Ignition System:
* Spark Plugs: Worn, fouled, or damaged spark plugs are a frequent cause. They prevent proper combustion, leading to misfires and jolting.
* Ignition Coils: These deliver the high voltage to the spark plugs. A failing coil can cause misfires in one or more cylinders.
* Ignition Wires (Spark Plug Wires): Damaged or cracked wires can prevent the spark from reaching the plugs reliably.
* Distributor Cap and Rotor (if applicable): Older Envoy models may have distributors. Cracks or worn components in these can disrupt the spark delivery.
Fuel System:
* Fuel Injectors: Clogged or failing injectors can deliver an inconsistent fuel spray, leading to rough running and jolting.
* Fuel Filter: A clogged fuel filter restricts fuel flow, causing similar problems to bad injectors.
* Fuel Pump: A weak fuel pump may not provide sufficient pressure, resulting in insufficient fuel delivery.
Engine Management System:
* Mass Airflow Sensor (MAF Sensor): This sensor measures the air entering the engine. A faulty MAF sensor can provide incorrect information to the computer, leading to poor fuel mixture and rough running.
* Throttle Position Sensor (TPS): This sensor tells the computer the throttle position. A malfunctioning TPS can cause erratic engine behavior.
* Oxygen Sensor (O2 Sensor): This sensor monitors the exhaust gases to help regulate the air/fuel mixture. A bad O2 sensor can lead to incorrect fuel mixtures and rough running.
* Computer (ECM/PCM): In rare cases, a faulty engine control module can cause a variety of problems, including jolting.
Other Potential Causes:
* Vacuum Leaks: Leaks in the intake system can disrupt the air/fuel mixture.
* Transmission Problems: While less likely to cause *engine* jolting, transmission issues can sometimes manifest as jolts felt throughout the vehicle. Consider this if the jolting is accompanied by other transmission symptoms.
* Engine Mounts: Worn engine mounts can allow excessive engine movement, leading to vibrations and jolts that might feel like they're coming from the engine itself.
Troubleshooting Steps:
1. Check Engine Light: If the Check Engine Light is on, have the codes read at an auto parts store or mechanic. This will give you a starting point for diagnosing the problem.
2. Listen for Misfires: Listen closely to the engine while it's running. A misfire often sounds like a sputtering or knocking.
3. Inspect Visible Components: Check spark plugs, wires, and other easily accessible parts for obvious damage.
Important Note: Diagnosing engine problems can be complex. If you're not comfortable working on your vehicle yourself, it's best to take it to a qualified mechanic for diagnosis and repair. Attempting repairs without proper knowledge can cause further damage.
