* Vehicle Speed Sensor (VSS): The VSS is crucial. It sends signals to the speedometer, the transmission (for shifting), and potentially the PCM (Powertrain Control Module) for engine management. A failing VSS can cause the speedometer to drop to zero, and because the PCM relies on this information for things like fuel injection timing and shifting points, a faulty VSS can also lead to rough running. The intermittent nature of the problem points to a connection issue (loose wire, corroded connector) or a sensor on the verge of failure.
* Ignition System: Intermittent starting and rough running are classic signs of problems in the ignition system. This could include:
* Distributor Cap and Rotor: Wear and tear can cause arcing and misfires, leading to rough running.
* Ignition Coil: A failing coil can provide inconsistent spark.
* Spark Plug Wires: Cracks or worn insulation can lead to misfires.
* Spark Plugs: Fouled or worn plugs are another common cause.
* Wiring Harness Issues: Age and wear can cause chafing or corrosion in the wiring harness, particularly around the engine bay. This can disrupt signals to various components, including the VSS, ignition system, and PCM.
* PCM (Powertrain Control Module): While less likely, a failing PCM could cause a variety of erratic symptoms. However, this is usually diagnosed after other more common problems are ruled out.
Troubleshooting Steps:
1. Check the VSS: Locate the VSS (usually on the transmission). Inspect the connector for corrosion or loose wires. Try wiggling the connector gently while the engine is running to see if it affects the speedometer or engine performance. If you have a multimeter, you could test the sensor's output signal (though this requires some technical knowledge).
2. Inspect the Ignition System: Check the distributor cap and rotor for cracks, wear, or corrosion. Inspect the spark plug wires for cracks and damage. Consider replacing the spark plugs and wires as preventative maintenance.
3. Check the Battery and Connections: A weak battery or corroded battery terminals can cause intermittent electrical problems. Clean the terminals thoroughly.
4. Check for Wiring Issues: Carefully examine the wiring harness in the engine bay for any signs of damage, chafing, or corrosion.
5. Professional Diagnosis: If you're not comfortable troubleshooting electrical systems yourself, take your Lumina to a trusted mechanic. They can use diagnostic tools to pinpoint the exact problem, saving you time and potential frustration.
Remember safety first! Always disconnect the negative battery terminal before working on the electrical system. If you're unsure about any repair, it's best to consult a professional.
