I recently encountered a guy who was not happy with the performance of
his A6/18T. So he removed the exhaust catalitic converter and installed
a straight section at that point. Now the engine has a rattle. No loose
retainer bolts or anything. It sounds like a bit of mettal oddly
rolling around in a chamber of sorts. The car went to the Audi dealer
(to the head mechanic) and no one had a clue as to what's causing the
rattle. Remember...just the converter was removed. He initially tried
to sell me this car, but the rattle could not be explained. The chief
mechanic says "other than that...the car works fine". I'm sorry. But
this is an AUDI A6 18T and that is just not acceptable. Just think of
the emotional stress of having to make excuses for that annoyance.
There could be long term damage.
I think that engine relies on being set up with the correct exhaust,
removing the cat does work with some cars to increase power but not with
all, it could be valve rattle, he could try replacing the cat & see if the
problem disappears, or to compensate the removal of the cat, an ecu re
programming may be required to set the ignition curve which may now be
different with reduced exhaust back pressure,
A common failure on the 1.8T is the wastegate actuator on the turbo going bad.
when that happens boost is mostly gone (along with performance). When it is bad
you will hear a rattling sound at mid range engine speeds.
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