Audi A8 (1994 - 2003) - 97 4.2 S8 Missing

Honest John Back Room » Technical matters
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Sun 7 Feb 2010 16:52
97 4.2 S8 Missing - Daedalus
Forum members may recall that over the years I have had issues with My Audi S8 missing randomly. I was talking to an Audi technician the other day who said it may be down to having the petrol tank at a low level, as this car had a factory induced missfire if the level of petrol was too low. I have never ever heard of this one before, but as I normally run the car on LPG and deliberatly keep the petrol level low (less weight to lug around) it may have some legs. Anyway I topped the car right up on Friday and so far no missing, but it can go for weeks with not missing anyway, but that could be because I had topped it up!!

By the way the petrol gauge is the only electrical thing on the car not working at the moment, in its 14th year and 189,000 miles is not too bad me thinks.
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Sun 7 Feb 2010 17:29
97 4.2 S8 Missing - Dynamic Dave
I topped the car right up on Friday and so far no missing but it can go for weeks with not missing anyway but that could be because I had topped it up!!


How old was the petrol in the tank prior to refilling? Petrol can go off if stored for a long time.
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Sun 7 Feb 2010 20:11
97 4.2 S8 Missing - Daedalus
DD

Yes I have had an issue with old petrol but it tends to make it hard to start. I use Shell V Power and since discovering that the old petrol does cause issues I have been running the car on petrol more often to ensure a reasonable turnover. Does anyone know how long V Power lasts before it goes off. I am tending to top up monthly at the moment but have been keeping the tank lowish.

Daedalus
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Sun 7 Feb 2010 23:37
97 4.2 S8 Missing - Dave_TD
See this from way back in 2003:

www.honestjohn.co.uk/forum/post/index.htm?t=15940

One post there from Cliff Pope with a lot of valuable information:

Petrol "goes off" if it is kept in circumstances where it can evaporate. The most volatile elements obviously evaporate first, leaving a residue that becomes progressively less volatile and so the engine harder to start. Also gummy deposits start to form, most seriously in fuel pumps, carburettors and injectors.
I think it is generally better to keep petrol in a full container or tank, when there will be less exposure to changing air owing to "breathing" as the air temperature changes. The gumming effect can be reduced by adding injector cleaner beforehand.
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Sun 7 Feb 2010 23:37
97 4.2 S8 Missing - CraigP
It loses one or two octane rating per year.

Does your lpg conversion not start on petrol anyway then switch to lpg once running? I was looking at a range rover on lpg and there were all sorts of conditions like ambient temperature had to be above 3 degrees c or it would choose to stay on petrol, or if you booted it, back onto petrol etc.
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