2008 BMW 135i Convertible Complete Maintenance Schedule & Service Guide

Key Takeaways

• The 2008 BMW 135i Convertible is powered by the N54 3.0-liter twin-turbo inline-6 engine, producing 300 horsepower and 300 lb-ft of torque .
• The vehicle uses BMW's Condition Based Service (CBS) system, which uses sensors and algorithms to calculate individualized service intervals based on actual driving conditions rather than fixed mileage for every item .
• BMW specifies spark plug replacement at 45,000 miles for the N54 engine, with many specialists recommending more frequent changes for modified or aggressively driven vehicles .
• Brake fluid requires replacement every two years as a fixed time-based interval .
• Automatic transmission fluid (ATF) replacement is specified at intervals of 100,000 miles .
• The original 4-year/50,000-mile BMW Ultimate Service free maintenance program has long since expired for this model year .

Understanding the BMW Condition Based Service (CBS) System

The 2008 BMW 135i Convertible utilizes the Condition Based Service (CBS) system, a sophisticated maintenance monitoring approach that represents a departure from traditional fixed-interval schedules. Sensors and special algorithms evaluate the different driving conditions of your BMW to determine current and future service requirements .

What CBS Monitors

On the Control Display or instrument cluster, you can view remaining times or distances for :

• Engine oil service
• Front and rear brake pads (separately)
• Brake fluid
• Vehicle check
• Legally mandated inspections depending on local regulations

Service Data in the Remote Control

Your vehicle continuously stores service-requirement information in the remote control while you are driving . Your BMW Service Advisor can read this data from the remote control unit and propose an optimized maintenance approach. Whenever you take your car in for servicing, you should hand your BMW Service Advisor the remote control unit that you last used .

Important: Make sure that the date is always set correctly; otherwise, the effectiveness of Condition Based Service CBS is not assured .

Factory Maintenance Schedule

The following represents verified OEM maintenance information for the 2008 BMW 135i Convertible based on BMW technical documentation and specialist sources.

Engine Oil Service

The CBS system determines engine oil change intervals based on driving conditions rather than a fixed mileage schedule.

Oil Specifications: The N54 engine requires BMW-approved synthetic oils. BMW recommends using engine oils that meet BMW's Longlife-01 (LL-01) specifications. BMW High Performance 5W-30 Synthetic Oil (P/N 07 51 0 017 866) is the factory-recommended oil .

Standard Scope of Engine Oil Service :

• Change the engine oil and oil filter
• After every 3rd engine oil change: replace the intake air cleaner air filter element (reduce replacement interval in dusty operating conditions)
• Parking brake: Check function and pre-burnish

Owner Guidance: Pelican Parts recommends changing synthetic motor oil every 5,000 miles . BMW tuning specialists recommend oil change intervals of 5,000–8,000 miles, with more aggressive driving or track use requiring more frequent changes (3,000–4,000 miles for track use) . The N54 twin-turbo engine places considerable thermal stress on engine oil, making a disciplined maintenance approach essential for longevity.

Spark Plug Service

BMW specifies spark plug replacement for 135i versions at 45,000 miles . The spark plug service is not shown in the CBS system and must be tracked separately . OEM replacement components are recommended for a proper ignition tune-up .

Owner Guidance: Replacing spark plugs ahead of the factory interval is recommended, especially if your BMW has been tuned for increased performance . Replacing ignition coils as preventive maintenance when performing spark plug service is also recommended . Turbocharged, direct-injected engines like the N54 place significant stress on spark plugs; aggressively driven N54s require more frequent changes .

Ignition Coils

Specialists recommend replacing ignition coils every 2nd time spark plugs are swapped . For stock vehicles, ignition coil replacement is typically recommended at 60,000–70,000 miles; for modified vehicles, a 30,000–40,000-mile interval is advised .

Brake Fluid Service

BMW mandates that brake fluid be replaced every two years . This is a fixed time-based interval critical for maintaining braking performance and preventing internal corrosion in the hydraulic system.

Automatic Transmission Service

BMW documentation specifies automatic transmission fluid (ATF) replacement at intervals of 100,000 miles .

Owner Guidance: BMW tuning specialists recommend automatic transmission fluid service every 60,000–80,000 miles for the N54 . The ATF fluid is a specialized oil optimized for the transmission's requirements, assisting in lubrication as well as operation of the brake bands, torque converter, and valves.

Manual Transmission Service

For the 2008 BMW 135i manual transmission, BMW specifies fluids conforming to MTF-LT-3 specification with a typical viscosity of 75W-90. Specialists recommend replacing manual transmission fluid every 60,000–80,000 miles as a conservative measure .

Differential Fluid

Specialists recommend differential fluid service every 60,000–80,000 miles for optimal longevity .

Microfilter (Cabin Air Filter)

The ventilation microfilter is not shown in the CBS system and must be tracked manually . The service schedule includes: "Replace ventilation microfilter" .

