Here's a systematic approach to troubleshooting:
1. Get the Diagnostic Trouble Codes (DTCs):
* OBD-II Scanner: This is the most crucial step. Buy or borrow an OBD-II code reader (available cheaply at auto parts stores). Plug it into the OBD-II port (usually under the dashboard) and retrieve the trouble codes. These codes provide clues about the specific problem areas. Write them down!
2. Interpret the DTCs:
* Online Resources: Use the DTCs you obtained to search online. Many websites (including forums dedicated to Ford vehicles) provide explanations of these codes and potential solutions. Be wary of generic advice; specifics for your Grand Marquis are needed. The year, make, and model are key.
3. Common Causes (based on typical Grand Marquis issues):
The bucking and engine light could stem from several issues:
* Spark Plugs and Wires: Worn-out spark plugs or faulty ignition wires can cause misfires, leading to bucking. These are relatively inexpensive and easy to replace.
* Mass Airflow Sensor (MAF): A dirty or faulty MAF sensor provides inaccurate air readings to the engine computer, causing erratic fuel delivery and rough running. Cleaning the sensor (carefully!) is a good first step; replacement might be necessary.
* Throttle Position Sensor (TPS): A malfunctioning TPS sends incorrect throttle position data to the engine computer, resulting in poor engine performance and bucking.
* Oxygen Sensor(s): Faulty oxygen sensors disrupt the fuel-air mixture, leading to poor engine performance and emission issues.
* Catalytic Converter: A clogged or failing catalytic converter can restrict exhaust flow, causing engine performance problems. This is more expensive to replace.
* Fuel Injectors: Clogged or failing fuel injectors can deliver inconsistent fuel to the cylinders, leading to bucking and rough running.
* Ignition Coil(s): A faulty ignition coil can cause misfires in one or more cylinders.
* PCV Valve: A clogged Positive Crankcase Ventilation (PCV) valve can cause excessive crankcase pressure, affecting engine performance.
4. Troubleshooting Steps (after getting DTCs):
* Start with the most likely and inexpensive solutions first (spark plugs, wires, MAF sensor cleaning). You'll often find the problem in this category.
* Check for vacuum leaks. A visual inspection of vacuum hoses is recommended; leaks can cause erratic engine behavior.
* Inspect your fuel filter. A clogged fuel filter can restrict fuel flow.
5. When to Seek Professional Help:
If:
* You're uncomfortable working on your car.
* You can't identify the problem after obtaining and researching the DTCs.
* The problem persists after trying simple fixes.
Take your Grand Marquis to a qualified mechanic. They have the diagnostic tools and expertise to pinpoint the issue accurately and perform the necessary repairs. Providing them with the DTCs will significantly speed up the process.
Important Note: Safety first! Always disconnect the negative battery terminal before working on any electrical components in your vehicle. If you're not comfortable working on cars, don't hesitate to seek professional help. Improper repairs can lead to further damage and potentially dangerous situations.
