1. Vacuum Leaks: Idle is when the engine relies heavily on vacuum for proper operation. A leak anywhere in the intake system can significantly disrupt the air/fuel mixture, leading to misfires, particularly at idle.
* How to Check: Visually inspect all vacuum lines and connections for cracks, splits, or loose fittings. Pay close attention to the lines connected to the intake manifold, brake booster, and PCV valve. Use a propane torch (with the engine running) and carefully spray the flame around suspected leak areas. A sudden change in engine RPM indicates a vacuum leak at that point. Caution: Be extremely careful using this method; keep the flame small and away from hot engine parts. A better method is to use a vacuum gauge; connect it to a vacuum port and observe the readings. A low or fluctuating vacuum reading signifies a leak.
2. Ignition System (Beyond Plugs and Wires): Even with new plugs and wires, other ignition components could be failing.
* How to Check:
* Distributor Cap and Rotor (if applicable): Inspect for cracks, carbon tracking, or worn contact points. Replace if necessary.
* Ignition Coil: While less likely to cause a misfire *only* at idle, a weak coil can struggle under the lower RPMs. A visual inspection for cracks or damage is a good start. You might need a multimeter to check the coil's resistance (consult a repair manual for specifications). A bad coil often causes multiple cylinder misfires, not just one.
* Ignition Control Module (ICM): The ICM controls the ignition system. Failure can result in intermittent misfires. Diagnosing ICM issues usually requires a scan tool or specialized testing equipment.
3. Injector on Cylinder 6: A clogged or malfunctioning injector on cylinder 6 would cause a misfire.
* How to Check: This is trickier without specialized tools. You could try swapping injectors with another cylinder (make sure you note which injector goes where!), but this is only a temporary diagnostic. If the misfire moves to the other cylinder, you've found the problem. A better approach is to use a fuel pressure gauge and a noid light. The noid light checks if the injector is getting a signal. Low fuel pressure also can cause misfires.
4. Sensor Issues: Several sensors influence idle operation. A faulty sensor can send incorrect information to the engine computer (ECM), resulting in a misfire.
* How to Check:
* Mass Airflow Sensor (MAF): A dirty or faulty MAF sensor can disrupt the air/fuel mixture. Clean it with MAF sensor cleaner (following the instructions carefully) or replace it if cleaning doesn't help.
* Throttle Position Sensor (TPS): This sensor tells the ECM the throttle plate's position. A malfunctioning TPS can cause idle problems. A scan tool can check for TPS voltage signals that are out of specification.
* Crankshaft Position Sensor (CKP): A bad CKP can cause misfires, especially at lower RPMs. Testing usually requires a scan tool or oscilloscope.
* Oxygen Sensor(s): While less likely to cause a single-cylinder misfire at only idle, a faulty oxygen sensor can impact the fuel mixture over time.
5. Compression Test: Low compression in cylinder 6 indicates a mechanical problem, like a worn piston ring, valve issue, or head gasket leak.
* How to Check: This requires a compression tester and familiarity with the procedure. Low compression in only one cylinder strongly suggests a mechanical issue.
Troubleshooting Steps:
1. Start with the easiest and cheapest checks: Vacuum leaks and visual inspection of the ignition system.
2. Use a scan tool (OBD-II reader): This will give you trouble codes which can greatly narrow down the problem.
3. If you lack experience, consult a mechanic: Misfire diagnosis can be complex; a mechanic has the tools and expertise to diagnose the issue efficiently.
Remember to always disconnect the negative battery terminal before working on any electrical components. Consult a repair manual for your specific year and model of Chevy Tahoe for detailed diagnostic procedures and specifications.
