Re: Octane vs. carbon buildup?

["Milan B. Milojevic" <milanb@earthlink.net>]

 From day one I have used 92 octane from Chevron, 76 Union and 
Mobil.  Nothing else was ever used.  I have also changed oil on a regular 
3,500 mile intervals (Castroll 20W50W).  All services were done on time by 
the dealer, as well as the oil changes, from which I have purchased the 
vehicle.  We have, what I consider, a good relationship.  You can well 
imagine my surprise at AudiUSA authorizing engine removal and disassembly 
to locate the source of the knock that occurred only when starting the car 
cold (for Southern California).  The noise only lasted a few seconds and 
was gone.  The condition was intermittent. There was no oil usage, the 
mileage was within the specs.  There was no lack of power nor any 
pinging.  In another words there was nothing to indicate excessive carbon 
build up.  Dealer's diagnostic equipment also indicated no 
problems.  Emissions were fine. Once the engine was opened it was like 
opening a Pandora's box.  There were mechanical problems on top of the 
carbon build up.  I won't go into those right now, but suffice to say that 
they were severe enough to warrant a factory rebuild engine.

In my limited technical opinion there should have been some signs to 
suggest carbon build up.  There should have been some sort of signal from 
O2 sensor indicating incomplete combustion.  NOX sensor also should have 
indicated something.  The realities of what we found once the engine was 
opened did not correlate to the information that the on board computers 
were showing.  Maybe someone more knowledgeable than me could explain all this.

With all of the above problems I still belive in the Audi product and would 
love to own a new one with 5 valve head geometry.  So far I have not seen 
any reports on the carbon build up in those engines.  Does anyone know if 
the new A6 and, or new A8 suffer from the similar faith.  Currently Audi's 
solution is to have regular chemical carbon removal and fuel 
additives.  This increases operating cost of the vehicle, not to speak of 
the inconvenience.

At 11:55 AM 10/19/99 -0700, you wrote:
>There may be no correlation, but I am wondering what octane (rating system 
>taken into account) level of gas is used by those who have had massive 
>carbon buildup, and by those that haven't?
>
>I've always used 92 octane (Chevron, btw) in my VW\Audi cars, because of 
>course with the higher compression they don't run too well on anything 
>less than premium.  At least in my experience.
>
>What then is the current wisdom on keeping your valves, pistons and 
>combustion chambers clean, besides Techron?  Does octane\quality of gas 
>play a role?
>
>Also did Audi issue a TSB on why those A6s have the carbon problem as 
>listed below?  Too cold a thermostat??
>
>=========================================
>
>AudiUSA just replaced my 1996 A6 engine at 49K for carbon build up and
>other mechanical problems.   I couldn't beleive the amount of carbon in the
>combustion chamber.  Anyway, when did you start using Audi Techron (milage
>wise)?  Is Audi Techron different from Chevron Techron, the inventor of the
>stuff?
>
>At 01:34 PM 10/18/99 -0700, you wrote:
>>I use the Audi Techron additive once a month and the inside of the engine
>>was clean at 287km when the head was pulled off. No carbon on the pistons,
>>or valves and I'm still running the original injectors. Can't hurt.
>
>______________________________________________________
>Get Your Private, Free Email at http://www.hotmail.com