* Vacuum Leaks: Many systems in a car rely on vacuum pressure generated by the engine. A significant leak can cause:
* Rough Idle: The engine's air/fuel mixture becomes disrupted, leading to an uneven idle.
* Hard Brake Pedal: The power brake booster uses engine vacuum to assist braking. A leak reduces or eliminates this assistance, making the pedal much harder to depress. This is the most likely culprit given both symptoms. Check all vacuum lines connected to the intake manifold, brake booster, and other vacuum-operated components for cracks, splits, or loose connections.
* Failing Power Brake Booster: This is a component that directly assists braking. A failing booster will typically result in a hard brake pedal, but might also indirectly contribute to a rough idle if it causes a substantial vacuum leak. A failing booster is usually diagnosed by checking vacuum at the booster itself and observing its functionality.
* Other less likely possibilities (but worth investigating if the above are ruled out):
* Throttle Body Issues: A dirty or malfunctioning throttle body can cause rough idling, but is less likely to directly cause a hard brake pedal.
* Mass Airflow Sensor (MAF) Problem: A faulty MAF sensor can affect the air/fuel mixture, leading to rough idling. Again, this is less directly connected to braking.
* Engine Misfire: A misfire could cause a rough idle, but would usually be accompanied by other symptoms like reduced power and potentially a check engine light.
Troubleshooting Steps:
1. Check Vacuum Lines: Start by visually inspecting all vacuum lines for cracks, loose connections, or damage. Pay close attention to lines connected to the brake booster and the intake manifold.
2. Test Brake Booster Vacuum: Use a vacuum gauge to measure the vacuum at the brake booster. A low reading indicates a problem, either with the booster itself or a leak in the system.
3. Check for Engine Codes: Scan the car's onboard diagnostic system (OBD-II) for trouble codes. This can provide clues to other potential problems.
4. Inspect the Throttle Body: Clean the throttle body if it's dirty.
5. Professional Diagnosis: If you can't pinpoint the problem, it's best to take your Lincoln LS to a qualified mechanic for proper diagnosis and repair. These are complex systems and incorrect diagnoses can be costly.
In short, focus your initial attention on the vacuum system, particularly the lines connected to the brake booster. A leak in this system is the most probable cause of both the rough idle and hard brake pedal. Remember safety first: if your brakes feel significantly impaired, avoid driving the vehicle until it's repaired.
