Ignition System:
* Worn Distributor Cap and Rotor: These components distribute the high voltage to the spark plugs. Wear causes poor spark delivery, leading to rough running and idle problems. Cracks or corrosion are common culprits.
* Worn Spark Plugs and Wires: Similar to the cap and rotor, worn plugs and wires result in weak or inconsistent sparks, causing misfires and the symptoms you describe.
* Ignition Coil: A failing coil won't provide sufficient voltage to the distributor, leading to weak sparks and poor engine performance.
* Ignition Control Module (ICM): This electronic component controls the ignition timing. Failure can cause erratic engine operation.
Fuel System:
* Vacuum Leaks: Leaks in vacuum lines can disrupt fuel delivery and air/fuel mixture, affecting idle and causing shaking. Check all vacuum hoses for cracks or disconnections.
* Clogged Fuel Filter: A dirty fuel filter restricts fuel flow, starving the engine and causing rough running.
* Failing Fuel Pump: A weak or failing fuel pump can't deliver enough fuel under pressure, leading to poor performance and stalling.
* Dirty or clogged carburetor (if equipped): If it's a carbureted model, a dirty carburetor can cause a variety of running issues, including poor idle and shaking. It might need cleaning or rebuilding.
* Throttle Body Issues (if fuel-injected): A dirty or malfunctioning throttle body can cause similar problems in fuel-injected models. Cleaning is often the solution.
Engine Mechanical Issues:
* Worn Engine Mounts: These mounts isolate the engine from the chassis. Worn mounts allow excessive engine movement, resulting in noticeable shaking.
* Valve Adjustment (if applicable): Incorrect valve adjustment can affect compression and lead to rough running. This is more likely if the engine is making other unusual noises.
* Low Compression: Worn piston rings, valve problems, or head gasket leaks can cause low compression in one or more cylinders, leading to misfires and shaking.
Other Possibilities:
* Alternator Problems: While not directly causing the idle issue, a failing alternator can cause voltage fluctuations, which can indirectly impact the ignition system and contribute to erratic engine behavior.
* Sensor Issues (if fuel-injected): Various sensors (e.g., Oxygen Sensor, Mass Airflow Sensor, Temperature Sensor) provide critical information to the engine's computer. A malfunctioning sensor can cause improper fuel mixture and timing, leading to rough running.
Troubleshooting Steps:
1. Visual Inspection: Start by visually inspecting the engine bay for obvious problems like loose wires, vacuum leaks, or damaged components.
2. Check Spark Plugs and Wires: Inspect them for wear and replace if necessary.
3. Check Distributor Cap and Rotor: Look for cracks, corrosion, or worn contacts. Replace if needed.
4. Listen for Unusual Noises: Unusual noises can pinpoint the source of the problem (e.g., ticking valves, knocking engine).
5. Check for Vacuum Leaks: Spray a carburetor cleaner around vacuum lines while the engine is running. A change in engine speed indicates a leak.
6. Check Engine Codes (if applicable): If the car has a self-diagnostic system, retrieve any trouble codes. This can significantly narrow down the possibilities.
Given the age of the vehicle, it's likely a combination of several of these issues rather than a single, isolated problem. A mechanic with experience in older vehicles would be best equipped to diagnose and repair this. Remember safety first – always disconnect the battery negative terminal before working on the electrical system.
