Most Likely Culprits:
* Crankshaft Position Sensor (CKP): This sensor tells the computer the engine's rotational position. A failing CKP sensor can prevent the engine from firing. They're prone to failure with age and heat. This is a very common cause of no-start issues in this era of GM vehicles.
* Cam Position Sensor (CMP): Similar to the CKP, a faulty CMP sensor will prevent the engine from starting. It signals the camshaft's position.
* Ignition System:
* Ignition Coil(s): Weak coils may not consistently provide enough spark to ignite the fuel-air mixture. These can be intermittent.
* Ignition Control Module (ICM): This module controls the ignition coils. A failing ICM can cause erratic or complete loss of spark. (Less likely given it sometimes starts, but still possible).
* Spark Plugs and Wires: Worn or damaged spark plugs or wires can prevent a consistent spark. This is a relatively easy and inexpensive check.
* Mass Airflow Sensor (MAF): A faulty MAF sensor will give the engine computer inaccurate readings of the incoming air, leading to a lean or rich fuel mixture. While a bad MAF might cause a rough running engine, it can also lead to no-start conditions, especially if the mixture is too lean.
* Fuel System:
* Fuel Pump: Though less likely given it sometimes starts, a weak fuel pump might struggle to provide enough pressure when the engine is hot or after sitting for a while.
* Fuel Pressure Regulator: A faulty fuel pressure regulator could lead to inconsistent fuel pressure.
* Clogged Fuel Filter: A clogged filter restricts fuel flow.
* Security System (Passive Anti-Theft System): If your Bonneville has a factory anti-theft system, a problem with it could prevent the engine from starting. This is less common but needs ruling out.
Troubleshooting Steps:
1. Check for Spark: This is the first thing to do. Remove a spark plug, ground it to the engine, and crank the engine. Look for a strong, consistent spark. If no spark, focus on the ignition system components (coil, ICM, wires, plugs).
2. Check for Fuel Pressure: You'll need a fuel pressure gauge to measure the pressure at the fuel rail. This requires some mechanical aptitude or a visit to a mechanic.
3. Scan for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to retrieve any diagnostic trouble codes stored in the car's computer. These codes can pinpoint the problem area.
4. Inspect Sensors: Visually inspect the CKP, CMP, and MAF sensors for any damage or loose connections.
5. Test Components (if needed): If you've identified a potential culprit (like a sensor), you might need a multimeter to test its resistance or voltage output to confirm it's faulty.
6. Consider the Battery and Alternator: While less likely to cause this intermittent issue, a weak battery or alternator can sometimes contribute to starting problems. Test their voltage.
Important Note: Given the age of the vehicle, preventative maintenance is key. Replacing worn spark plugs and wires, checking the fuel filter, and inspecting other wear items are always a good idea.
If you're not comfortable performing these checks yourself, take the car to a qualified mechanic. They have the tools and expertise to diagnose the problem accurately. Mention the intermittent nature of the issue – it will help them focus their investigation.
