* Fuel pressure regulator: This regulator controls the fuel pressure in the fuel rail. A faulty regulator might allow fuel pressure to drop too low when the engine is off, preventing it from starting, but still allow some fuel to reach the cylinders when starting fluid is introduced (providing a temporary boost). Test the fuel pressure at the rail with a gauge. It should hold pressure when the engine is off.
* Clogged fuel injector(s): While less likely given the starting fluid works, a clogged injector(s) could restrict fuel flow enough to prevent starting. Cleaning or replacing the injectors is a possibility. A fuel injector cleaner additive might be a cheaper preliminary attempt, but it's not guaranteed to resolve the issue.
* Fuel pump relay (despite being checked): Even though you checked the relay, it's possible the relay is intermittently failing. Try swapping it with another relay of the same amperage rating (to rule out a bad relay) to see if it resolves the issue. A simple visual inspection might not be enough to catch an intermittent problem.
* Wiring issues: Check all wiring related to the fuel pump and injectors for any damage, corrosion, or loose connections. A broken wire somewhere in the circuit can prevent the pump from engaging or injectors from firing, yet still allow for enough fuel to reach the cylinders when starting fluid is directly injected into the intake.
* Crankshaft Position Sensor (CKP Sensor): While less likely given your description, a faulty CKP sensor can prevent the engine from starting, but starting fluid bypasses this issue. This sensor is essential for the engine's ignition timing.
* Computer (PCM) issue: The Powertrain Control Module (PCM) controls fuel delivery. Although less probable, a faulty PCM could be sending incorrect signals related to fuel delivery. This would require professional diagnostics.
Troubleshooting Steps:
1. Fuel pressure test: This is the most crucial step. Rent or borrow a fuel pressure gauge and test the pressure at the fuel rail.
2. Check all wiring: Carefully inspect all wiring related to the fuel system for any damage or corrosion.
3. Swap the fuel pump relay again: Try another relay, just to be certain.
4. Consider a fuel injector cleaner: This is a less expensive and less involved solution compared to replacing injectors, but it's not a guaranteed fix.
5. Consult a mechanic: If you're not comfortable performing these steps yourself, take your Grand Marquis to a qualified mechanic for diagnosis and repair.
Using starting fluid is a temporary solution and not a diagnostic tool. While it helps pinpoint the problem to fuel delivery, further investigation is required to find the exact cause. Always be extremely cautious when using starting fluid, as it is highly flammable.
