There is a little gremlin hiding under the hood of your BMW. If it’s not inspected and maintained properly, it can cause major engine failure.
Wondering what this little hand grenade is?
Keep reading.
The mystery component is the engine belt tensioner. This handy little gadget keeps tension on the drive belt and allows the rotational energy of the engine to be transferred to critical components under the hood.
If you don’t quite know what a engine belt is check out this article.
So what’s the big deal with these tensioners?
Why is this component so critical to keep an eye on?
In this article I’ll show you everything you need to know about BMW belt tensioners.
- How to check your vehicles belt tensioner
- What a shop would typically charge
- A general walk through of the repair
- Some stories along the way
What’s a belt tensioner?
Many critical components on your BMW need to be spun to function; I’m talking about your steering, heat and A/C, even the recharging of your battery. This is why an engine belt is more important than your morning coffee, (depending on who you ask.)
This belt wraps around all of these pulleys and is driven by the engine crankshaft, turning rotational power into the driving force of critical systems under the hood and beyond.
You can thank the engine belt for turning components such as your A/C compressor and your power steering pump. Without these critical parts, you’ll be sweating to the oldies, trying to turn a stiff steering wheel while hot air blows in your face.
The belt tensioner keeps the belt tight so it can properly grip the pulleys of each component. A good way to think about this is wearing a belt three sizes too big. Wearing an oversized belt will cause problems, often embarrassing ones. If the belt isn’t tight, it slips on the pulleys, causing screeching noises, steering issues, and more.
Remember the days of the screeching car first thing in the morning? No worries, just rev the engine until the belts get so hot they grip. (Yeah, that was terrible advice.)
Whats The deal with bMW belt tensioners specifically?
This problem is especially common in BMW E90 3-series (2005-2011), but the same failure mode happens in many BMW models with similar belt tensioner designs. If you own a BMW with a belt tensioner, this applies to you.
Why these specifically?
The design of these belt tensioners allows for side-to-side wear which misaligns the pulley. Eventually, the engine belt will ride off that pulley into the promise land.
And by the promise land I mean one or two places typically.
Version 1 – The tensioner fails and the belt is thrown off of the pulley and basically shreds while whipping around under your hood. Most cars you lose power steering, and while that’s sinking in the battery light comes on, believe it or not, this is the best-case scenario.
Version 2 – Basically all of version 1 with a sinister plot. When the belt fails the rotating engine pulley pulls the shredded pieces of belt behind it, eventually the belt is pulled into the front crankshaft seal.
When this happens, it usually causes catastrophic engine failure. The belt makes the engine jump timing and essentially become a boat anchor.
How to check your engine belt tensioner
The good news is when these start to fail they usually show signs for a while before they completely fail. Grab a flashlight and do this quick check every once in a while and you may just thank me later.
This quick tip assumes you have some mechanical knowledge, if not that’s OK! You’ll still learn what to say to your mechanic.
Step – 1: Grab a flashlight and get the hood open
Step – 2: Shine the light at the front of the engine and take a look at the engine belt, look for the tensioner (this will look like an aluminum arm with a pulley on it).
Step – 3: Inspect the pulley position, does it look off? Like cocked to one side? Or is the edge of the engine belt riding right at the front edge of the pulley or further off?
If the pulley looks cocked or the belt looks like it’s riding off of the pulley the tensioner is absolutely starting to fail. You need to get it replaced ASAP.
How much does it cost to have this replaced?
This is a top question I get asked at my shop about this and, well, basically every other job. Typically, the tensioner job should range from $450-$600, could be more or less, but that’s a pretty close general idea.
You or your mechanic should be checking this at every service interval. If you have your BMW at a shop and they don’t know what you’re talking about when you bring this up, I’d rethink letting them work on it.
Can I replace this myself?
You can, just like I can paint my house in a week. Maybe I’ll get it done, but it will look like Beavis and Butthead did it. I’m not a painter, and if you’re not a mechanic or someone with decent mechanical ability, leave this to the pros like I do.
This isn’t to say people with some mechanical knowledge can’t get this done in a weekend. Just be sure to replace the tensioner, bolt, and belt (obviously). These need to be torqued properly as well to specified torque and then angle torqued.
Here is a quick rundown of the process
Remove the engine belt
Using a T-60 Torx socket and a long ratchet, release the belt tension and remove the belt. I suggest taking a picture of the routing before you end up desperately hunting Google for the diagram in a panic.
Pull out the engine belt and check all of the pulleys on each component, give them a spin. You want to check for bearing noise, roughness, and general “this ain’t right feelings.”
You may need to remove some stuff to get to the tensioner, I repeat, this is a generic procedure.
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Unbolt the tensioner
Using a female Torx socket, unbolt the center bolt in the tensioner assembly and pull the unit out. Notice how this bolt feels very light and isn’t steel? That’s BMW for you; this baby is aluminum and requires a new bolt and very specific torque if you want it to be right.
Now that the tensioner is out clean the area and inspect, you want to make sure the threads into the block are not damaged. You also want to make sure the mating surfaces are clean and flush.
Install the new tensioner
The tensioner will only go in one way; you will see a “block” section that slides into the engine block. It’s important to note, the bolt doesn’t really take all of the load; the indent in the engine block does. The bolt’s job is to hold it flush against the engine.
Install the bolt and TORQUE TO EXACT SPEC! This is where the Elmer Fudd garages and DIYers get it wrong.
BMW uses torque-to-yield bolts that need to be torqued to initial spec, THEN torqued to a specific angle. You need a good torque wrench that can do this, if you don’t have it I suggest leaving this job to the pros.
Am I being paranoid? You are here to read quality content, if you want to crank the bolt down and pray that’s fine, but I’m here to give you quality.
Do it right, here is the torque.
| Renew screw. Thread repairs exclusively with Helicoil. Jointing torque and angle of rotation must be observed without fail. | ||||
| Jointing torque | 25 Nm | |||
| Angle of rotation | 90° |
Here is the torque spec, keep in mind this is for a 2006 BMW 325xi, make sure you find the correct spec for your exact situation.
Button it up
Install the new belt and any other components, and you are all set. Run the engine and check your work. As long as you routed the belt properly, replaced the bolt, and torqued properly, you should be good to go.
It’s happened at my shop
In the early days, we had a BMW at the shop that needed a new engine belt and tensioner. The catch was this particular car also had a large oil filter housing leak that had been dropping oil bombs onto the belt for months.
The customer refused to repair this portion even after being warned several times about the risk. Well, long story short, the car came back with a blown engine and a customer who thought it was our fault.
Totally preventable (for the most part)
These things fail, not “if” but “when.” The good news is they usually look sick before they completely fail. Follow the quick guide above to check your belt and tensioner. I’ve seen a few of these; it happens more often than you think to unmaintained cars.
