I just broke down yesterday in my 2004 A4 with 100k miles. I heard a tapping noise and pulled over in 10-15 seconds and turned off the engine. It started but would not turn over. Towed to a shop. I'm concerned it's the timing belt - and if it is - does this mean there will be valve damage? In other words - is there ALWAYS valve damage when the timing belt breaks while driving - even though I turned off the engine right away?

 

The likelihood of incurring absolutely zero valve damage when the timing belt breaks is very very low. At a conservative 2,000 RPMs, the pistons would have each moved to top dead center at least 300-500 times in 10-15 seconds. At least one set of valves would be open in at least one cylinder. The end result has been frustratingly nicknamed "valve salad." Not to mention the high probability that the corresponding piston face is probably dinged up pretty good.

I have heard of one instance where a TB broke and there was zero valve/ piston damage. Cross your fingers, and you may be the second!

 

Thanks Brad. Just wondering if you have any advice. Within the past 6 months, I replaced the fuel pump, transmission, front and rear brake pads and rotors. Now the timing belt and possible (but probably) valve damage. I guess my options at this point are: 1) fix it (would you say $2k-$4K?) 2) find a good used engine, 3) sell it for whatever I can get? 4) salvage! Thanks.

 

One way is to put a new timing belt on with the timing correct (if it is indeed the belt that failed) and before going any further, do a compression test. If they're all even you may have gotten lucky. Unsure what the correct cranking compression pressure should be on that engine but you should have at least 120 psi or more.
Another way is, with the valve cover off so you can see the cam lobes, turn the engine until the valves are closed on cyl 1 and then do a leak down test with air pressure. Then do the same for cyl 2, etc. You don't necessarily need an actual leak down gauge, only an air hose adapter that screws into the plug hole. Listen at the throttle body and the tail pipes for air hissing. There will always be hissing heard from the crankcase to some extent from leakage past the piston rings.

Either way, if all seems good, put the engine back together far enough so you can start and run it, even just briefly. If it is smooth, finish reassembly and you're good to go.

P.S. I've seen probably 1 out of 10 cars with an interference engine that broke or slipped the timing belt that didn't bend the valves in the past 30+ years of being an auto tech. Hoping you have the 1 of 10!