Street

Last part first. Car is headed to Audi service.

Was seeing as much as 42 mpg on the highway and now I'm rarely getting 34.

My daily commute on surface roads dropped from 34 to 30. Service advisor on the phone says could be the cold weather (it's balmy here compared to the east) or my driving style since the car is now has 1,400 miles and I'm driving it harder. Not really.

Just tried to push the MPG up on a freeway trip and 34 MPG.

I also noticed roughness in the idle. Maybe an injector failed?

in2dwww

Fuel consumption will vary with temperature and fuel quality (bio-blend and cetane levels). With just over 1k miles on the car, my consumption increased also. This is normal for my Mercedes BlueTec and TDI's I have owned in the past. In the winter, more cold starts, more idle time, even more city driving will impact the results. When the weather gets above 72 F, it's normal to see extremely good numbers (see below).

Driving style is imperative. Don't be a light foot off the line. When at cruising speed, keep momentum by using ACC if you have it or keep your foot planted on the pedal and let the car do it's fuel restriction on downhill runs. The real-time consumption indicator is a good screen to keep in the instrument cluster.

The roughness in the idle is either a DPF burn in progress (slightly higher rpms) or a heat-cycle. The new EGR design and the start-stop feature are probably forcing an artificial load on the engine to increase operating temp as quickly as possible. I've noticed three different idle positions so far, the high and low idles are smooth, and the middle idle happens at random times and hovers near 900 rpm. I'd say it's rough to the ears - it sounds rough but doesn't feel rough. Open the door during that "rough" idle and the harmonics change drastically, which leads me to believe it's a DPF burn.

If an injector failed, it would be hard to start. Since the injectors are electronic, you'd also get a few fault codes.

My Fuelly shows the full effect of winter vs summer:

http://www.fuelly.com/driver/in2dwww/jetta - 30,723 miles logged (55.8 best tank)
http://www.fuelly.com/driver/in2dwww/passat - 7,052 miles logged (48.5 best tank)
http://www.fuelly.com/driver/in2dwww/a6-quattro 1,020 miles logged (35.1 best tank)
http://www.fuelly.com/driver/in2dwww/e320 - 32,911 miles logged (34.2 best tank)

Superfly_A6

Here's mine: http://www.fuelly.com/driver/superflya6/a6-quattro
I've also noticed that the last tank returned 10.3L/100km. not going to worry about it for now. Lets see what happens down the road

in2dwww

WOW!!!

$5.19
Avg. Price/Gallon

Superfly_A6

^^^ Diesel shot up to $1.35 - $1.40/L in the last month = $5.29/Gallon. It is usually $1.20 - $1.25/L = $4.65/gallon. In Canada Gas Prices are higher. For example if you drive across the border to Niagara Falls NY, you can fill up for about a $1/L ($3.78/Gallon) for regular gasoline, where in Ontario, you would pay $1.25-1.30/L ($4.80/Gallon)

Street

Update to OP. Now at 1,900 miles.

Car went in to Service. All diagnostics are good.

Recently doing 24 MPG with mixed city/hwy cycle. Even when I try to stretch the MPG on the freeway, instantaneous MPG is around 30 and dips to teens. On same runs using same throttle, was seeing 42.

My S4 gets the same MPG on the same cycle.

in2dwww

It's going to boil down to driving style. Don't lightfoot acceleration, but do coast more to the red lights and nearly lift off going downhill.

When the weather warms up, you will notice a huge improvement.

Also, check your tire pressures.

in2dwww

Also, did you fill up at the same station again? I've found with my other diesels that I get laggard results from one particular station near me. I filled up my BlueTec and TDI from the same QuikTrip station (Audi considers QT a top tier fuel vendor) and both cars had low fuel economy results.

Shell is usually the most expensive but returns better results (I think the have higher than posted cetane levels). Although a full tank from Shell is probably $4 more than QT, if you net just 2.2mpg more from the Shell, you're at break even.

Strangely, a B10 or B20 fuel will net better mpgs in winter, but be careful... too much or too high of a biodiesel blend and you could have fuel pump troubles.

Here's some reading.. some of which is relevant to Canada, trucks, older models, and some newer TDIs:
http://www.thedieselstop.com/forums/f24/b20-mpg-185507/

http://forums.vwvortex.com/showthrea...7#post79639247

in2dwww

You might want to try BP diesel if it's available in your area:
http://forums.tdiclub.com/showthread.php?t=369830

Street

Thanks for the tips on fuel. But I still fear something else is amiss.

Recent MPG on the same route, same traffic has dropped to low 20's from mid 30's. This route is 2/3 highway and 1/3 surface roads.

I haven't reset the long term trip computer since new and it was staying pretty flat, as you would guess it should. In the last 3 weeks the total MPG dropped 0.5 MPG.

It has been very cold recently. Anyone know how much diesel the auxiliary cabin heater burns per hour?