gnosis

There's lots of stories on here about engine damage caused by cam belt breakages and Variocam problems, but very few people mention the circumstances that led up to the point where their engine stopped and/or damaged itself.

Can the people who've experienced a broken cam belt or Variocam problems post here and describe your breakage, condition of the belts, chain, tensioners, runners, pulleys etc., what your engine was doing at the time of failure, how much and how hard you use the car (i.e. is it a daily driver or a track car), how long since the last belt change/maintenance, etc.

I'd like to come away from this thread with a picture of how these parts break, rather than just recommendations that you should change them religiously at a certain mileage.

Why? Because after a long lapse of maintenance where I've run on the same cam and balance shaft belts/pulleys for 5 years and 90,000km, and the same Variocam gear since new (7 years and 160,000kms), I can see no wear at all in any of these parts. I want to replace them to be safe, but I'm curious as to why my stuff looks pristine and others are experiencing horrible looking wear and failures.

So far, this is what I've found:

Cam belts: Watch out because the tensioner might fail and then the belt will break.

Variocam: The plastic guides on the Variocam actuator wear, and the chain stretches around 4mm in length. These things together put a strain on the cam sprocket teeth, which break off and end up in your sump. Once about four teeth in a row break off, the chain will slip/jam/break and valve/piston/head/camshaft damage will occur. A broken oil pipe on the actuator unit can contribute to this failure, by presumably limiting lubrication.

The Variocam story is, I would say, rather complete. The cam belt story is kinda vague. So it's the cam belt area I'd like to see the most information on.

Clayton

Jay Wellwood

Don't know the answers yet for the 968, but.....

We 928 owners observe the religion when it comes to our timing belts, rollers, and water pumps.

"Thou shalt replace thy timing belt every 40,000 miles or 4 years, whichever comes first. Thou shalt replace the water pump and rebuild the tensioner at the same time.

If thou doesn't ahere to thy religion, woe beit to thy 928 motor valves and thy checkbook when thy timing belt breaks/strips, or the water pump seizes.

sayporsha

Clayton,

Timing belts (and balance shaft belts) do break when a tensioner or roller fails. But what kills most belts is time, wear & tear. Even if you never drove your car, the elements would be at work wearing out your belts.

Your cam (a.k.a. timing) belt is basically a rubber band with reiforcing cords inside - it's a wear item. With each heat cycle of the engine, it gets stretched & slackened as the engine expands & contracts. It travels a very circuitous route, and that at wildly varying speeds. It really takes a beating during track use, where rapid fluctuations in engine speed essentially cause it to yank back and forth on the toothed sprockets. Judging by your avatar, your car sees some track duty.

If your timing belt looks pristine, that's a good thing. However, a visual inspection of your belts should only give you peace of mind if you are between belt change intervals. After 5 years and 90,000km your baby has earned a new one.

daytrader968

Well, I just had a timing belt failure in November (still sorting out the damage at the moment!):

The car was approaching four years and 40,000 miles on the belts and I was planning to do belts/rollers/waterpump this spring. At the moment of failure, the car was in reverse as I was backing out the driveway, revs were low and I had just started to engage the clutch. Then the car stalled out. Just a half hour previously, I had arrived home from a two hour drive that included some stop/go city driving and some highway time. It drove flawlessly the whole time!

After inspection, I found the cam belt shredded all its teeth around the crankshaft pulley. Only one other tooth on the belt was missing. All the rollers were freely spinning (ie none were seized). Cam belt tensioner is in fine working order. Balance belt, of same age, is in fine condition.

My car is a daily driver and sees maybe 10,000 miles per year, on average. Probably only drove about 5,000 this past year. Has about 103k mi total. It's never seen a track. I drive the car with spirit, but try to keep it smooth. No pushing the redline or street racing or anything.

I inspected the variocam pads and camshaft sprockets last year, along with the belts. All looked fine.

The only cause of my belt failure seems to be age/mileage or just a fluke. I can't find anything else that failed, causing the belt to fail. I was aware of the maintenance needs of the engine, and I didn't think I was being excessive in the interval I chose to leave it considering the Porsche maintenance book says every 60k. I guess 3yr/30000 miles will be my interval in the future?!? Part of me wishes something else had caused the belt to fail because it's a little unsettling.

Good topic, I look forward to seeing any other responses.

gnosis

I wonder how many failures occur in cars that see temperatures below freezing each year. It could be a contributing factor to belt fatigue. Let's see what happens after we get a few more posts. Hopefully we'll be able to plot locations on a map and see how many cars are driven (or even just stored) in the tundra and/or extreme heat conditions.

Clayton

UKKid35

This is what you don't want to see, a belt with missing teeth:

http://www.928.org.uk/~ukkid35/proje...ture%20002.jpg

This is what may have caused it, sprockets with worn teeth:

http://www.928.org.uk/~ukkid35/proje...ture%20030.jpg

This is what teeth that are about to fail look like:

http://www.928.org.uk/~ukkid35/proje...ture%20034.jpg

gnosis

Informative photos, Paul. I've heard that some belts can look fine until you turn them inside out like your last photo shows.

Clayton

Scott M.

Paul;

In your case, the belt failure is related to age and the oil deposit w/in the belt covers.

The worn cam sprockets is due to a mal-adjusted timing belt.

I save all the old t-belts after a job (40+), wall of shame so to speak, and the only shredded teeth belts I've seen came from engines that look like your pics.

Like oil and water don't mix, neither do oil and belts.

Now slap a stroker crank in it.

Scott
93 GTS 6.5L
89 GT
92 euro 968
74 911
...

UKKid35

Thanks Scott

You're right about the age of the belt, it was probably five years and one month old, but I'm not convinced that the seemingly minor oil deposits would have found their way on to the belt (sounds like you have 40 times as much experience as me though).

However there was no oil at all in the tensioner, the boot was cracked and solid, I didn't mention this previously because I'm not sure whether it is relevant to the four cylinder cars.

The belt tension was adjusted by a "Porsche Specialist" nine months before the belt failed, however I wasn't given any feedback on the condition of the belt or tensioner or the sprockets. Perhaps the belt was overtightened then in order to kill the belt tension warnings, or maybe the sprockets were damaged earlier in the car's history (or both!)