Vehicle Check

BMW specifies a comprehensive Vehicle Check as a standard scope service . The Vehicle Check includes :

• Brief diagnostic test and Check Control message verification
• Indicator and warning light checks
• CBS display reset
• Tire inspection, pressure adjustment, and Tire Pressure Monitor reset
• All exterior and interior lighting system checks
• Safety belt condition and function checks
• Windshield wiper and washer jet position checks
• Tire tread depth, wear pattern, outer condition, and inflation pressure checks
• Battery state of charge check (magic eye) and charging if required
• Power steering reservoir fluid level check
• SRS airbag unit visual inspection
• Convertible-specific: Open the convertible top and activate rollover protection system via diagnostic link
• Coolant fluid level and concentration check
• Windshield washer system check
• Brake system connections and lines leak, damage, and positioning check
• Underbody inspection (transmission, rear axle, fuel lines, exhaust system)
• Steering components clearance, leaks, damage, and wear check
• Road test with brake, steering, shock absorber, and transmission checks

N54 Engine Common Service Considerations

The 2008 135i Convertible uses the N54 twin-turbo engine, which is found in the 2008-2010 135i models . The N54 engine has several well-documented service considerations:

Carbon Buildup on Intake Valves

The N54 engine is prone to carbon buildup on the intake valves due to its direct-injection design. Symptoms include rough idle, reduced power, and decreased fuel economy. The recommended cleaning method is walnut blasting. Regular maintenance and inspection at higher mileages are recommended.

High-Pressure Fuel Pump (HPFP)

The N54 engine is known for high-pressure fuel pump issues. BMW has historically addressed this with warranty extensions and recalls on earlier model years. A forum example shows HPFP replacement at approximately 59,500 miles and again at 83,800 miles on a 2008 135i .

Valve Cover and Air-Oil Separator

The OEM air-oil separator can clog and build up pressure over time, causing valve cover gasket leaks. This is a known issue on N54 engines.

Cooling System

Proactive replacement of cooling system components, including the water pump and thermostat, is recommended to prevent failures at higher mileages.

Recommended Maintenance Planning

For 2008 BMW 135i Convertible vehicles, consider the following service items based on factory and specialist recommendations:

Frequently Asked Questions

How often should I change the oil on my 2008 BMW 135i Convertible?

The CBS system determines intervals based on driving conditions. Pelican Parts recommends changing synthetic motor oil every 5,000 miles . BMW tuning specialists recommend 5,000–8,000-mile intervals, with more frequent changes (3,000–4,000 miles) for track use . The N54 engine requires BMW LL-01 approved synthetic oil.

What engine oil should I use in my 2008 BMW 135i Convertible?

BMW recommends BMW High Performance 5W-30 Synthetic Oil (P/N 07 51 0 017 866) . Oils meeting BMW Longlife-01 (LL-01) specifications are required. Approved viscosity grades include SAE 5W-30 and 5W-40. Always replace the oil filter when changing the oil.

When should I replace the spark plugs on my 2008 BMW 135i Convertible?

BMW specifies spark plug replacement for 135i versions at 45,000 miles . Replacing spark plugs ahead of the factory interval is recommended, especially if your BMW has been tuned for increased performance . Replacing ignition coils as preventive maintenance when performing spark plug service is also recommended .

How often should I change the brake fluid?

Brake fluid must be replaced every two years, per BMW's factory maintenance requirements .

Does the 2008 BMW 135i Convertible require transmission fluid service?

BMW documentation specifies ATF replacement at intervals of 100,000 miles . BMW tuning specialists recommend automatic transmission fluid service every 60,000–80,000 miles . For the manual transmission, BMW specifies MTF-LT-3 specification with 75W-90 viscosity.

What is the Condition Based Service (CBS) system?

CBS is BMW's onboard system that uses sensors and algorithms to evaluate driving conditions and calculate individual service requirements . It monitors engine oil, brake pads, brake fluid, vehicle check, and legally mandated inspections. Service data is stored in the remote control key and can be read by your BMW Service Advisor .

What engine does the 2008 BMW 135i Convertible use?

The 2008 BMW 135i Convertible features the N54 3.0-liter twin-turbo inline-6 engine producing 300 horsepower and 300 lb-ft of torque .

Are the spark plugs shown in the CBS system?

No. The spark plug service is not shown in the CBS system and must be tracked separately .

Is the microfilter (cabin air filter) shown in the CBS system?

No. The ventilation microfilter is not shown in the CBS system and must be tracked manually .

What was the original warranty on the 2008 BMW 135i Convertible?

The 2008 BMW 135i Convertible came with BMW Ultimate Service, providing 48 months or 50,000 miles of free scheduled maintenance, along with 48-month unlimited-mile roadside assistance . These programs have long since expired for this model year.