High-Probability Issues:
* Crankshaft Position Sensor (CKP): This sensor tells the computer the crankshaft's position, essential for proper fuel injection timing. A faulty CKP will prevent the engine from starting even with spark and fuel. This is a very common culprit in no-start situations.
* Camshaft Position Sensor (CMP): Similar to the CKP, a bad CMP sensor prevents the computer from knowing the camshaft's position, leading to improper valve timing and a no-start condition.
* Fuel Injectors: Even if you *think* you have fuel, a problem with the injectors (clogged, faulty, low fuel pressure) can prevent fuel from being sprayed into the cylinders in the correct manner. Low fuel pressure could indicate a problem with the fuel pump or a clogged fuel filter.
* Ignition Control Module (ICM): While you have spark, the ICM controls the timing and distribution of that spark. A faulty ICM can provide weak or improperly timed spark, enough to create a visible spark but insufficient for starting.
* Computer (PCM/ECM): The Powertrain Control Module (PCM) or Engine Control Module (ECM) is the "brain" of the engine. A faulty PCM can cause a myriad of issues, including a no-start condition. This is less likely but possible.
Less Likely, but Possible Issues:
* Timing Belt/Chain: If the timing is off by even one tooth, the engine won't start. Check for proper timing alignment (though a drastically off timing would usually show other symptoms, like bent valves).
* Security System: Some 1991 Luminas might have an aftermarket or factory anti-theft system that's preventing the engine from starting. Check for any related warning lights or clues.
* Low Battery Voltage: Although you have spark, a weak battery might not provide enough power to crank the engine effectively, even if it produces sufficient spark for a brief moment. Check battery voltage with a multimeter.
* Starter Motor: Though unlikely, if the starter isn't engaging properly it won't be able to turn the engine over fast enough, even if everything else is working correctly.
* Compression: Very low compression in one or more cylinders could prevent a start. This requires a compression test to diagnose.
Troubleshooting Steps:
1. Check Battery Voltage: Ensure the battery is fully charged.
2. Verify Spark: Make sure the spark is strong and consistent at all cylinders.
3. Fuel Pressure Test: Test the fuel pressure to rule out low fuel pressure.
4. Check CKP and CMP Sensors: These are the most common causes of this problem. Check for proper voltage and signal using a multimeter and a service manual.
5. Listen Closely While Cranking: If you hear no fuel pump prime when you turn the key, that is a strong indication of a fuel pump or relay problem.
6. Consult a Repair Manual: A repair manual specific to your 1991 Chevy Lumina is invaluable. It will provide detailed diagnostic procedures and wiring diagrams.
Remember to always disconnect the negative battery terminal before working on any electrical components. If you're not comfortable working on your vehicle's electrical system, it's best to take it to a qualified mechanic.
