drive belt - what causes it to break?

I've got a 99 Carrera with a rebuilt engine. It now has about 5,000 miles on the clock (the car has a total of 30,000) and today the belt broke. What happens that might cause this? I want to be somewhat prepared for what the mechanic tells me because he's the guy I bought it from in the first place. Bought the car with 25,000 miles and this rebuilt 996 engine. He gave me a one year warranty. I'm already betting money he'll claim that it's not part of the rebuild so I'll be paying for a new belt installation (btw, what does that typically cost?). I just had the fuel filter replaced because it was running rough and stalled a few times. He claims there was crap in the line and that I probably got some bad/cheap fuel. So that was $300 a week ago and then $200 for an oil change and when I went to get it, he was fiddling in the back and told me that he was adjusting the idler pulley. Any possible connection? Don't you think he would have noticed a bad belt? Anyway, any thoughts/advice is appreciated.

An idler would absolutely have an impact on the life of your belt.

Serpentine belt cost $40 and 5 minutes to change. Your mechanic is taking you for a ride big time on those oil and fuel filter prices. You really need to learn some very basic DIY work. You could have changed the oil, fuel filter, Belt yourself for less than $100.... I own a 99 model 996. I am 73yrs of age and do all my own basic maintenance work. Bad cheap fuel my ***. He is taking you for a ride and will continue to do so if you let him.

There are no idler pulleys that need adjustment.

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Belts get old, and belts that aren't used a lot still get old. Unless it was replaced when the engine was rebuilt.

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A bad out of the box serpentine belt can fail. Do you have the old belt? If so what is its condition? Is it just old? Missing sections of the small V's? Frayed on one or both edges? Did the belt break or just come off a pulley? One possibility is if the belt was not renewed when the engine was rebuilt the belt was just old and failed due to age if not miles. But be sure the belt was not rubbing. A sign is if the belt edges are sharp, or worse, frayed. A "rebuilt" engine might have some accessory drive alignment issues and this can have a belt gone in no time. If the belt is otherwise ok, no signs of rubbing, it just came off that could be a failing tensioner. I do not know what a belt replacement job costs. I've done a couple myself (on my Boxster) and all it cost was an hour of my time and I work slow and a new belt. The 996 might be a bit more involved. While I have a 996 (Turbo) I've never replaced this belt myself. I've had the belt replaced a couple of times and I do not recall the labor. It was replaced with some other work so the tech might have just replaced it without adding any labor to the ticket. Oh, I know of no idler adjustment. Whether there is a connection or not to the belt failing I can't say. Ask him what he was adjusting? Alignment? Or tension?

He knew something was chewing up the belt, that's why he was fiddling with it. As said above, idlers are not adjustable, he was making up ****. He is also fleecing you with BS services that should have been covered by the engine warranty. He might have chaged you for a fuel filter but actually replaced something else that was causing the stalling issue. Oldest trick in the book, blame one part so the customer pays for the "warranty" work. Get another mechanic.....fast!!

Find another mechanic or get more acquainted with your engine.

Post a photo of the underside of the belt. We may be able to tell you if it looks old and worn. The belt is driven by the crank pulley. The things that cause it to break are age and wear. Things that can make it fail prematurely are accessories (alternator, A/C, water pump, etc.) that are not turning freely, exposing the belt to excessive tension and higher than normal temperatures.

Thanks everyone. I think after reading your comments and watching a few videos, I'll try to do this myself - at least I'll check each of the pulleys to be sure nothing is bad there. Regarding the belt itself, it flew out at about 60 MPH on a 2-lane highway. I walked back to find it but couldn't.

Here's a great video about belts and how new EPDM belts don't normally really show wear like old belts, they don't crack. Start at 6:45, but at 8:00 it shows how to measure wear on an EPDM belt. Changing it on the 996 is easy, good luck! Tech Garage - Ep 1511 - YouTube

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This DIY tech article may be of some assistance as well with the replacement. Pulley DIY here as well and the rest of our articles can be found here for anything else you're needing. Good luck!

I finally bought two belts (thinking it would be good to have a spare) and went to do it myself. The original belt was actually there in the bay all knotted up. It was the center idler pulley that sheared off. The belt is in perfect condition - not a scratch. I'm trying to figure out now how to replace the idler pulley. This is the one with which you adjust the tension. Doesn't look like it should be hard and I'll take a look at those videos to get my bearings (no pun intended). Thanks again everyone.

So now you have two spare belts!!

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There's a hex bolt at the end of that "L" arm of the tensioner, hidden behind the power steering pump. To access that, you need to remove the throttle body. To tighten the bolt on that idle pulley, you need to slip a 15mm wrench between the "L" arm and the pulley to counter the rotation. The photo shows how to remove the pulley from the "L" arm.