Owner's Problem Description
The owner reports that their 1992 Cadillac Deville's brakes are completely unresponsive, causing the vehicle to accelerate unexpectedly when shifting from park to drive. The brake pedal feels extremely hard, akin to a brick, giving no feedback or stopping power. The owner replaced the master cylinder and brake booster about one to two months ago but currently cannot install the brake drum on the right rear side. This issue raises significant safety concerns.
Professional Technical Analysis
This analysis involves a 1992 Cadillac Deville with an automatic transmission, experiencing critical brake failure and unintended acceleration. The vehicle has approximately 150,000 miles, which is significant for wear-related issues. The brake system's hard pedal indicates a possible failure in the brake booster, particularly if there is a vacuum leak. The brake booster should maintain a vacuum of approximately 18-22 inHg during operation. When the brake pedal feels like a brick, this often suggests that the diaphragm inside the booster is ruptured or that there is a significant vacuum leak in the booster hose. A malfunctioning booster results in insufficient assist, leading to a loss of braking efficiency. Additionally, the vehicle's idle issue, where the engine revs up unexpectedly, could be linked to a vacuum leak affecting both the brake booster and the engine's air-fuel mixture. This could cause the engine to run lean, leading to an elevated idle speed. It is essential to check the brake booster vacuum hose for integrity and ensure it is properly connected. A malfunctioning throttle position sensor (TPS) could also contribute to high idle, but the primary focus should be on vacuum integrity first. The inability to install the brake drum on the right rear side suggests that there may be additional mechanical issues in the rear brake assembly, which should also be inspected for proper functionality and alignment. The current condition of the vehicle poses significant safety risks, and immediate attention is required to address these critical braking and idle issues.
Possible Causes
Most common causes (ordered by frequency):
- Brake Booster Failure: The brake booster may have an internal rupture or significant vacuum leak, preventing adequate brake assist. Check the booster hose for cracks or disconnections, which should maintain a vacuum of 18-22 inHg. A failure here leads to a hard brake pedal feel and can cause loss of braking function. Inspect the vacuum connections and ensure they are secure and free of leaks. - Hard brake pedal, inability to stop vehicle without shifting to park, vacuum gauge readings showing low vacuum.
- Vacuum Leak: A vacuum leak in the brake booster hose or intake manifold can affect both braking performance and idle speed, causing high idle and loss of brake assist. The vehicle should be tested for vacuum integrity using a gauge to confirm readings. This is especially crucial given the symptoms of unintended acceleration and high idle. - High idle speed, engine revving without accelerator input, presence of vacuum leak in the booster hose.
- Throttle Position Sensor (TPS) Malfunction: If the TPS is faulty, it may send incorrect signals to the PCM, causing the engine to rev unexpectedly. This could exacerbate the high idle issue observed. Confirm TPS operation and check for proper voltage readings (typically 0.5V at closed throttle and around 4.5V at wide open). - Erratic engine idle, possible stored codes for TPS malfunction, inconsistent throttle response.
- Rear Brake Assembly Issues: The inability to install the brake drum on the right rear side suggests potential mechanical issues, possibly due to misalignment or damaged components within the rear brake system. Inspect the brake shoes, springs, and adjusters for wear or damage to ensure proper assembly. - Difficulty in reassembly of rear brakes, visual inspection showing worn components or misalignment.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial Inspection: Begin with a visual inspection of the brake booster and associated vacuum lines. Look for any signs of cracks, disconnections, or damage. Use a vacuum gauge to test the brake booster hose while the engine is running; it should maintain a vacuum of 18-22 inHg. If the vacuum is low, check for leaks or faulty connections. This step is crucial in diagnosing the brake pedal feel and idle speed issues.
- STEP 2 - Brake Booster Testing: Disconnect the brake booster hose and use a hand-held vacuum pump to apply vacuum to the booster. Monitor the hold; it should maintain vacuum for a substantial period without dropping significantly. Any rapid drop indicates a leak or failure within the booster unit. Confirm the integrity of the booster itself, ensuring it is functioning correctly.
- STEP 3 - TPS Inspection: Check the throttle position sensor for proper operation. Use a multimeter to measure the voltage output while slowly moving the throttle. Ensure readings are within specification (approximately 0.5V closed and 4.5V wide open). If the readings are inconsistent or out of range, the TPS may need replacement. Additionally, check the wiring harness for damage.
- STEP 4 - Rear Brake Assembly Check: Inspect the right rear brake assembly for mechanical issues. Remove the brake drum and check the condition of the brake shoes, springs, and adjusters. Ensure all components are aligned properly and functioning as intended. If any components are worn or damaged, they should be replaced to ensure proper braking performance.
