* Failing Oxygen Sensor: A crucial part of the fuel injection system, a faulty sensor can lead to poor fuel mixture and trigger the light.
* Mass Air Flow Sensor (MAF) Issues (if equipped): While less common in '87 models, if it has a MAF sensor, a malfunction can cause incorrect air-fuel ratios.
* Problems with the Ignition System: This could include faulty spark plugs, wires, distributor cap, rotor, or even the ignition coil itself. Misfires are a common culprit.
* Vacuum Leaks: Vacuum lines are vital for many systems; a leak can disrupt engine performance and trigger the light.
* EGR Valve Issues: The Exhaust Gas Recirculation valve helps control emissions. A malfunctioning EGR valve can cause various problems.
* Fuel System Problems: This could be anything from a clogged fuel filter to issues with the fuel pump or injectors.
* Temperature Sensors: Incorrect readings from coolant temperature sensors can affect the fuel mixture.
* Transmission Issues (depending on whether it's an automatic): While less directly related to the engine, transmission problems sometimes trigger the light.
* Alternator Problems: A failing alternator can affect the electrical system, indirectly impacting engine performance.
How to Diagnose:
Since you don't have OBD-II, you'll need to use older diagnostic methods:
* Check the simplest things first: Inspect wires, connectors, and vacuum lines for obvious damage or leaks. Look at the distributor cap and rotor for cracks or wear. Check the spark plugs.
* Consult a Repair Manual: A factory service manual for your specific year and model is invaluable. It will contain diagnostic trouble codes (DTCs) and troubleshooting procedures.
* Use a Test Light or Multimeter: You can use these tools to test components for voltage, continuity, and ground connections.
* Listen for Unusual Sounds: Unusual noises from the engine (knocking, ticking, etc.) can offer clues.
Important Note: Without a specific code, figuring out the cause of the check engine light in a 1987 Grand Marquis requires methodical testing and a good understanding of the vehicle's systems. It's often best to seek the help of a qualified mechanic familiar with older vehicles if you're not comfortable troubleshooting yourself.
