1998 BMW 750iL Has Rough Idle

Troubleshooting Causes of Rough Idle in Your 1998 BMW 750iL and How to Restore Smooth Performance

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Reviewed by
Andrew Hajjar
Written by RepairPal Car Genius
Updated January 14, 2025
0%
21% chance it's the
Spark Plugs
0%
15% chance it's the
Ignition Coil
0%
64% chance it's
Something else
The most common reasons a 1998 BMW 750iL has rough idle are a vacuum leak, an issue with the spark plugs, or a problem with the ignition coil.
  • Spark Plugs: Worn or fouled spark plugs can lead to misfires, poor fuel economy, and difficulty starting the engine.
  • Ignition Coil: A faulty ignition coil can cause misfires, rough idling, and difficulty starting the engine due to inadequate spark delivery to the spark plugs.
Proper diagnosis will determine the exact cause of your symptom, saving you money on unnecessary repairs. The diagnostic experts at RepairPal Certified shops have the right tools and experience to solve any symptom.

How much does it cost to fix a 1998 BMW 750iL has rough idle?

Common Repairs

The total repair cost will vary based on the condition of your vehicle and the number of repairs needed to fix your symptom. Proper diagnosis will save you money by finding the exact repairs needed to fix your car right the first time.

Here are a few common symptoms of worn-out brake pads:

  1. Low brake pad warning light is on: Not all vehicles are equipped with this system, but if yours is, this dashboard indicator light will likely be the first signal it’s time to replace your brake pads.
  2. Brakes start to squeal regularly: This is often one of the first signs of worn brake pads and is caused by a metallic piece built into brake pads for this exact purpose. This part contacts the brake rotor to make a squealing or whining noise when the pads are low.
  3. Brakes make loud grinding noise: If not addressed early enough, your brake pads may get so worn that the metal backing plate of the pad grinds against the metal rotor.
  4. Brake pads look thin: You may be able to visually check your brake pad thickness to determine if they need replacement.

Most often, a brake system inspection will just be a routine portion of manufacturer specified maintenance, and will be performed at mileage intervals. In those cases, there will likely be no symptoms. However, when a brake system makes a funny noise, causes vibration, is hard to use, will not slow the vehicle, prevents the vehicle from driving, makes the vehicle pull to one side, or leaks brake fluid, the vehicle will need a visit dedicated to the inspection of the brake system.

What are the initial diagnostic steps to take for a rough idle in a 1998 BMW 750iL?

When addressing a rough idle in your 1998 BMW 750iL, it's essential to adopt a systematic diagnostic approach that begins with the most accessible components. Start by checking the spark plugs and coilpacks, as these are critical for proper ignition; worn or faulty parts can lead to misfires and an unstable idle. Next, inspect the fuel mixture, as an incorrect ratio can significantly impact engine performance. Adjusting the mixture screw or evaluating the idle jet size may be necessary to achieve optimal fuel delivery. Additionally, the idle control valve should be examined, as it regulates engine speed and can become clogged over time. Cleaning or replacing this valve can often resolve idle issues. It's also crucial to verify the engine timing, ensuring that it aligns correctly at top dead center (TDC), as improper timing can lead to rough idling. Lastly, don't overlook the possibility of vacuum leaks, which can disrupt the air-fuel mixture and cause instability. By following this structured approach, you can effectively diagnose and remedy the rough idle in your BMW, enhancing both performance and driving experience.

What are the common problems that could cause a rough idle in a 1998 BMW 750iL?

When dealing with a rough idle in a 1998 BMW 750iL, it's essential to consider several common problems that could be affecting your vehicle's performance. One of the primary culprits is faulty spark plugs, which can lead to misfires and an unstable idle. Additionally, vacuum leaks can disrupt the air-fuel mixture, further exacerbating idle issues. Another critical factor is the coolant temperature sensor; if it malfunctions, it may send incorrect signals to the engine control unit (ECU), resulting in rough idling or even stalling. Carbon accumulation on the intake valves is also a concern, particularly in direct-injection engines, as this buildup can hinder engine efficiency and power. Lastly, it's important to rule out transmission problems, as they can sometimes mimic engine hesitation during idle. By systematically diagnosing these potential issues, DIYers can effectively address the rough idle and restore their BMW's performance.

How urgent is it to address the rough idle in a 1998 BMW 750iL to prevent further damage or safety risks?

Addressing a rough idle in your 1998 BMW 750iL is not just a matter of convenience; it is essential for maintaining the vehicle's performance and safety. A rough idle can stem from various issues, such as a malfunctioning idle control valve, which can disrupt the engine's ability to maintain a steady state. If left unchecked, this can lead to more severe problems, including stalling or poor acceleration, which not only affects drivability but also poses safety risks on the road. Moreover, a rough idle may indicate deeper issues, such as problems with the transmission control module or low fluid levels, which can escalate into costly repairs if not addressed promptly. By diagnosing and repairing the rough idle early, you can avoid the potential for increased repair costs and ensure that your BMW remains safe and reliable. Therefore, it is crucial to take immediate action to resolve this issue, safeguarding both your vehicle and your driving experience.

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1998 BMW 750iL Rough Idle: Causes & Repairs | RepairPal