2006 marked the introduction of the 335i and the first turbocharged engine BMW offered in their compact sports sedan segment since the 1970’s. The N54B30 quickly received recognition in the automotive world as a marvel of contemporary engine design by its combination of the tried-and-true straight-six layout assisted by two tiny true-twin-turbochargers. The basic design is one that immediately stands out as similar to the recipe for success chosen by both Toyota and Nissan in their straight-six twin-turbo monsters that have been used for two decades in some of the highest horsepower cars on the planet. BMW followed suit and gave European enthusiasts a ‘2JZ of their own’ with the N54, but they are not without their pitfalls. Before you make an effort to contact a dealership or private seller, there are a few things you should know specifically about the N54.
The N54 suffered from a few common issues that affected some critical engine systems and occasionally resulted in some catastrophic failures. Due to the commonality of the problems and the narrow timeframe in which they generally happen, the N54 has (unrightfully) been called by many an unreliable engine. While there is no shortage of video depicting these issues we will address, we must first make it clear that the N54 is, or can be, an incredibly reliable power plant. Owning an N54 can be immensely rewarding and provides nearly endless opportunities to make impressive horsepower and torque outputs with minimal investments thanks to its design.
First and foremost, owning a BMW specific scan tool, like the Schwaben Professional BMW scan tool, is an absolute must. The N54 is a performance engine, after all, so staying current on maintenance and staying informed about issues as they occur are both critical to retain engine dependability. Full access to these complex computer systems can only be achieved with a BMW-specific scan tool so we cannot stress enough how important it is that you should keep one on-hand should you prefer the N54 as your engine of choice.
This brings us to common mechanical issues. The N54 was, by all accounts, an experiment by BMW to introduce forced induction back into regular production. Now, many of their production engines are turbocharged, so we can safely assume the experiment was successful. Unfortunately, it was not without its hiccups in the early N54s. The N54 suffers from leaky injectors, a weak HPFP (high-pressure fuel pump), a failure-prone water pump, carbon buildup, brittle charge pipes, and the notorious wastegate rattle. While even just one of these problems can result in the dreaded ‘limp mode’ or even failure of engine operation entirely, none are specifically difficult to either prevent or repair. BMW even extended warranties on known issues, like the wastegate rattle, and have been diligent in their commitment to fixing the issues for the customer. This is where the most important step in your car buying decision comes into play: with the N54 powered 335i, you need to ensure maintenance records and ownership history are both impeccably documented and available before you purchase the car. Doing some light history reading is the best way to walk into your 335i N54 buying experience well informed and confident.
With that said, the N54 is also generally the preferred engine to modify for high horsepower over the N55. The biggest difference between the two engines is in the forced induction setups. The N54 features true twin-turbos that pressurize the exhaust gasses of three cylinders each. In contrast, the N55 uses a twin-scroll turbocharger that operates nearly identically than the true-twin setup in the N54, but scrolls slightly faster. The reasons BMW chose to switch from the twin setup to the twin scroll version are predominantly cost savings in manufacturing, reduced exhaust reversion, slightly reduced spool time, and space savings in the engine bay.
While the N55 does make marginally more power than the N54 in stock form, mod-for-mod an N54 will outperform an N55 and is, therefore, the more popular model chosen by tuners. Both engines have their benefits and drawbacks so it will hinge upon your intentions with the car. For a general rule, if you intend to modify your 335i, the N54 has better potential, but the N55 is considered more reliable for those who wish to remain stock. Again, the N54 is or can be incredibly reliable in its stock form as long as all maintenance has been meticulously performed, so we definitely would not tell anyone to exclude that engine from their search if modification is low on their list. Similarly, the N55 does have excellent potential for upgraded performance and can provide a superb platform for anyone interested in upping the horsepower.
