* Ignition System: Even after a tune-up, components like:
* Spark Plugs: Incorrect gap, damaged insulator, or a plug not seated properly. Did they use the correct plugs for your engine?
* Worn Plug Wires: Internal cracking or damage causing poor conductivity. Sometimes the problem isn't *all* wires, just one.
* Ignition Coil (or coil pack): A failing coil pack can cause misfires in specific cylinders. A coil pack test is crucial.
* Ignition Control Module (ICM): Less common, but a faulty ICM can prevent a specific cylinder from firing correctly.
* Fuel System:
* Injector: A clogged or failing injector in cylinder 6 won't deliver enough fuel for proper combustion. An injector flow test is necessary.
* Fuel Pressure: Low fuel pressure can starve the engine of fuel, causing misfires. A fuel pressure gauge check is needed.
* Fuel Filter: A clogged fuel filter restricts fuel flow. Replacing it is a cheap and easy check.
* Engine Mechanical Issues:
* Valve Problem: A bent or burned valve in cylinder 6 will prevent proper combustion. This would require a compression test to confirm.
* Low Compression: A leak in the cylinder (worn rings, head gasket, etc.) significantly reduces combustion pressure, causing a misfire. A compression test is essential.
* Vacuum Leak: A large vacuum leak can affect the air/fuel mixture, leading to misfires. This is harder to pinpoint but a visual inspection of all hoses and connections is a good start.
* Sensor Issues:
* Crankshaft Position Sensor (CKP): A faulty CKP sensor can cause erratic firing.
* Mass Airflow Sensor (MAF): A malfunctioning MAF sensor can provide the engine computer with incorrect air readings, affecting fuel delivery.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine plug wires for cracks, damage, and proper connection. Inspect the spark plugs for fouling, damage, or incorrect gap.
2. Compression Test: This is crucial to rule out mechanical problems in cylinder 6. Low compression indicates a significant internal engine issue.
3. Fuel Pressure Test: Measure fuel pressure to ensure the system is delivering sufficient fuel.
4. Injector Test: Either a professional flow test or a simple visual inspection to ensure they are spraying properly.
5. Ignition Coil Test: Test the coil pack or individual coil for cylinder 6. This can often be done with a multimeter.
6. Scan for Codes: Use an OBD-II scanner to read any trouble codes stored in the engine's computer. This can provide valuable clues.
It's important to systematically check these areas. Since the problem persists after a tune-up, focusing on the ignition system, fuel system, and potential mechanical issues is crucial. Ignoring the mechanical possibilities could lead to significant engine damage. Taking your truck to a qualified mechanic is strongly recommended if you're not comfortable performing these tests yourself.
