. Compression Test: Perform a compression test to check the compression in each cylinder. Low compression indicates a mechanical problem.
4. Fuel Pressure Test: Check the fuel pressure to ensure it's within the manufacturer's specifications.
5. Trouble Codes (if applicable): If your vehicle has a diagnostic port, use a scan tool to read any trouble codes stored in the ECM.
Important Note: If you're not comfortable working on your engine, it's best to take it to a qualified mechanic. Improper diagnosis and repair could cause further damage. Remember safety first – disconnect the negative battery terminal before working on the ignition system.
