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A8 / S8 (D3 Platform) Discussion Discussion forum for the D3 Audi A8 produced from 2003-2010 and Audi S8 produced from 2006-2010

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Old 03-24-2014, 08:51 PM   #1
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Default Solved my starting problem and you wont believe the answer!

OK Was having issue getting car to start after sitting a while. (cranks fine so not the battery) Swapped out fuel filter (no help) looking at new fuel pumps($1000) and almost pulled the trigger until a friend told me 93 octane gas has higher ignition point than 87 ,89 and 91 and often has starting issues. Went back to 87 and problem vanished. Cannot believe but he was right. it was the octane level in the gas which i was trying for first time.
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Old 03-24-2014, 09:05 PM   #2
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Surprised discovery. However, I am shocked that you're running 87 octane in your D3! My understanding is 93 is blended with ethanol which I would not use, but 87 would be too low for Audi. No carbon problem? No knocks on the engine?
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Old 03-24-2014, 09:40 PM   #3
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Hmmm... that doesn't seem right to me. The car is designed for 91 octane so I don't see how 93 would cause an issue. I have been running 93 octane in my car for the past year and it has never had so much as a hiccup (knock on wood.) I'm wondering if it was more an issue with bad gas?? I have heard that some stations that have lower volume of sales can have issues with the premium gas going bad from sitting too long.
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Old 03-24-2014, 09:44 PM   #4
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In Australia we are told that the engine must use 95 Octane or higher (98 or 100 more usually)
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Old 03-24-2014, 10:23 PM   #5
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Quote:
Originally Posted by rfmad View Post
OK Was having issue getting car to start after sitting a while. (cranks fine so not the battery) Swapped out fuel filter (no help) looking at new fuel pumps($1000) and almost pulled the trigger until a friend told me 93 octane gas has higher ignition point than 87 ,89 and 91 and often has starting issues. Went back to 87 and problem vanished. Cannot believe but he was right. it was the octane level in the gas which i was trying for first time.
You are right. I don't believe that this solved your problem. This was pure coincidence. You should search for a real reason for not starting. It may come back in a bad moment.

A8 is designed for premium gas. Only in a case of emergency regular gas can be used. There are three systems of octane number measuring. Same gas in North America has lower number (91 - 94) than in Europe or Australia (95 - 100). Some ethanol (10%) doesn't hurt. It makes octane value higher (pure ethanol has octane value 100), but a bit less energy (pure ethanol has about 15% less energy than pure gasoline).

Don't use regular. You are saving a few bucks at the pump but your car burns more of it for the same drive and it hurts your engine on the long run.
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Old 03-24-2014, 11:49 PM   #6
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I agree that it's a coincidence. Your engine was designed for premium fuel. The reason is to inhibit predetonation which will kill your motor over the long haul. Although there are knock sensors to retard timing, it does not always pick up small knocks. Use of regular gas would be ok for temporary emergency use, and I think you are playing with fire if regular gas is all you plan to use.
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Old 03-25-2014, 05:11 AM   #7
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Excerpts from the manual (3.0T but I'm guessing it would be the same for most any Audi engine):

"The fuel recommended for your vehicle is unleaded premium grade gasoline. Audi recommends using Top Tier Detergent gasoline with a minimum octane rating 91 AKI (95 RON)."

"Your vehicle may also be operated using unleaded regular gasoline with a minimum octane rating of 87 AKI (91 RON). However, using this octane fuel will slightly reduce engine performance."

The manual also notes that use of gasoline below 87 AKI should never be used as it will harm the engine and recommends filling up at busy gas stations. My take on this is, while the engine will adjust for the lower octane of "regular" gasoline, besides a performance disadvantage, there could be a risk that a given purchase of regular octane gas could fall below 87 AKI, especially at a less visited station (inspectors can't be everywhere all the time). Bottom line, why take a chance? My view is Audi recommends "Top Tier" premium, so that's what I use. (93 AKI in Massachusetts). Besides, I feel that although regular gas might be used safely, I invested in a high performance automobile, so even if the difference is "slight", why would I skimp on lower performing fuel that could hamper its capabilities?

The OP should figure out what caused his issue and return to using premium fuel.
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Old 03-25-2014, 11:07 AM   #8
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Default Fuel issues

The quality of fuel can be a issue but your problem doesn't seem right. You might have a fuel system pressure problem. Do you get air suction taking off gas cap? Maybe you got bad tank of gas? Water in fuel?
I live at 6000 feet and use regular gas all the time in my 2006 A8.
If temperature is below zero or above 100 then I go to premium fuel. This is my 2nd A8 and I have gone 60k driving on regular fuel without problem. At higher altitude you need less octane because there is less oxygen.
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Old 03-25-2014, 11:17 AM   #9
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Quote:
Originally Posted by rfmad View Post
OK Was having issue getting car to start after sitting a while. (cranks fine so not the battery) Swapped out fuel filter (no help) looking at new fuel pumps($1000) and almost pulled the trigger until a friend told me 93 octane gas has higher ignition point than 87 ,89 and 91 and often has starting issues. Went back to 87 and problem vanished. Cannot believe but he was right. it was the octane level in the gas which i was trying for first time.
Ummm odds are that isn't the solution to the problem...
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Old 03-25-2014, 01:13 PM   #10
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Switching fuel grades has nothing to do with your problem or its solution. In the US, the scale is R+M/2, so our 91 Octane fuel is comparable to 95 RON from elsewhere in the world.

You don't run into ignition quality issues until you get into "real" high octane race fuels (or the purple aviation stuff). There is no perceptible difference in ignition quality between 87 and 93, especially not in a relatively high compression, modern engine.
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Old 03-25-2014, 01:13 PM
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