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Solved my starting problem and you wont believe the answer!

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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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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, 07:15 PM   #11
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Octane rating is a measure of how well the fuel resists self ignition caused by the heat and compression in the combustion chamber. So lower octane fuel is actually easier to ignite.

A high compression engine needs higher octane ratings to prevent the heat and compression in the cylinder causing the fuel to self ignite prematurely to the spark firing (aka pre-ignition, detonation, knock, pinging etc). Running low octane fuel will cause this to happen so the ECU "hears" it and retards spark timing in order to help reduce the problem by lowering combustion chamber temps, and in doing so robs you of power and economy.

High Octane fuel does not contain any more energy than lower octane rated fuel of the same type.
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Old 03-25-2014, 07:49 PM   #12
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Just to add to the octane discussion (not that I expect anybody to learn from this ).

Pre-ignition is not the major problem. Detonation is. Difference between normal burning and detonation is speed of flame advancement. Normal burning advance 20 m/s while detonation advance 2000 m/s creating extreme thermal and mechanical shock. It happens in parts of combustion chamber further from the spark plug when already hot mixture gets further compressed by regular burning. By retarding ignition point ECU keeps maximum pressure lower in order to suppress detonation, hurting efficiency.
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Old 03-25-2014, 09:39 PM   #13
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Problem sounds like a bad crankshaft position sensor. It is the same problem that my E500 had and all V8 Benzes have gone or will go through. Well documented problem.
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Old 03-26-2014, 03:53 AM   #14
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^^^ I was thinking those thoughts also, but I am by no means an expert on these.
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Old 03-26-2014, 06:45 AM   #15
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Quote:
Originally Posted by mishar View Post
Just to add to the octane discussion (not that I expect anybody to learn from this )....
Not so. I learned something from your post.
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Old 03-26-2014, 07:09 AM   #16
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Not so. I learned something from your post.
Thanks, but I believe you already use premium.

Last edited by mishar; 03-26-2014 at 11:01 AM.
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Old 03-26-2014, 07:20 AM   #17
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I'm guessing just the mere fact that "any" fuel was added (after the car had been sitting for an extended period of time) most likely stirred up whatever residue or trash that was in the tank and allowed it to start.

While I've not experienced this with an Audi, I have experienced with one other car, and it was definitely dirty gas.
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Old 03-26-2014, 09:46 AM   #18
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Well i used 2 tanks of premium and had the prblem for 2 weeks when my 2nd tank of premium was 3/4 gone I filled up again with 87 and the problem was gone and has not returned in 6 days.

I replaced the fuel filter as well with no success. So right now I'm going with slow firing 93 as the issue. I had this problem only when I put 93( 2 different stations) in the car before and after when I used 87 the issue went away.

This is my 3rd A8L so im no rookie ii have out over 350,000 miles on a8ls. All on 87 with not 1 engine issue.
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Old 03-26-2014, 10:00 AM   #19
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none and this is my 3rd a8l all used 87 with not one engine issue

btw its been several days now and not one start issue since went back to 87

go figure
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Old 03-26-2014, 10:05 AM   #20
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Quote:
Originally Posted by pmdeaver View Post
I'm guessing just the mere fact that "any" fuel was added (after the car had been sitting for an extended period of time) most likely stirred up whatever residue or trash that was in the tank and allowed it to start.

While I've not experienced this with an Audi, I have experienced with one other car, and it was definitely dirty gas.

Thats not the facts .....car was cranked multiple times everyday during the 2 weeks or so I used 93 octane on 2 separate fill ups( tank a week average).

When the 2nd tank of 93 was down to 1/4 tank( about 12 days after issue fill up) i returned to 87 and issue went immediately away.

I Agree it all makes no sense but that exactly what has happened. The issue has disappeared when i returned to 87.
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