B7 S4 Sudden increase in fuel consumption

[andrewcooke71]

I have a 2006 B7 S4.
Recently I noticed a big increase in fuel consumption. *I don't use the on board readings to give me my fuel consumption. *I reset my trip each time I fill up and calculate with how many miles I have done. *I was getting just over 17 mpg and then all of a sudden this dropped to 14 mpg, with no change in driving habits or routes driven. *The distance I can go now on a little over 7/8ths of a tank (15 gals) is about 200 - 210 miles, whereas I was getting 240 on under 7/8ths (14gals).
I have no engine check light and I took it to an Audi dealer who ran some diagnostic and could find no faults of reason for the sudden increase. *They said it was good enough to pass CA smog.
I have noticed that the idle is sometimes quite rough, though not always. *The other thing I saw was that after I pulled up to a traffic light my fuel guage dropped about 1/8th, maybe a little more, and the trip mileage readout dropped like a stone, whilst idling at the light.
Does anybody have any ideas what may have caused this sudden increase?

[kgw]

VVT's? Variable Valve Timing solenoids, usually the intake cam(s), when they get clogged up with deposits from dirty oil, or an engine that is on short trips (doesn't get warm enough to drive out moisture, etc.). When they get really clogged, or die, they will set an engine light.

I am assuming that the V8's have them, btw.


"I have noticed that the idle is sometimes quite rough, though not always. *The other thing I saw was that after I pulled up to a traffic light my fuel guage dropped about 1/8th, maybe a little more, and the trip mileage readout dropped like a stone, whilst idling at the light.
Does anybody have any ideas what may have caused this sudden increase?"