Owner's Problem Description
The owner reports a hard ride and a clunking noise from the rear suspension of their 1993 BMW 740iL, particularly when going over larger bumps like potholes. Despite the noise, the vehicle drives fine and the tire wear appears normal. An alignment was performed 18 months ago, and previous repairs included rebuilding the upper control arms in the front. The mechanic did not find any issues in the rear suspension during the last inspection.
Professional Technical Analysis
The 1993 BMW 740iL is equipped with a self-leveling rear suspension system, which consists of hydraulic shock absorbers and nitrogen-charged spheres (commonly referred to as 'bombs'). The symptoms described, including the hard ride and clunking noise when traversing larger bumps, suggest that the nitrogen spheres may be failing or have lost their charge. This can lead to inadequate damping during suspension travel, causing the clunking sound. Given the vehicle's age and the typical lifespan of these components, it is reasonable to suspect that they have deteriorated, especially if they have not been replaced in recent years. The absence of any other serious symptoms such as uneven tire wear or steering issues further points toward the rear suspension components as the likely source of the noise. Additionally, the previous inspection did not reveal any significant issues, indicating that the problem has developed relatively recently. A thorough inspection of the rear suspension, including the shock absorbers and nitrogen spheres, is essential to confirm the diagnosis and determine the appropriate course of action for repairs.
Possible Causes
Most common causes (ordered by frequency):
- Nitrogen Sphere Failure: The nitrogen-charged spheres in the self-leveling suspension may have lost their charge, leading to insufficient damping. These components typically degrade over time, and failure can result in a hard ride and noise due to inadequate shock absorption during suspension travel. - Clunking sound when driving over bumps, hard ride quality, potential visual inspection revealing leaks or damage to the spheres.
- Shock Absorber Wear: The rear shock absorbers may be worn out, causing them to fail to provide adequate damping. This wear can lead to increased noise and a firmer ride, especially over uneven surfaces. - Increased noise levels when going over bumps, visual inspection showing leaks or physical damage to the shock absorbers.
- Control Arm Bushings Failure: Worn control arm bushings can lead to clunking noises when the suspension is compressed or extended. This could be particularly relevant if the upper control arms were rebuilt, as the lower bushings may still be original and worn. - Clunking when hitting bumps, especially noticeable during acceleration and deceleration, inspection may reveal cracked or deteriorated bushings.
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 - Visual Inspection: Start with a thorough visual inspection of the rear suspension components, including shock absorbers and nitrogen spheres. Look for any signs of leaks, physical damage, or corrosion. Pay attention to the condition of the mounting points and bushings for any signs of wear or deterioration.
- STEP 2 - Shock Absorber Testing: Conduct a bounce test by pushing down on the rear of the vehicle and observing the rebound. If the vehicle continues to bounce excessively, it indicates that the shock absorbers may need to be replaced. Additionally, measure any visible leakage from the shock absorbers.
- STEP 3 - Nitrogen Sphere Pressure Check: Check the pressure in the nitrogen spheres if accessible. This may require specialized tools or equipment. If the spheres are found to be low or empty, they will need to be replaced to restore proper suspension performance.
- STEP 4 - Road Test: Perform a road test to replicate the conditions under which the noise occurs. Pay attention to any clunking sounds when going over bumps and assess the overall ride quality. Document any observations regarding the handling and performance of the vehicle during the drive.
