1. Security System Interference:
* Check the Passlock System: The 1999 DeVille likely uses a Passlock system, a theft deterrent that prevents starting if the car doesn't recognize the original key's transponder chip. Even if you *have* the original key, the system might be malfunctioning.
* Try multiple key turns: Try turning the key to the "on" position (without cranking) several times in quick succession. This sometimes helps reset the system.
* Battery Voltage: A low battery can interfere with the Passlock system's ability to read the key. Jump start the car or use a battery charger to bring the voltage up.
* Key Programming: If none of the above works, the key's transponder might be faulty or the Passlock system itself may need reprogramming. This requires a professional scan tool capable of communicating with the Cadillac's PCM (Powertrain Control Module) and Passlock system. A locksmith or qualified mechanic specializing in GM vehicles would be needed.
2. Fuel Delivery Problems (Despite the Security System):
* Fuel Pump: Even if the Passlock is working, the fuel pump could be failing. This is a common issue in older vehicles. You should be able to hear the fuel pump prime when you turn the key to the "on" position (before cranking). Listen carefully near the fuel tank. If you don't hear it, the pump might be faulty, the relay is bad, or there's a wiring problem.
* Fuel Pump Relay: Locate the fuel pump relay (consult your owner's manual or online resources for its location) and try swapping it with a known good relay of the same amperage (a different relay of similar size might work as a test).
* Fuel Filter: A clogged fuel filter can restrict fuel flow to the engine. Replace the filter – it’s a relatively inexpensive part and a routine maintenance item.
* Fuel Injectors: While less likely if you have spark, a problem with the fuel injectors (clogged, faulty, low fuel pressure) would prevent fuel injection. Diagnosing this requires specialized tools and knowledge.
* Fuel Pressure Regulator: This regulates fuel pressure to the injectors. A faulty regulator could lead to low or no fuel pressure.
* Wiring and Connectors: Inspect the wiring and connectors related to the fuel system for any damage, corrosion, or loose connections. Pay close attention to the wiring leading to the fuel pump and injectors.
3. Diagnostic Tools:
* OBD-II Scanner: A scan tool can read diagnostic trouble codes (DTCs) stored in the PCM. These codes provide valuable clues about what's wrong. A basic OBD-II scanner might not pick up all the Passlock-related issues. A more advanced scan tool capable of communicating with the Cadillac's specific systems is preferable.
Important Notes:
* Safety: Work on the vehicle in a well-ventilated area. Gasoline is flammable.
* Professional Help: If you're not comfortable working on cars, it's best to take the vehicle to a qualified mechanic. Trying to diagnose and fix complex electrical and fuel system issues without the right knowledge and tools can be dangerous and potentially lead to more damage.
* Year-Specific Information: Always consult a repair manual or online resources specific to a 1999 Cadillac DeVille for detailed diagrams, component locations, and troubleshooting procedures.
The combination of no fuel injection and a functioning spark points toward a problem with fuel delivery or a security system issue blocking fuel delivery. Systematic troubleshooting, starting with the simpler checks, will help you pinpoint the cause.
