1. Basic Checks (Do these FIRST):
* Visual Inspection: Look for obvious issues like loose spark plug wires, damaged or cracked spark plug wires, broken or loose vacuum lines, or anything obviously amiss around cylinder 3.
* Check the Spark Plug: Remove the spark plug from cylinder 3. Inspect it for:
* Excessive wear: Worn-out plugs misfire.
* Cracking or damage: A damaged insulator can cause misfires.
* Fouling: Oil fouling (black, wet) suggests a valve seal or piston ring problem. Fuel fouling (wet, black/brown) suggests a rich fuel mixture.
* Gap: Make sure the gap is correct (check your owner's manual or a reliable online resource for the specification). A gapped plug is crucial.
* Check the Spark Plug Wire: Inspect the wire for cracks, breaks, or corrosion. A simple visual inspection isn't always enough; you may need to use a wire tester to verify continuity.
* Check the Ignition Coil (Coil-on-Plug System): The Astro likely uses a coil-on-plug (COP) system, meaning each cylinder has its own ignition coil. If you have a spare or can borrow one, swap the coil for cylinder 3 with a coil from another cylinder. If the misfire moves to the other cylinder, the coil is likely the culprit. Otherwise, the problem is elsewhere.
2. More Advanced Diagnostics (If basic checks don't solve the problem):
* Compression Test: A low compression reading in cylinder 3 indicates a problem with the cylinder's mechanical integrity (worn piston rings, blown head gasket, damaged valves). This requires a compression tester and is a more involved procedure.
* Fuel Injector Test: A clogged or malfunctioning fuel injector can cause a misfire. You can test the injector's resistance (with a multimeter) or use a fuel pressure gauge to check fuel pressure. This requires more advanced knowledge and tools.
* Vacuum Leak Test: A significant vacuum leak can disrupt the air-fuel mixture, leading to a misfire. A vacuum leak detector can pinpoint leaks in the intake manifold or vacuum lines.
* Scan Tool with Live Data: A more advanced scan tool that can show live data, such as misfire counts, fuel trim, and oxygen sensor readings, can provide valuable insights into the root cause. This is highly recommended if basic checks fail.
* Check the crankshaft position sensor (CKP) and camshaft position sensor (CMP): These sensors are crucial for ignition timing; a faulty sensor can cause misfires.
3. Possible Solutions based on findings:
* Replace Spark Plug and Wire: The most common and easiest fix.
* Replace Ignition Coil: If swapping coils reveals a faulty one.
* Repair Vacuum Leak: If a vacuum leak is identified.
* Replace Fuel Injector: If the injector is faulty.
* Engine Overhaul (Worst Case): Low compression usually indicates significant internal engine damage.
Important Considerations:
* Safety First: Always disconnect the negative battery terminal before working on the ignition system.
* Professional Help: If you're not comfortable performing these diagnostics or repairs, it's best to take your Astro to a qualified mechanic. Misfires can lead to further engine damage if left unresolved.
* Year and Engine: The specific procedures might vary slightly depending on the exact engine in your 1999 Astro (4.3L V6 is common). Consult a repair manual specific to your vehicle for detailed instructions.
Remember to systematically go through the checks above. Starting with the simplest and cheapest solutions (spark plug and wire) before moving to more complex and costly repairs is the most efficient approach. A repair manual for your specific vehicle model and year will be invaluable.
