Hissing Sound:
* Vacuum Leak: The most likely culprit is a vacuum leak. The hissing sound is often a telltale sign of air escaping from the intake system. This can be caused by:
* Loose or cracked vacuum hoses: Inspect all vacuum hoses for cracks, splits, or loose connections. Pay special attention to those near the intake manifold and PCV valve.
* Damaged intake manifold gasket: A damaged gasket can allow air to leak into the intake manifold, disrupting air/fuel mixture.
* Faulty PCV valve: The PCV valve helps regulate engine vacuum and prevent crankcase pressure buildup. If it's stuck open or malfunctioning, it can cause a vacuum leak.
* Boost Leak (If Turbocharged): If your Century has a turbocharger, a boost leak can cause a similar hissing sound and rough idle.
Rough Idle and Stalling:
* Vacuum Leak: As mentioned above, a vacuum leak disrupts the air/fuel mixture, causing the engine to run poorly and potentially stall.
* Spark Plugs and Wires: Worn spark plugs or faulty ignition wires can cause misfires, leading to rough idle and stalling.
* Fuel Injectors: Clogged or faulty injectors can disrupt fuel delivery, resulting in a rough idle and potential stalling.
* Air Filter: A clogged air filter restricts airflow, impacting the air/fuel mixture and causing rough idle.
* Throttle Body: A dirty or malfunctioning throttle body can affect the engine's idle speed.
* Mass Airflow Sensor (MAF): This sensor measures air flow into the engine. A faulty MAF can give inaccurate readings, leading to poor performance.
* Oxygen Sensor (O2 Sensor): The O2 sensor monitors exhaust gases and adjusts the fuel mixture. A faulty O2 sensor can cause incorrect fuel delivery, resulting in a rough idle.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Visually inspect all vacuum hoses for cracks, splits, or loose connections. Use a spray bottle with soapy water and spray around hoses while the engine is running. If bubbles appear, you've found a leak.
2. Inspect Spark Plugs and Wires: Check for worn spark plugs or damaged wires. Replace as necessary.
3. Check Fuel Injectors: If you suspect a fuel injector problem, you may need to have them tested or cleaned.
4. Clean Air Filter: Ensure the air filter is clean and free of debris.
5. Clean Throttle Body: Clean the throttle body to ensure smooth operation.
6. Check MAF and O2 Sensors: If you suspect these sensors are faulty, you may need to have them tested or replaced.
Important Note:
* Be cautious when working around the engine. Always disconnect the battery before working on electrical components.
* If you're not comfortable performing these checks, take your car to a qualified mechanic for diagnosis and repair.
