AndyZ

Hi Guys--

As I'm preparing to get ready for my 90k timing belt service, I learned about a new one-- PaulW-- this one's (possibly) for you:

A Porsche mechanic I know also has a 98 A8-- In talking with him about timing belt changes on the A8 (70k miles, changed belt @ 60k) he stated that he gets a thrumming sound @ idle, and that he was 99.9% sure that this was caused by a worn profile on the camshaft sprockets. He was VERY concerned that this may eventually cause a skipped tooth due to the belt "catching" each time it engages a new tooth on the sprocket. He plans on changing the belt and sprockets again very soon.

Any thoughts? Should we also be changing our timing sprockets?

Thanks All,

Z

PaulW

What exactly do you mean? Do you mean the height of the teeth of the sprocket is worn?

I.E. the component shown in this picture?

<img src="http://www.audipages.com/Tech_Articles/enginemechanical/tbp31.JPG">

Is he suggesting that the timing belt are wearing the teeth on it? I would say that would almost be impossible. Look at the relative hardness of the cam sprocket versus the rubber belt. It isn't going to happen, the belt doesn't have diamonds in it.

I would say that his noise is either a pulley he didn't replace on the timing belt side, or on the serpentine side (my guess), or that his belt is tensioned too tight.

The sprocket does not need replacement due to the belt wearing it out.

Or does somebody else see something that I'm missing?

AndyZ

I couldn't imagine the belt causing that much wear either, which is why I posted the question--

However, I remembered your earlier posts re: noise after belt replacement. Erring on the side of caution, I decided to share all this--

cuatrokoop

Over-tensioned belt...Would put my money on it if I was a betting man ;-)

s4master1

if that were possible it would wear out the sprocket for the crank first
he needs one of the bearings running with the belt