* Vacuum Leaks: This is a very common cause. Vacuum leaks disrupt the air/fuel mixture, especially when the throttle plate closes (you let off the gas). Check all vacuum lines for cracks, loose connections, or missing lines. Pay close attention to the lines connected to the intake manifold, brake booster, and other vacuum-operated components. A hissing sound near the intake is a good indicator.
* Throttle Position Sensor (TPS): The TPS tells the computer the throttle's position. A faulty TPS can cause erratic fuel delivery, especially during transitions like releasing the accelerator. It's a relatively inexpensive sensor to replace.
* Mass Airflow Sensor (MAF): While less likely to cause stumbling specifically on deceleration, a dirty or faulty MAF sensor can affect the air/fuel mixture, leading to rough running. Clean it with MAF sensor cleaner (following instructions carefully), or consider replacement if cleaning doesn't help.
* Idle Air Control (IAC) Valve: This valve regulates air flow at idle. A dirty or malfunctioning IAC valve can cause rough idling and stumbling when transitioning from acceleration to deceleration. Cleaning it (with carburetor cleaner) is often effective, but replacement might be necessary.
* Fuel Pressure Regulator: A failing fuel pressure regulator can cause excess fuel pressure, leading to a rich mixture and rough running, possibly more noticeable when decelerating.
* Distributor Cap and Rotor (if applicable): Cracks or corrosion in the distributor cap and rotor can cause intermittent misfires, which can manifest as stumbling. Inspect them carefully for any damage.
* Spark Plugs and Wires: Worn-out spark plugs or faulty spark plug wires can cause misfires that might be more noticeable under certain conditions, such as deceleration.
* Computer Problems (PCM): While less likely, a failing PCM can cause various problems, including those described. This is usually diagnosed last after other components have been checked.
Troubleshooting Steps:
1. Start with the easiest and cheapest: Check for vacuum leaks first. This is the most common cause and relatively easy to diagnose.
2. Inspect the spark plugs and wires: Look for obvious signs of wear or damage.
3. Clean the MAF and IAC valves: This is a relatively simple process and often resolves issues.
4. Check fuel pressure: This requires a fuel pressure gauge and some mechanical knowledge.
5. If the above don't resolve the issue: consider testing the TPS, fuel pressure regulator, and possibly even the distributor cap and rotor (if equipped).
If you're not comfortable working on your vehicle yourself, take it to a qualified mechanic. They'll have the tools and expertise to diagnose and repair the problem efficiently. Providing them with the information about the stumbling specifically during deceleration will help them narrow down the possibilities.
