possible cause of timing belt break?

marko · 07-21-2004, 07:03 PM

I'm in the process of putting everything back together after bent valves from a timing belt failure. (I've now promoted myself to novice mechanic from Rookie, after 2 weeks of removing all that crap)

12 teeth sheared off at the crank sprocket causing the timing belt to freeze in that position while the rest of the engine was turning. The rollers, tensioner, and water pump spin freely. I noticed the balance belt was very very loose when I first took it apart. Also the upper balance shaft TDC mark on the sprocket flange was at about the 2 o'clock position when it should be at 12 o'clock. Could this have caused the timing belt failure?

Granted, the belt is about 6 years old and has 127k miles on it, but I would think something would have to "seize" the belt a little for the crank sprocket to shear off 12 consecutive teeth. It also happened at idle.

(also is it possible to put the balance shaft sprocket on wrong by 30 degrees or would it have to be on wrong by 180 degrees.)
Thanks.

AFJuvat · 07-21-2004, 07:51 PM

Overstretched belt - tension too loose

or it was oilsoaked

AFJuvat

marko · 07-21-2004, 07:55 PM

It was really loose, about 5 or 6 inches play from top to bottom. Could that have interfered with the timing belt?

AFJuvat · 07-21-2004, 08:23 PM

I suspect that your timing belt itself was loose as well
That old, it probably stretched to the limit

Floppy balance shaft belt can cause damage as well

AFJuvat

marko · 07-21-2004, 09:17 PM

The timing belt itself was on there pretty tight. I'm just hoping that it was the balance belt that casused the problem so that I can feel a better when I ever git to start it back up again.
Hoffentlich geht es alles ok.

jimbilly · 07-21-2004, 10:05 PM

127k is about 100k more than recommended, so the teeth probably just got weak and broke off. Have you tried prying other teeth off to see how well they're attacthed? I've seen teeth shear off on vw's with old belts.

SoCal Driver · SoCal Driver's Garage

Timing belt was too old and too many miles. You are damn lucky that it lasted as long as it did. As this is an S2 how is the chain tensioner pad? This is a much a failure point as the timing belt. In fact having the plastic pad disentigrate and jam in the cam teeth is the most likely cause of timing belt failure where the teeth shear off.

Difference in the key ways on the balance shaft pulleys is 170 degrees (190 degrees the "other"direction). It's possible to get the pulleys back on wrong. If they are you will get a very high vibration in the engine.

marko · 07-21-2004, 11:25 PM

Thanks guys.
I changed the chain tensioner pad a couple of months ago, the pad only, that I got from Zims, it still looks brand new. (the original still looked in great shape when I changed it). The tensioner itself was still pretty stiff, so I decided not to change that.
I just had a second look at the belt and it still looks good, but obviously not where the teeth got sheared off. No oil on it, not dry rot, no cracks or anything. I tried like you said to pull some of the teeth off. I could pull them off, but it needed alot of twisting and torqing, they didn't come off easily. I'm no materials expert but it looks to me that alot of force was needed to shear those teeth off, especially 12 consecutive ones. That's why I suspected something may have seized the belt. If it was something in the head, I won't be able to check that since it's sitting in my garage nice and shinny from the machine shop. On that subject I'd like to know how they check valves, only one looked slightly bent, the 7 others they said were bad looked perfectly straight to me, but then again, I'm no machinist either.

marko · 07-21-2004, 11:44 PM

They did find a broke spring set in the head, but I don't think that would have seized the cam. I had two sets of springs break 3 months after I bought the car with 29k miles on it, and it just made the engine run like crap.

jm951 · 07-22-2004, 07:40 AM

It's not too unusual to see lots of teeth sheared off. I've seen similar a couple of times. When the engine is rolling over at a midrange rpm, there's lots of inertia with the rotating assemblies. Let one weak tooth go, and it becomes a chain reaction.

Britwrench · 07-22-2004, 08:13 AM

Lack of maintenance caused the belt failure. As JM951 says, when one tooth goes, the rest follow along. Obviously changing the belt at the recommended intervals is way cheaper then repairing the damage caused by a broken belt.......

marko · 07-22-2004, 11:24 AM

Ok,
That makes sense, I'll go along with that.
I'll have to call Paul Weir's in Seattle and let him know what a great job he did on the 60k belt change.

jimbilly · 07-22-2004, 12:24 PM

Umm, you know that Weir has been out of business for a couple of years?

marko · 07-22-2004, 12:28 PM

Ya, I tried just tried the number and got a disco recording. He did my belts around 1998 or 99.

Britwrench · 07-22-2004, 12:39 PM

Yep, he went bankrupt several years ago. We bought most of the tools and parts at the liquidation auction. I also worked there and your belts were done on 07/14/99 at 86,646 miles.

jimbilly · 07-22-2004, 12:44 PM

Hey, maybe you did the pre-purchase inspection on my white 944S2!, my friend Mark Peterson brought it there before he bought it with about 65k showing on the broken odo.

marko · 07-22-2004, 01:32 PM

HUH? "Brit" wrench. Is that Mark?

Britwrench · 07-22-2004, 01:44 PM

Yep, it's me!!

marko · 07-22-2004, 02:03 PM

HEEY....it was 5 years ago. I apologized, the belt has nearly 101k not 126k on it. (my car has 187k on it now...THAT I can remeber, cuz I just looked at it yesterday)

SoCal Driver · SoCal Driver's Garage

So now that we have all have had a shot at you; what is the recommended belt change interval?

Are you going to change your belt at that interval next time?

If not please let us know so some one can change the Porsche crests on your car to Volvos.