Key Benefits of the N54:
High-performance potential
Can be made to be reliable
Less expensive upfront cost
Key Drawbacks of the N54:
Lots of required maintenance to make it reliable
Suffered from many mechanical issues when new
Expensive to maintain in the event of worst-case catastrophic failure
Key Benefits of the N55:
More reliable out of the box than N54
100RPM faster turbo spool from Twin Scroll charger
Only available in newer cars, so these will ideally be in better overall condition than most N54s
Key Drawbacks of the N55:
Not as high of a potential for performance
Expensive up front and during ownership
Still needs extensive maintenance
Now, assuming you have made a decision as to which one you would like to purchase, we can move on to the pre-purchase inspection and test drive. It is critical, once again, that you obtain all service records and history on the car you choose. This holds true for any used car purchase, but especially for N54/N55 cars. Gaps in the service history or a high number of owners can be enormous red flags. Assume any service not present in the history of the car has not been performed and proceed with discretion.
When you have selected a car that seems to check all the boxes, you can set up a time to go drive the car. We always recommend that you bring a jack, jack stands, and jack pad adapters for the E90 along with you to inspect the car. This is to check the suspension, inspect for fluid leaks under the car, and evaluate the condition of bushings, bearings, and mounts. Be sure to check the front wheels for any play that could indicate worn suspension components and inspect the brake pads, rotors, and lines for signs of excessive wear. These cars are, like all E9X models, prone to fluid leaks from a few key spots.
The main leaks are:
Oil - valve cover gasket, oil pan gasket, oil filter housing gasket
Coolant - all hoses
Look at the coolant as well and ensure it is still Genuine BMW blue coolant. Seeing another color can indicate something went wrong with the cooling system and the car was repaired by a non-BMW specific shop. This isn’t necessarily a bad thing, but it could raise questions if you do not see coolant system service on the car’s history. As well, the water pump and thermostat both notoriously fail around 80-90,000 miles, so these should be replaced upon purchase immediately if no service records for their replacement exists.
With the hard stuff under the car checked, double check the fluid levels. Oil fluid level is checked from inside the car through the car’s computer, while the rest of the fluids are observed traditionally in the engine bay.
Turn the ignition on and listen to the engine as it warms up. Watch the coolant temperature to ensure the system is functioning as it should. Test the A/C and Heat as well as any interior functions like windows, power locks, radio functions, steering wheel controls, and cup holders that fold out from the passenger side dash. These are all weak points to the E9X and many will most likely require service or replacement. The number of interior functions still in working order will indicate how well cared for the car was and should be congruent with the physical condition you saw overall during your initial overview.
If everything is to your liking, take it for a test drive. Give the car a hard drive, make sure you accelerate to 60mph and stop hard from 60mph. Feel for any abnormal vibrations, pulls to either side under braking, and listen for any squealing from the brakes or engine. A well-maintained car will have a good alignment, will not pull to either side under braking, and will ride smoothly without excessive vibrations.
Be sure to listen intently to the car on the drive. If the owner rides with you, make sure you tell him (politely) to keep his mouth shut. You need to hear every gear change, listen to the engine through the revs, and be mindful of any noises from hard braking. Take the car on the highway, through some city driving, and make a few stops where you turn off the car and back on for some hot starts. If you drive the car hard enough, which we highly suggest you do, any hidden problems mechanically will make themselves known.
Indicators of suspension/brake wear:
Brake fluid, Calipers, Rotors, and Pads: Pulls to one side, soft pedal feel, pulsing pedal feel, or brake shimmy.
Worn Ball Joints/Thrust Arm Bearings: clunks, shimmy, ‘death wobble’.
Poor Alignment: Pulls to one side under driving, excessive tire wear
Drivetrain: excessive deflection, noticeable clunkiness on shifts, vibrations that are not wheel speed dependent.
Wheel/Tire imbalance: Wheel speed dependent vibrations.
If you have completed your test drive and everything is in acceptable condition, then hopefully you can agree on a price with the owner and take home a new-to-you E9X 335i or 335is with the confidence that you purchased the best one for the money. Any N54 or N55 driver will tell you the potential of the cars in which you find those engines can be one of the most rewarding to drive, tune, and live with on a daily basis. With this article, and of course all parts you may need, Turner Motorsport is here to help you on your journey to build, own, and love your E9X N54 or N55 car for years to come. Be sure to check out the full catalog of parts, upgrades, and accessories we have available to keep your BMW happy and exceed your performance goals.
