* Increased Engine Load: When you're stopped in gear with your brake applied, the engine is working harder than it would in neutral. The transmission is resisting the engine's torque, putting a significant load on the engine. This is especially noticeable in older vehicles with less sophisticated engine management systems. A weak or worn engine might struggle under this increased load, leading to a rough idle.
* Vacuum Leaks: Vacuum leaks can disrupt the engine's idle speed control. Many systems rely on vacuum, including the brake booster, and a leak can affect the mixture and idle speed, making it rough, especially under load.
* Brake Booster Issues: The brake booster itself uses vacuum from the engine. A failing booster can create a significant vacuum leak or place an unexpected load on the engine, especially when braking. If the booster is failing, you might notice a harder brake pedal as well.
* Throttle Position Sensor (TPS): A faulty TPS can send incorrect information to the engine control unit (ECU), leading to poor idle control.
* Idle Air Control (IAC) Valve: The IAC valve regulates air flow at idle. A dirty, sticky, or failing IAC valve can cause rough idling, particularly noticeable under load.
* Dirty or Faulty Fuel Injectors: Clogged or malfunctioning fuel injectors can lead to inconsistent fuel delivery, resulting in a rough idle, especially under increased load.
* Ignition System Problems: Worn spark plugs, faulty ignition wires, or a failing distributor (if equipped) can all contribute to a misfire, leading to a rough idle. This would likely be more pronounced than just under braking.
* Low Engine Compression: Low compression in one or more cylinders means the engine isn't burning fuel efficiently, causing a rough idle. This issue is often independent of the braking.
Troubleshooting Steps:
1. Check for vacuum leaks: Inspect all vacuum lines for cracks, holes, or loose connections. Pay particular attention to lines connected to the brake booster. Use a vacuum gauge if possible.
2. Inspect the brake booster: Look for any signs of leaks or damage. If you suspect a problem, have it checked by a mechanic.
3. Check engine codes (if equipped): If your Dodge Colt has an OBD-I system (less likely, given the age), try to retrieve any diagnostic trouble codes (DTCs) that might point to the problem.
4. Clean the IAC valve: Remove and thoroughly clean the IAC valve with appropriate cleaner.
5. Check the spark plugs and wires: Inspect them for wear, fouling, or damage. Replace if necessary.
6. Check the throttle position sensor (TPS): This might require specialized tools and testing to ensure it's within specifications.
7. Consider fuel system issues: Fuel filter replacement, fuel pressure testing, and injector cleaning or replacement may be necessary if other causes are ruled out.
It's important to note that diagnosing this issue may require some mechanical knowledge or the assistance of a qualified mechanic. Start with the simpler checks (vacuum leaks, IAC valve) before moving on to more complex diagnostics. The fact that it's worse with the brake depressed strongly suggests a vacuum leak related to the braking system or a weak engine struggling under increased load.
