Mechanical Issues:
* Oxygen Sensor(s): A malfunctioning oxygen sensor provides inaccurate information to the engine's computer about the air/fuel mixture. This often leads to a richer (more fuel-heavy) mixture, resulting in significantly poorer gas mileage. This is a common culprit in older cars.
* Mass Airflow Sensor (MAF): The MAF sensor measures the amount of air entering the engine. A faulty MAF sensor can cause the engine to run rich or lean, affecting fuel efficiency.
* Spark Plugs & Wires: Worn spark plugs or damaged ignition wires can cause misfires, leading to incomplete combustion and wasted fuel.
* Fuel Injectors: Clogged or faulty fuel injectors can spray an incorrect amount of fuel into the cylinders, leading to poor fuel economy.
* Air Filter: A clogged air filter restricts airflow, forcing the engine to work harder and consume more fuel. This is a cheap and easy thing to check first.
* PCV Valve (Positive Crankcase Ventilation): A faulty PCV valve can cause excessive crankcase pressure, leading to oil leaks and affecting fuel efficiency.
* Vacuum Leaks: Leaks in the vacuum system can disrupt the engine's air/fuel mixture, leading to poor fuel economy.
* Transmission Problems: Issues with the automatic transmission (if equipped) can cause it to slip or operate inefficiently, resulting in higher fuel consumption. This is often accompanied by other symptoms like slipping gears or rough shifting.
* Tire Pressure: Under-inflated tires increase rolling resistance, leading to reduced fuel economy. Check your tire pressure regularly.
Other Potential Causes:
* Driving Habits: Aggressive driving (rapid acceleration, hard braking) significantly impacts fuel economy. Have your driving habits changed recently?
* Increased Weight: Carrying extra weight in the car will reduce fuel efficiency.
* Fuel Quality: Using lower-quality fuel can sometimes lead to slightly decreased fuel economy.
Troubleshooting Steps:
1. Check the obvious: Start with the easiest and cheapest checks: tire pressure, air filter.
2. Check for error codes: Use an OBD-II scanner (available at most auto parts stores) to read any diagnostic trouble codes (DTCs) stored by the car's computer. These codes can pinpoint potential problems.
3. Consult a Mechanic: If you're not comfortable diagnosing the problem yourself, take your Cavalier to a trusted mechanic for a thorough inspection. They have the tools and expertise to diagnose the issue accurately.
Without more information about the specific symptoms (e.g., engine performance issues, unusual noises, warning lights), it's difficult to pinpoint the exact cause. However, the list above covers the most common reasons for a sudden drop in fuel economy.
