* Worn or damaged spark plug: Even if you replaced them, a spark plug could be faulty right out of the box or damaged during installation. A cracked insulator, incorrect gap, or electrode damage can cause a misfire. Double-check the plug and consider replacing it again with a known good one.
* Worn or damaged spark plug wire: The wire leading to cylinder #3 might be cracked, frayed, or corroded, preventing proper spark delivery. Inspect it carefully for damage. A simple visual inspection may not be enough; you may need to use a spark tester to verify its operation.
* Faulty ignition coil: The ignition coil for cylinder #3 could be failing. These coils often wear out over time and can cause intermittent or consistent misfires in the cylinder they serve. This is a common issue on these engines.
* Injector problem: A clogged or malfunctioning fuel injector for cylinder #3 will prevent the proper amount of fuel from reaching the combustion chamber. This can lead to a lean condition and a misfire. An injector cleaning or replacement may be necessary.
* Valve problem: A burnt, sticking, or otherwise damaged intake or exhaust valve in cylinder #3 will affect combustion. This is less likely immediately after a tune-up, but possible if there was pre-existing valve damage. A compression test can help diagnose this.
* Damaged piston or rings: Though less probable after a tune-up, a problem with the piston or rings in cylinder #3 could lead to a misfire. This usually involves low compression and often other symptoms beyond just a misfire. A compression test would be needed here.
* Vacuum leak: While less directly related to a single cylinder, a vacuum leak can affect the engine's overall performance and contribute to a misfire. Check all vacuum lines for cracks, leaks, or disconnections.
* Wiring issue: A break in the wiring harness affecting cylinder #3's components (coil, injector, sensor) is also a possibility. Carefully check the harness for damage.
Troubleshooting Steps:
1. Re-check the spark plug and wire for cylinder #3: This is the easiest and quickest check.
2. Check the ignition coil for cylinder #3: You can try swapping it with another coil (if possible) to see if the misfire follows the coil.
3. Perform a compression test: This helps determine the mechanical health of the cylinder.
4. Use a scan tool: A OBD-II scan tool can give you more specific trouble codes, potentially pointing you directly to the problem.
If you're not comfortable performing these checks yourself, take it to a qualified mechanic. They have the tools and expertise to diagnose the problem accurately and efficiently. Remember to specify that the misfire is isolated to cylinder #3, as this will help them focus their diagnosis.
