BMW Serpentine Belt Sucked Inside Engine?

It’s definitely possible, and this is how it happens.

For years, there have been rumors and anecdotes floating around the BMW enthusiast community and online message boards. Incredulous owners have reported accounts of being told by their BMW mechanic that not only did their BMW serpentine belt fail, but it had also been sucked into the engine. 

Yes, you read that correctly. Not mangled up and entwined among the moving ancillary components or into the various spaces and valleys found around the assembled long-block – no, yanked into the actual internals of the engine. These reports are often met with (understandable) skepticism from their fellow BMW owners and enthusiasts, leaving everyone asking the question…

“Can a BMW serpentine belt really be pulled into the engine?”

As strange as it may sound, a BMW serpentine belt getting sucked inside the engine is not only possible, it is actually a fairly common occurrence. We see it all the time. In fact, we saw another one just recently on a new customer’s ’08 BMW 535i and we took the opportunity to get the photos that you see here. In this post, we’ll shed some light on this phenomenon and tell you which BMW models can be affected by this, how and why it happens, and what you can do to prevent this from happening to your BMW.

First things first, let’s establish which BMW models are susceptible. The simplest way to put it is any model with the 3.0-liter I6 “N5X” engine (engine codes N52 and N53), including the turbocharged versions (engine codes N54 and N55). This engine can be found in 2004 – 2013 “E9X” 3-Series models (E90/E91/E92/E93 generation), the 2004 – 2010 E61/E62 generation of the 5-Series, as well as some F10/F11 5-series cars up to the 2015 model year.

How does the belt get pulled into the engine?

The serpentine belt on an engine is driven by a pulley on the front of the crankshaft, which protrudes out of the front of the engine. Surrounding the protruding end of the crank where it sticks out from the engine block is something called the crank seal, or more precisely the “front main seal.” This is a hard sealing ring that basically keeps the engine oil in and outside elements out.

On N5X engines, the crankshaft pulley that drives the serpentine belt sits a little closer to the engine block than on other engines, creating a slightly narrower gap between the pulley and the engine. So when the belt fails or comes off the pulley, it can sometimes get pinched in that gap and the immense rotational force of the crankshaft spools it up into that gap, forcing it past the front main seal and into the engine internals. When that happens, it is a very big problem.


The belt gets chewed up and shredded in the process and bits of belt material get strewn all over the place. It can clog up the screens in the oil pick-up tube and generally wreak havoc throughout the engine and must be thoroughly, painstakingly cleaned out.

Aside from breaking or damaged belts that haven’t been changed due to long-term neglect, what causes the serpentine belt to come off the pulley in this way can usually be blamed on one of two things. The first one is the belt tensioner pulley going bad. As the bearing in the tensioner pulley wears out, the pulley develops a little bit of play and begins to tilt slightly out of alignment from the force exerted by the tension of the belt. The longer that is left unaddressed, the more pronounced it becomes, until eventually it is bad enough to walk the belt off the pulley entirely.

The second main cause of a belt coming off is oil from a leaking oil filter stand gasket. Due the location of the oil filter stand, when the gasket begins to leak the oil is deposited right on to that belt. The oil makes the belt and pulleys slick and can also cause the belt rubber to swell and deform, which can result in the belt slipping off.

How To Prevent a Serpentine Belt from Failing or Coming Off

Simply put, the best prevention for all of this is to find a good BMW repair shop and diligently keep up with the regular maintenance on the car. A competent, qualified BMW mechanic who will be familiar with these cars and know what to look for will be able to spot these kinds of things early during your routine service visits, alert you to them, and make appropriate recommendations.

The reason a serpentine belt would ever come off and wind up getting pulled into the engine like this ultimately boils down to either the car not being serviced properly by a competent BMW specialist, or not listening to their recommendations. When your Bimmer mechanic recommends some service or repair (such as, in this case, replacing a belt tensioner or repairing a leaking oil filter stand), it is because they are experts on BMW’s and they want to prevent larger problems from occurring.

In fact, your mechanic is actually trying to save you money. While replacing a belt tensioner (or other recommended repair) does make an otherwise routine service visit more expensive in the short term, it pales in comparison to the cost of repairs from putting it off too long and allowing something catastrophic to happen. Waiting for something major to fail is far more expensive than the relatively low serpentine belt replacement cost. In this case, that increased cost amounts to at least an additional 15 hours in labor and several thousand dollars on top of the cost of repairing the initial underlying cause. And if the engine also jumps time from the belt coming off, you can add a very costly cylinder head job to that expense, or possibly even a full engine replacement.

If you need to find a European car repair and maintenance shop you can trust with your BMW, we invite you to come in and see why Autoscope is the premier BMW specialist in DFW. Give us a call or set up an appointment online today! Don’t forget to like and share this post to social media!

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