C.R.Brown

I'm getting about 15mpg driving in [D] on around town, no expressway casual driving. That seems excessively low to me.

I'm going to recheck my tires from a week ago to rule them out. I also came across <a href="https://forums.audiworld.com/c6a6/msgs/46561.phtml">this post by JWJ</a> mentioning possible Oxygen sensor problems as a cause. Now the car is an '05 with 70k miles and AFAIK is not throwing any O2 errors but cannot be certain. But looking at my exhaust tips that may be something to look at...

<a href="http://img142.imageshack.us/my.php?image=img0124rs1.jpg" target="_blank"><img src="http://img142.imageshack.us/img142/8596/img0124rs1.th.jpg" border="0" alt="Free Image Hosting at www.ImageShack.us" /></a>

J. Patterson

Never changed O2 sensors on our A6 (only 20K miles) but on my 01 TT I had the same symptoms as you at about 75000 miles. No CEL, no codes,
just lousy milage. Especially in town.

I recently replaced the pre cat O2 sensor and mileage has improved by 10--15 % .

Good luck! of course we have 4 O2s on our A6s 2 pre cat and 2 post cat. I would do only pre cat first.

pierreb

tips look fine to me.

gk1

<ul><li><a href="https://forums.audiworld.com/c6a6/msgs/49748.phtml">https://forums.audiworld.com/c6a6/msgs/49748.phtml</a</li></ul>

C.R.Brown

I'll have to check the service history to see if a radiator flush has ever been performed. Although I doubt it.

MP4.2+6.0

If so, that's typical, but a tad rich perhaps. As they go toward tan--rare to see on a newer car--it is leaned out.

From my experience, perhaps the 02 sensors cxould lead to lower mileage is my reaction. But first I would do the air filter, and change it or at least blow it out at about the 15,000 typical mid point of the change cycle. Then I would change the plugs if you are toward the later part of the maintenance cycle (like 25K miles into it or more). Likely more cost effective and much easier labor wise. Then you an look at the plugs too for signs of blackness indicating richness, or maybe oil contamination which could suggest a ring or valve issue. Those could also affect mileage.

O2 sensors are meant to be lifetime now with their auto heater built in. They can affect mileage, but then they have to be dead or pretty messed up. You would expect to see a CEL light at some point if rich, since the rear sensor likely would be seeing a failure--the front one(s) before the cat is the operative one that changes the mixture. More commonly if they go, they just get slower to react early on.

C.R.Brown

I guess as a diagnosis list I'm going to...

1. check vag-com for any codes or info
2. check air filter/replace
3. pull a plug and check/replace

further...
4. replace coolant
5. replace front O2 sensors and then rear

MP4.2+6.0

Coolant--you might be a little early for it, but not too much. The newer stuff is perhaps a bit longer lived than where you are, but it won't hurt, will extend cooling internals that much longer and it's a cheap item. Use distilled water only with it when mixing.

Sensors: I would not replace the rear 02 sensors ever unless you get a specific code. They have no motor or performance related function. Their only function is to monitor the catalytic converter. Basically if what they see is the same as what the sensor in front of the converter sees, then the rear sensor is telling the computer the converter must not be doing anything and it throws the check engine light. Thus replacement absent a code error to a rear sensor is a waste of $$.

By the way something else simple I would definitely do if you haven't is run a bottle of Techron through. If you want to do a bit more, open up the intake boots just in front of the electronic throttle and clean the throttle plate; might smooth out idle performance a bit.

Finally you could look at the MAF and clean it or replace if need be. 02 sensors should correct the mixture for MAF misreads short of a CEL being triggered, but still a fouled MAF means subpar performance and it takes a fraction of a second for the O2 downstream to always be trying to catch up to the MAF upstream. If you have access to a VAG COM and log the MAF reading, as a rule of thumb if you aren't seeing about 80% of the rated horsepower in MAF flow (grams/sec), MAF is probably letting you down. Thus on my C5 with 300HP, I am looking for 240 g/sec up near redline. Indeed I saw that + with a replaced MAF, while I saw only in the low 200's with a marginal MAF. Can affect mileage in that you get deeper and longer into the gas pedal to get the acceleration you want. Definitely is a performance hit of course. If this is a mystery to you, just skip absent a code or at most clean the MAF, but only with an electronics cleaner, specialty MAF cleaner, rubbing alcohol or something similar that dries completely residue free.

C.R.Brown

The last couple of tanks have been at a Kroger but just filled up at Chevron (or at least a station that switched over to being a Chevron). And I ran a bottle of fuel line cleaner, Gumout, in my last tank.

I'll try to check all these things tonight. Tomorrow I put 900 miles on the expressway.

C.R.Brown

the filter really needed to be replaced but didnt have one prior to my trip so we blew it out with an air compressor. Thanks to mtm_speed's help at Ringtec we pulled a spark plug but it looked fine. According to him the coolant isn't an issue as the Audi stuff G12 (iirc) should last 100k. We also added some special fuel additive I forget the name. X44 something?

Anyway the drive from Atlanta to Grand Rapids was smoth as silk. Left with a full tank and added 17 gals. outside Lexington, Kentucky. Averaged 27mpg with an average speed of 70mph with my cruise set at 80.

Using only 30 gallons of gas and costing about $150. Not bad.