Fuel System:
* Fuel Pump: A weak fuel pump may struggle to maintain pressure at idle or under low engine load. When slowing down, engine demand for fuel decreases, and a weak pump might not be able to keep up, leading to fuel starvation and stalling.
* Fuel Filter: A clogged fuel filter restricts fuel flow, creating similar symptoms to a weak fuel pump. The restriction is more noticeable at low engine speeds.
* Fuel Pressure Regulator: This regulator controls fuel pressure in the system. A faulty regulator can cause erratic pressure, leading to stalling, especially at idle.
* Carburetor Issues (if equipped): If it's a carbureted model, problems like a clogged idle circuit, dirty jets, or a failing accelerator pump could cause stalling at low speeds.
* Fuel Injector Issues (if fuel injected): Clogged or malfunctioning fuel injectors could deliver insufficient fuel when slowing down. This is less likely in a '85 Fleetwood but still possible.
Electrical System:
* Ignition System: Problems with the ignition coil, distributor cap, rotor, or ignition wires can cause misfires or complete engine failure, particularly under low load conditions. A failing ignition module is also a possibility.
* Idle Air Control (IAC) Valve: This valve regulates airflow at idle. A faulty IAC valve can cause the engine to stall when slowing down, as it struggles to maintain the correct idle speed. Especially crucial in vehicles with electronic fuel injection.
* Throttle Position Sensor (TPS): This sensor tells the computer the throttle position. A faulty TPS can provide incorrect data, leading to erratic fuel delivery and stalling.
* Mass Airflow Sensor (MAF) or Air Flow Meter (AFM): These sensors measure the amount of air entering the engine. A faulty MAF/AFM can cause the computer to deliver incorrect fuel amounts, leading to stalling, particularly at low RPM. (The presence of a MAF or AFM depends on the specific fuel injection system.)
* Alternator: While unlikely to be the *direct* cause of stalling, a failing alternator might not provide sufficient power to the ignition system or fuel pump at low RPM, leading to stalling. Check your battery voltage at idle.
* Vacuum Leaks: Vacuum leaks can disrupt the proper functioning of various engine components, leading to unstable idle and stalling. Check all vacuum lines for cracks or disconnections.
Other Potential Causes:
* Transmission Issues: While less likely, transmission problems can sometimes cause the engine to stall when slowing down, particularly if there's a connection between the transmission and engine's control systems.
* Catalytic Converter: A severely restricted catalytic converter can cause back pressure, leading to stalling. This is less likely to be specific to slowing down, but it's a possibility.
Troubleshooting Steps:
1. Check for obvious vacuum leaks.
2. Inspect the fuel filter and consider replacing it. It's a relatively inexpensive and easy fix.
3. Test the fuel pressure. This requires a fuel pressure gauge.
4. Inspect and clean the ignition components (distributor cap, rotor, wires).
5. Check the condition of the battery and alternator.
6. Inspect and clean the IAC valve (if applicable).
7. Have a mechanic check for any codes from the onboard diagnostics system (if applicable). 1985 might or might not have a fully developed OBD system.
Given the age and complexity, a qualified mechanic familiar with older Cadillacs would be beneficial for accurate diagnosis and repair. The symptoms you've described point to multiple possibilities, and a systematic approach is necessary to pinpoint the exact cause.
