1. Fuel System:
* Fuel Pressure: This is the most likely culprit after you've ruled out the pump relay and inertia switch. You'll need a fuel pressure gauge to check the pressure at the fuel rail. This requires a specialized fitting that screws onto the fuel rail (usually a Schrader valve, like on a tire). The pressure should be within the specifications listed in your owner's manual (typically around 30-40 PSI, but this can vary). Low or no pressure indicates a problem with the fuel pump, fuel filter, or a clogged fuel line.
* Fuel Injectors: While less common, a faulty injector or multiple injectors could prevent starting. This requires a bit more specialized equipment to diagnose, typically a noid light or a scan tool that can monitor injector pulse.
* Fuel Pump itself (even after relay check): The relay might be fine, but the pump itself could be failing to deliver sufficient pressure, even if it's making a noise when you turn the key. A pressure gauge test will confirm this.
2. Ignition System:
* Spark Plugs and Wires: Inspect the spark plugs for wear, fouling (soot or oil), or damage. Also, check the spark plug wires for cracks, fraying, or loose connections. A simple way to check for spark is to remove a plug, ground it (against the engine block), and crank the engine. You should see a strong blue spark. If not, the problem could lie in the:
* Distributor (if equipped): If your Grand Marquis has a distributor, check the rotor and cap for cracks, corrosion, or worn contacts. A faulty distributor could prevent spark delivery to the cylinders.
* Ignition Coil: This provides the high voltage for the spark plugs. A bad coil will prevent sparking. A visual inspection for cracks or damage is a start; testing with a multimeter would be more definitive.
* Crankshaft Position Sensor (CKP): This sensor tells the computer where the engine is in its rotation. A faulty CKP will prevent the engine from getting spark. This requires a specific scan tool to check for correct sensor signals or replacement to test.
* Cam Position Sensor (CMP): Similar to the CKP sensor, but for the camshaft. A bad CMP can also prevent the engine from starting. Testing requires a scan tool or replacement of the sensor.
3. Other Possibilities:
* Mass Airflow Sensor (MAF): While unlikely to prevent *cranking*, a severely faulty MAF sensor could prevent the engine from starting due to incorrect air/fuel mixture.
* Engine Control Module (ECM): A failing ECM is less likely, but possible. This is a more difficult issue to diagnose and requires professional-level tools.
Troubleshooting Steps:
1. Fuel Pressure Test: This is the HIGHEST priority given your previous checks. Get a fuel pressure gauge and check the fuel pressure.
2. Spark Test: Check for spark at the plugs as described above.
3. Visual Inspection: Carefully inspect all ignition components for any obvious signs of damage or wear.
4. Check Engine Light (CEL): Is the Check Engine Light on? If so, get the diagnostic trouble codes (DTCs) read using an OBD-II scanner. This might provide clues about the problem.
Important Notes:
* Safety First: Always disconnect the negative battery terminal before working on any electrical components.
* Professional Help: If you're not comfortable working on your car's electrical system, it's best to take it to a qualified mechanic. Incorrect diagnosis and repair attempts can lead to further damage.
By systematically checking these areas, you should be able to narrow down the cause of your no-start problem. Remember to check fuel pressure first – that's the most likely culprit given your prior troubleshooting.
