uglybread88

Hello all. Owner of a 2004 A6 3.0L, just about 78,000 miles and I have some recent fuel economy concerns. I've owned the car for less than a year now and have noticed a significant drop in MPG (according to the dash.) I know that this isn't the most reliable source so I'm currently in the process of calculating an actual MPG using a good old pencil and paper.

Anyway, I've been trying to be much more hands on and been doing small jobs myself. I recently changed the oil, oil filter, spark plugs, air filter, tires, alignment, had a set of new flex pipes welded in, and did the front motor mount (snub mount) myself. After all of this, I seem to have somehow lost some MGPs?!? Currently averaging 14MPG with local city driving when I remember getting at least 17-18MPG when I first purchased the car. Again this is based off the display on the dash, so accuracy is probably not spot on but I'd expect it to be close. According to the readings, when accelerating through 1st and 2nd gear, I'm getting a whopping 6MPG!!!! This just doesn't seem right. I'm no speed demon and have been trying to baby the car as much as possible.

Really hoping that I'm not stuck with this horrendous mileage and that I'm missing something to help improve my economy. I've had suggestions to take a look at the o2 sensors but am not sure where to begin with that or if that is even a feasible solution. Something about dumping more gas into the engine then I need due to a faulty sensor or some bad connection. No check engine lights though. Could purchasing a VAG-COM and getting some readings help? Not sure if no lights means no problems or if I could get some new info regardless of what the dash says.

Also after doing some reading and STF I've thought about cats causing some loss in efficiency (**fingers crossed** please don't be the cats, please don' t be the cats ). Might make sense considering I started to notice the drop in MPG after the new flex pipes were welded in. You can imagine my surprise when one fixes an exhaust system and gets a WORSE gas mileage! Ugh.

Apart from the gas stuff, would also like some opinions on timing belt/accessory belt. I'm getting a high pitched noise from that area (I know that there is a lot of stuff going on there) and am curios if anyone has any suggestions on where to look first/if its something I can attack myself.

Thanks in advance for the input.

SloopJohnB@mac.com

high pitched noise is probably a failing bearing on an idler pulley or accessory. You can often isolate it with a long screwdriver held handle to ear and tip to area/bolt holding pulley or on the accessory itself or use a mechanic's stethoscope.

Mileage drop could be 02 sensor but generally as long as the MIL isn't on they're ok…they can get lazy but still function…about the only way to be sure is to scope the sensor signal and look for 1Hz function at 2K rpm (three crossovers minimum). Unless a cat has collapsed and is backing up pressure, shouldn't affect mileage.

You could always disconnect the cats and see if mileage improves but you still need a pre-cat 02 sensor to properly run the engine…somewhat difficult if the pre-cat sensor goes away when you disconnect the cat. Another way is to measure back pressure prior to the cat, hopefully accessing with an unused bung.

uglybread88

Thanks for the input Sloop. Definitely have seen some movement in the front accessory belt and bearing sounds like the solution. Think that it would be easier/not too much more expensive to replace the whole pulley compared to the bearing?

As for the o2 sensor, not sure how to check the signal. Very interested to find out how and see if this is why my car is eating so much gas. Thanks.

SloopJohnB@mac.com

Depends on how it's made; the idler pulley bearings come with new pulleys, so technically the pulleys/bearings are replaced. It could also be a serp belt tensioner pulley bearing…unload the serp belt tensioner, pin it, then try spinning the various pulleys and check by feel for play (should be essentially zero) and noise (should be none) and shouldn't be 'grainy' or rough spinning. Also check the water pump 'feel.'

Even though the TB was done 30K ago the PO might not have replaced the WP (he should have) or the WP might be going bad.

You can google how to test an 02 sensor. I managed to get hold of a relatively cheap LCD graphing DVOM and it works like an oscilloscope.

Extech mini multiscope II model 381265…damn…it's discontinued. IT was a very good buy…nearest Fluke is about $900.

Try this one…ought to do the trick. Helluva price.

http://www.adafruit.com/products/468...FS8OOgoduB0A6w


You can also search for iPhone or iPad oscilloscope apps….
Here's one for $10 but you need to buy an A-D adapter, oh read about it.

https://itunes.apple.com/us/app/osci...388636804?mt=8

uglybread88

Finally figured out my MPG problems!! So I ended up driving home the other day and had a check engine light pop on with some bad misfiring going on. Thankfully I had just ordered a vag-com and it was delivered that day!! (What luck)

So I plugged it in and diagnosed my issue. Had a misfire in cylinder #2. Had this happen once before in cylinder #5 so my first guess was coil pack. Got in there and moved the coil pack from #2 to #1 to see if the problem would follow and it did. Got a ride over to Autozone and got their last pack (today just seemed to be my day) otherwise I would have replaced them all. Is it normal that these coils are going after only 78,000 miles?

Anyway one quick $16.50 fix later and my MPG is up big time. Also feeling a much smoother ride overall. Thanks again for your help Sloop. Still going to investigate the o2 situation just for some peace of mind. As for anyone else having some economy issues, check out the coil packs! Have had two go since December and I would like to think changing them all is a smart move.

Last couple things. Is there anyway to check the efficiency of these packs? Any warning signs that they are on their way out or is it just best to replace them all? Are there better packs that have better performance? The only one Autozone had was a Duralast but I imagine that there are higher quality packs that I can check out. Thanks again everyone!

SloopJohnB@mac.com

Like most electronic equipment, an oscilloscope often provides insight into their operation if not their remaining life. Since you have a new one you can scope it and save the trace…I would recommend amperage draw and voltage (although that is more a function of spark plug gap and condition but at least it's a baseline with new plugs…) and then compare it with your other coils. There's a lot of information online as well. If you don't have a scope there is a neat app out there for iPad...