dvs_dave

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.

mishar

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.

tashakes

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.

1Wasserwerks

^^^ I was thinking those thoughts also, but I am by no means an expert on these.

snagitseven

Not so. I learned something from your post.

mishar

Thanks, but I believe you already use premium.

pmdeaver

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.

rfmad

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.

rfmad

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

rfmad

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.