This article gives detailed instructions on replacing the tensioner pulley, idler pulleys and main accessory (serpentine) belt in a BMW vehicle with the N54/N55 turbocharged engine. Even though we used a 2011 335i to perform this repair, this article can be applied to all 1,3,5 and X1 vehicles with either the N54 or N55 turbocharged engine with minor modifications to the repair steps.
A BMW N55 engine squeal repair includes replacing the tensioner and idler pulleys, as well as the main accessory belt.
We continue to hold the belief here at the Repair Guide that the most annoying sound that is ever generated from the motors in our BMWs is engine squeal due to worn pulleys and/or drive belts. It is a nerve grating sound that is almost impossible to pin point due to all of the other ambient noise emanating from the engine. But if you are experiencing engine squeal in your N54/N55 turbocharged powerplant, it is most likely due to a failing belt tensioner, idler pulleys or main accessory belt.
A BMW N55 engine squeal is usually caused by a failing belt tensioner or idler pulley. The belt tensioner does exactly as its name implies; it keeps pressure on the main accessory (serpentine) belt as it weaves its way through the alternator, power steering pump and ac compressor. As the belt tensioner spring ages this pressure begins to decrease, allowing the belt to slip and causing a squeaking sound from the engine. The idler pulleys (there are two on the N54 and N55 engines) have the job of redirecting the main accessory belt so it doesn’t get bound up as it travels around the front of the motor. Age also takes it toll on their bearings, causing them squeal as they reach the end of their service life. The two belt tensioner pulleys are also susceptible to bearing failure.
The main accessory belt, also known as the serpentine belt, can be another source of BMW N55 engine squeal. As the belt ages it can stretch, causing it to slip on the pulleys producing a chirping sound. This is most noticeable when the engine is cold, or when it is first started.
Diagnosing the exact culprit of your BMW N55 engine squeal is very difficult due to all of the back ground noise going on around the pulleys and the belt. Failing pulleys and belts can squeal when cold, hot, or anywhere in between. Trying to pin point exactly which component is failing is usually an exercise in futility. The best plan of action in curing your BMWs engine squeal is to replace all of the pulleys and the belt together in one repair.
Replacing the serpentine belt, tensioner and idler pulleys in your N54/N55 turbocharged engine is actually quite simple and can done by any home mechanic with average ability. The pulleys are easily accessed from the top of the engine and the belt can be replaced without having to remove any major components. It is a great money saving repair that avoids an expensive trip to the dealership or local foreign repair shop.
1. Main accessory (serpentine) belt (required) – Unlike many other BMW engines, there is only one drive belt on the N54/N55 turbocharged six cylinder engine driving all of the accessories. Always use an OEM quality belt on your BMW…cheaper belts do not last as long and simply are not the worth the couple of dollars saved.
2. Belt tensioner (required) – There are two different belt tensioners depending on which engine you have in your BMW…either the N54 or newer N55. If you are not sure which engine you have, use our engine model number tool above courtesy of realoem.com. Simply type in the last seven digits of your VIN in the serial number search box and your engine model will be given to you along with other important information about your vehicle.
3. Idler (deflection) pulleys (required) – There are two idler pulleys on the N54/N55 engine…an upper and a lower. Both of them must be replaced when doing this repair. Like the belt tensioner above, the idler pulleys are engine specific…if you don’t know what engine you have use the engine model number tool above.
As discussed at the beginning of this article, replacing the belts and pulleys on your N54 or N55 turbocharged engine is actually quite simple compared to other BMW models. The belt and pulleys are easily accessible from the top of the engine without any need to raise the vehicle or remove the cooling fan. It is a little bit of a tight squeeze getting the new belt on the lower pulleys so exercise patience during this step.
- Remove the two torx bolts anchoring the intake duct to the suction hood at the front of the car. Unsnap the intake duct from intake muffler and remove it from the vehicle.
- Identify the belt tensioner, the upper and lower idler pulleys and the main accessory belt on the front of the engine. Note that the belt tensioner on the N54/N55 engine also has two pulleys.
- Using a T60 torx bit, rotate the belt tensioner clockwise to release the pressure on the main accessory belt.
- With the pressure off of the belt, reach down and pull the belt off of the belt tensioner pulley.
- Pull the belt free from the rest of the pulleys and remove it from the vehicle.
- Use a small flat blade screwdriver or metal pick to remove the protective cap off of the top belt tensioner pulley. Using a T50 torx bit, remove the top pulley from the belt tensioner. It is easier to extract the main belt tensioner unit from the engine if you remove the top pulley first.
- Grasp the belt tensioner unit and remove it from the vehicle.
- Next, unsnap the protective cap off of the upper idler pulley and use a T50 torx bit to remove it from the vehicle.
- Finally, repeat the above step on the lower idler pulley and remove it from the vehicle.
Installing the new pulleys and accessory belt in your BMW N54/N55 turbocharged engine is simply the above steps in reverse with no special procedures. Take your time installing the new belt ensuring it is routed correctly around the pulleys (see routing diagram below). We have provided a handy checklist below with the exact reassembly steps along with important torque values for the pulley bolts.
BMW N55 Engine Squeal Reassembly Checklist
***The following steps were performed on a 2011 335i…other models may vary.
- Install the new lower idler pulley and torque its mounting bolt to 50 Nm (37 ft-lb).
- Install the new upper idler pulley and torque its mounting bolt to 50 Nm (37 ft-lb).
- Install the new belt tensioner (with its lower pulley only) into the engine. Remember that due to clearance issues, its usually easier to install the belt tensioner in separate pieces.
- Install the belt tensioners upper pulley and torque its mounting bolt to 50 Nm (37 ft-lb).
- Install the new accessory belt. Pay careful attention to how it sits in the pulleys…you want to make sure that its ribs are lines up correctly.
- Use a T60 torx bit to turn the belt tensioner clockwise. Slip the new accessory belt onto the top belt tensioner pulley.
- Replace the intake duct back into the engine.