* No Fuel Pressure: The pump might be humming, but not building sufficient pressure. A pressure gauge is needed to confirm this. Low pressure can be due to:
* Clogged Fuel Filter (despite replacement): It's possible the new filter is faulty or there's a blockage *before* the filter (e.g., in the fuel line).
* Weak Fuel Pump (despite humming): The pump might be struggling to overcome a restriction or might be failing internally despite making noise.
* Fuel Pressure Regulator: This component regulates fuel pressure; a faulty one can prevent sufficient pressure reaching the injectors.
* Clogged Fuel Injectors: The injectors might be clogged, preventing fuel from entering the engine.
* Ignition System Problems: Even with fuel, the car won't start without a functioning ignition system:
* Spark Plugs/Wires: Check for spark at the plugs. They might be fouled, worn out, or the wires could be damaged.
* Ignition Coil: This provides the high voltage to the spark plugs. A faulty coil will prevent sparking.
* Distributor (if applicable): Older cars like the 1990 Grand Prix might use a distributor. Issues with the distributor cap, rotor, or pickup coil can prevent proper spark delivery.
* Crankshaft Position Sensor (CKP): This sensor tells the engine's computer where the crankshaft is, which is critical for timing the spark. A faulty CKP will prevent starting.
* Other Potential Issues:
* Immobilizer System (if equipped): Some 1990 models might have an immobilizer. If the key isn't recognized, the car won't start.
* Battery/Alternator: Although less likely if the pump hums, a weak battery could still prevent starting. Check battery voltage.
* Computer/ECM Issues: A problem with the engine control module (ECM) could prevent the car from starting, even if all other components are working.
Troubleshooting Steps:
1. Check Fuel Pressure: This is the most important step. Rent or borrow a fuel pressure gauge to measure pressure at the fuel rail. Your car's repair manual will specify the correct pressure.
2. Check for Spark: Use an inline spark tester or remove a spark plug, ground it, and crank the engine to see if you get a spark.
3. Inspect Fuel Lines and Filter: Look for any visible damage, kinks, or blockages in the fuel lines leading to and from the filter.
4. Verify Battery Voltage: Make sure the battery is properly charged.
5. Consult a Repair Manual: A repair manual specific to your 1990 Grand Prix will provide detailed diagnostic procedures and specifications.
If you're not comfortable performing these checks yourself, take it to a qualified mechanic for diagnosis. Trying to troubleshoot fuel system problems without the right tools and knowledge can be dangerous due to the flammability of gasoline.
