[spaladugu03]

Hi all, just bought a brand new belt tensioner, idler pulley and belt kit from FCP euro and did the install as I noticed a wobble on my old tensioner. Bought the car at 55k, its now at 67k and no indication that this service was ever performed. Everything was torqued down to spec +- w/e variance my torque wrench has.

The brand new belt tensioner has pretty much the same wobble, if not worse on idle. Noticed this while the car was in park and when cold and also fully warmed up. But when revving the car above 1k RPM its not moving anymore?

Wanted to see if this was normal to have a wobble on idle on the brand new tensioner. A lot of the youtube videos I saw don't have a wobble on the brand new ones.

Uploaded videos of my issue for context:
Old tensioner with wobble:

Brand new FCP Euro tensioner with new belt/ Idler pulley with Wobble:

[LuxoM3]

So I've had quite a few BMWs over the years... that idler pulley is connected to a semi-moving arm that is tensioned by a spring. The pulley will move front/back - that is normal. Wobble is a side to side movement which indicates a bad/worn bearing or loose arm or both.

Given the mileage you have, the new parts - I am going to say, this is nothing to worry about.

For reference, I'm a 2015 435i at 80k, still on stock pulley, arm, and tensioner. No recommendations from my BMW indy shop for replacement.

[spaladugu03]

Hey Luxo, does your current tensioner have as much play as either of the videos I posted above? I'm thinking maybe something else in the loop is causing this. Harmonic balancer maybe?

[LuxoM3]

Originally Posted by spaladugu03

Hey Luxo, does your current tensioner have as much play as either of the videos I posted above? I'm thinking maybe something else in the loop is causing this. Harmonic balancer maybe?

So I went outside in the heat, it's like 99+ degrees right now...

My 2015 435i with 80k miles - has a similar forward/backward movement to both your videos - let me correct that, I have more. If I had to scrutinize it, maybe an easy 1/4" of movement.

But if I compare it to my wife 2021 X3 and son's 2008 328i... it's A LOT more movement, but on their motors, the tensioner arm is A LOT shorter too - so their tensioner springs are doing less work.

I'd personally stop worrying about this. A tensioner-based belt system causes a lot less wear on bearings and belt systems because it allows for tension variation. The spring has a lot more give than bearings and a belt. So on your car and mine, it's doing it's job.

As a worst case - failure - what would happen? You won't have AC and alternator power.
But the cost to repair is cheap and easy.


P.S. Note belt diagram. I have to think the belt routing also makes this belt/pulley movement more visible. This isn't as highly tensioned like on say the E36 M3 where the belt loops around almost 60% of the pulley surface. So what I am saying is, more contact surface, more tension... less contact surface, less tension, and more movement.

[spaladugu03]

Thanks for your help on this Luxo. Makes a lot of sense and my only concern was if this was expected behavior on a brand new tensioner. I'm mostly afraid of the belt coming off and causing damage while I'm in the middle of driving or worse, getting sucked through the front crank seal.

I also noticed that it moves a lot less at anything higher than idle rpm(~650?). When I'm doing an oil measurement for example the rpm kicks up around 1k rpm and it doesn't move at all. Little less worried after seeing that.

[LuxoM3]

Originally Posted by spaladugu03

Thanks for your help on this Luxo. Makes a lot of sense and my only concern was if this was expected behavior on a brand new tensioner. I'm mostly afraid of the belt coming off and causing damage while I'm in the middle of driving or worse, getting sucked through the front crank seal.

I also noticed that it moves a lot less at anything higher than idle rpm(~650?). When I'm doing an oil measurement for example the rpm kicks up around 1k rpm and it doesn't move at all. Little less worried after seeing that.

Like I said, don't over think it.

It's a poly-ribbed belt with some serious groves. You have nearly ZERO side-to-side/lateral movement. Plus if it REALLY slipped off - I have yet to hear about ANY serpentine belt falling or spinning off in my 35+ years of working on cars. They will fail, but we are talking gross lack of maintenance, which I don't think you are that type.

Plus, it won't get sucked into the crank seal. [ laughing ]. If the belt breaks, it pretty much falls off and at worse, flies off left or right along the path it was spinning (into the airbox, fender well, etc.) - and it won't general enough force to cause much damage. The notion of it getting sucked into the crank seal would mean all the kinetic energy moving in one direction would somehow need change course and be turned 90 degrees - which physically is impossible.

Enjoy your car man. Life has enough worries on it's own!