Ignition System:
* Worn Distributor Cap and Rotor: These components distribute the high voltage to the spark plugs. Cracks or corrosion can cause misfires leading to rough idling and stalling. Inspect them closely; replacement is cheap and easy.
* Worn Spark Plug Wires: Similar to the cap and rotor, cracks or worn insulation can lead to misfires. Look for cracks or areas where the wire is close to metal.
* Faulty Spark Plugs: Worn, fouled, or incorrectly gapped spark plugs are a major cause of rough idling. Check the gap and consider replacing them.
* Ignition Coil: A weak or failing ignition coil can't provide sufficient voltage for consistent sparking.
* Ignition Module (or Electronic Ignition Control Unit): This controls the timing and spark delivery. A failing module can lead to erratic performance.
Fuel System:
* Dirty Fuel Filter: A clogged fuel filter restricts fuel flow, leading to poor performance and stalling. This is a cheap and easy replacement.
* Vacuum Leaks: Leaks in the vacuum lines can disrupt the fuel/air mixture, causing rough running. Inspect all vacuum lines for cracks or loose connections.
* Fuel Pump: A weak or failing fuel pump can't supply enough fuel pressure. You'll likely hear a change in the fuel pump sound or a lack of fuel pressure when cranking.
* Clogged Fuel Injectors (if equipped): If your truck has fuel injection, clogged injectors can cause a misfire. This usually requires professional cleaning or replacement. (Carburetor equipped trucks are more common for that year)
* Carburetor Issues (if applicable): If you have a carburetor, various problems can arise: clogged jets, worn out accelerator pump, incorrect float level, etc. Carb rebuild or replacement may be necessary.
Other Potential Issues:
* EGR Valve: A faulty Exhaust Gas Recirculation valve can affect the air/fuel mixture.
* Mass Airflow Sensor (MAF) or Throttle Position Sensor (TPS) (if equipped): These sensors provide critical information to the engine's computer. A faulty sensor will send incorrect information, leading to poor performance. Less common in 1986 models, but possible on later-year versions or aftermarket additions.
* PCV Valve: A Positive Crankcase Ventilation valve that's stuck closed or clogged will affect crankcase pressure and engine performance.
Troubleshooting Steps:
1. Start with the simple and cheap: Check spark plugs, wires, distributor cap and rotor, and the fuel filter. These are the most common and easiest fixes.
2. Listen to the engine: Are there any unusual noises? A ticking, knocking, or whining sound can provide clues.
3. Check for vacuum leaks: Spray carb cleaner around vacuum lines while the engine is running. A change in idle speed indicates a leak.
4. Check fuel pressure (if you have the tools): This will help determine if the fuel pump is working correctly.
5. Consult a repair manual: A repair manual specific to your 1986 Ford F-150 will provide detailed diagrams and troubleshooting procedures.
If you're not comfortable working on your vehicle yourself, take it to a qualified mechanic. They'll have the tools and expertise to diagnose and repair the problem efficiently. Providing them with as much detail as possible about the symptoms will help them pinpoint the cause quickly.
