Fuel-Related Issues:
* Fuel Pressure Regulator: A faulty fuel pressure regulator might be allowing excessive fuel pressure to build up, causing the engine to run fast and continue firing even after the ignition is turned off.
* Clogged Fuel Injectors: Clogged injectors can cause a rich fuel mixture, contributing to the high idle and dieseling.
* Fuel Return Line: A blockage in the fuel return line can also cause fuel pressure to build up, leading to the same symptoms.
Ignition Issues:
* Ignition Timing: Incorrect ignition timing can lead to the engine running fast and continuing to fire after shutdown.
* Distributor Cap & Rotor: Worn or damaged distributor cap and rotor can cause misfires and spark leakage, potentially contributing to the dieseling effect.
* Spark Plugs: Worn or fouled spark plugs can cause misfires and a rich fuel mixture, which can lead to dieseling.
Vacuum Leaks:
* Intake Manifold Gasket: A leak in the intake manifold gasket can allow unmetered air into the engine, throwing off the air-fuel mixture and causing a fast idle.
* Vacuum Lines: Cracked or disconnected vacuum lines can also cause a vacuum leak and lead to a high idle and dieseling.
Additional Considerations:
* Choke Mechanism: On older vehicles, a sticking choke can cause a high idle.
* Fuel Vapor Recovery System: Problems with the fuel vapor recovery system can cause fuel to be drawn back into the engine, leading to a rich mixture and dieseling.
* Engine Temperature: A very hot engine can sometimes experience dieseling due to residual heat igniting the fuel.
How to Diagnose and Fix the Issue:
1. Check for Fuel Pressure: Test the fuel pressure with a gauge. If it's excessively high, suspect the fuel pressure regulator.
2. Inspect the Fuel Injectors: Visually inspect the fuel injectors for signs of clogging. Consider cleaning or replacing them if necessary.
3. Examine Vacuum Lines: Carefully inspect all vacuum lines for cracks or disconnections.
4. Check Intake Manifold Gasket: Look for leaks at the intake manifold gasket.
5. Inspect Ignition System: Inspect the distributor cap, rotor, spark plugs, and ignition timing for any issues.
Important Note: If you're not comfortable diagnosing and repairing these issues yourself, it's best to take your vehicle to a qualified mechanic for proper diagnosis and repair. They can use specialized tools and knowledge to pinpoint the exact cause of the problem.
