Hi guys, there seems to be differing opinions on the cause of the rattle so I'll share what I've learnt - I currently have a naked allroad BAS on a chain in my brother's driveway (I strung it up, it had it coming).
Firstly and most crucially, at 167,000kms (I'm in New Zealand but the car is a Jap import) my timing chain guides/slippers etc SHATTER when dropped a foot onto concrete, I couldn't believe it. These things are now so brittle that I'd urge everyone to replace them ALL. For some reason cars imported from Japan suffer greatly from heat damage so I guess for lots of you guys this may not be the case. I had one broken and one lifting off the guide. Note that the early engines have an oil boss with o-ring on the top middle guide (079 109 469AC) so don't buy 079 109 469AD like I did lol).
Secondly, the cam adjusters (with sprocket attached) require oil pressure to maintain a given cam position. To avoid the cam being thrown forwards and backwards by the valves as they pass over the nose, there is a spring-loaded sliding pin inside them that engages in a hole locking the cam at the full advance (idle) position.
Two things happen with this pin.
(1) It elongates its hole allowing for a slight rocking of the cam as it operates before oil pressure takes charge. This is the noise which Audi say is "normal". As the hole elongates over time this will get worse and may stress your fragile guides.
(2) It fails to engage completely leading to the situation I've shown in a video at
https://www.youtube.com/watch?v=mBZxFCqzrl8.
The noise from (1) is annoying, the noise from (2) sounds like a washing machine full of steel plates. See
http://img202.imageshack.us/img202/3434/0nhn.jpg for the inside, showing both an elongated hole (which on my other bank gives just a tiny amount of cam rock without oil pressure) and the offending pin.
Regarding (2), I just pulled my adjuster apart and the pin DOES engage into the hole but the wear in the elongated section has formed a little ramp for the pin to walk up! If the elgongation was full depth, no problem it'd lock but rattle as in (1) above. But the damned thing gets shallower the further from the orginal hole it gets :(
So having just ordered a pair of these
http://www.ecstuning.com/Audi-All_Ro...ster/ES284546/ for US$536 EACH (best price I can find) I think an accumulator is ESSENTIAL if you don't currently have any noise. It should completely eliminate further wear because with oil pressure this pin is withdrawn and does not wear the hole.
But as painful as it is if I own another high mileage 4.2 I'll bite the bullet and do the guides and tensioners definitely - not sure if the tensioners needed replacing but they were cheap so silly not to :)
Cheers!!
Geoff
Wellington, New Zealand
2004 allroad 4.2 V8
2000 BMW 540 Motorsport
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