Purchased a used 03 a4 a few weeks back and had the check engine light for the o2 sensor. This was my first audi so I wasn't really aware of what it was or what it did. After doing some research I realized it was pretty important so I had it replaced immediately. Before doing so, my car was getting about 19mpg average city+highway which is about right since the car is stated at 16/24 (20 average). I got it replaced and was told by multiple people my fuel efficiency will increase, but now my car is averaging about 12/15mpg still driving the same city+highway mileage. I fill up and it says about 320/340 miles until fill up, but drops to about 260 by the time am almost empty. Anyone know what the issue could be, do you think the mechanic used a cheap generic part or maybe installed something wrong, or is there something I am missing and should take a look at next time I have a service? Thanks

 

Where did you get the work done, dealership?

Yes, O2 sensors can vary in quality and effect on the car

Tune up 101

- O2 Senors , what brand was used?
- Spark Plugs, when was the last time they were changed?
- Spark Wires, when was the last time they were changed?
- Air Filter, same as above.

Spark Plugs should be properly gapped, This is something you can do and replace. I presume you have a petrol engine?

As all of those help with how the engine runs and then comes down to MPG.

Some other considerations.

Is it an ATX? ATX is always hungry compaired to 5 speed
When was the catalytic convertor replaced?
Have you gotten any codes pulled with a scanner?

Hope this helps. Your milage does seem a bit low, and a full tune up may be needed.

Report back... keep us posted

 

I took it to several shops which all gave me really high quotes on the o2 sensor replacement considering the part is relatively cheap and should take less than an hour. I still had to pay about $250 to get it replaced and I am assuming (at least I hope they used the recommended bosch one). When I got the car with the check engine light, I had the deq slip that said the o2 sensor and a couple misfire codes which the mechanic said were caused by the o2 sensor. I had him check everything it could have been for the misfire and he said the spark plugs/wires and everything looked normal and that the engine didn't seem like it needed a tune up. I think maybe the air filter was recommended sometime soon but not necessary, and he said it could have possibly used a fuel injection cleaner as well but he couldn't tell and I had no idea if it was done recently before my purchase.

It is the 3.0L quattro automatic, I don't have any records from previous repairs, but I did take it to a couple shops to have a complete check over and most of them all said the same minor things although unfortunately the last place I took it to get the alignment done said I do need to spend $2000 to get my control arms replaced. I just bought this car (as-is) about a month ago and already have $1000 in to tires and repairs already. It just seems weird that basically one day before getting the o2 sensor replaced I was getting almost 20mpg, and now I am getting 12-15mpg and that is all the guy did! I have no idea, but my "new" car is turning in to a big headache.

 

Funny two other places didn't pick up on the alignment, if the new tire place that put them on didn't recommend one. Check again in four months

As for your mileage. That is an odd ball one.

You have a 66L gas tank. When you are near empty how much gas do you fill up at the pump with? I presume you are using 91 octane as well?

 

First audi I have owned, so again not sure what the norms are for babying my car but I just fill up with regular gasoline which I think was 87-89 octane, don't really pay as much attention as I should I suppose. Should I use the mid level or premium gasoline or does it really not matter if there is no noticeable difference? Also, I fill up and get maybe just about 16gal each time which is roughly 58-60L of gas.

Don't know if it makes a difference but my last oil change (2-3 weeks ago) was with regular oil, they offered full synthetic (they said it wasn't necessary if I didn't want it) but it was maybe $60+ when my normal oil change is free from being a member of a car care plan so I stuck with that. I guess I went in a circle with my tires/alignment as I noticed it was pulling bad to the left, had the mechanic check and he said the tires were shot. Then when I went to get my tires on they said it was because my alignment was bad which is initially what I assumed was the issue. Either way they both needed done so I got it taken care of. I still need the control arms which I posted in another thread about since the guy quoted $1500 in parts but what I found online was only $400 total for the same MPN that was on my estimate from the shop.

Anyways my mpg is still sucking, I drive back and forth downtown everyday for school which makes up about 60% or more of my driving and I still only get maybe 15mpg total. I planned on getting a new exhaust system in a couple months to make my engine sound better, any recommendations on one that will beef up my mpg at the same time?

 

Good on you for searching online for replacement control arms. I use ebay for a lot of stuff for my cars. Name brand stuff, but with good discounts compared to buying here in town.

As for the alignment, get one if you haven't yet. Eating through new tires sucks. I was recommended to get an alighnment on my very first used car, I didn't I did a balancing instead, not knowing that was a dumb idea. Anywho, lack of alignment cost me about 50K of life OFF the tires. So, unless the tires are showing some signs of wear, I generally will get an alignment done once every 1.5 years - preventative maintanance isn't always a bad thing.

So lets talk about your MPG - 91 Octane is recommended for your engine. The ECU that chats with the O2 sensors and all the other bits and bobs is disigned to run optimilly on 91 octane. You get the best performance and mpg with this. Running anything less, will cause the engine to act sluggish and get poor mpg.

Another way to look at it, would you eat BigMacs as your normal meals and expect a good workout at the gym? Nope. Same goes with the gas.

I've had enough cars to know that cutting corners on gas isn't wise. So, hope you don't take my rant as a chastizing Just passing on good experience. And you do have a sweet ride to fuel up with!.

As for the exhaust. Headers, CAT and catback are the main items.

Borla has some nice catbacks, nice sound.
OBX catback, good quality, don't know about the sound
Magnaflow - another popular brand, nice sound
Greddy - another very very nice sound and brand I've heard of

I don't know if you can get your hands on a Brullen catback, but I have one on my mazda mx6 and it makes my friends sign happily when they hear it.

Highflow CAT - that is magnaflow territory, probably some other decent ones.

Headers - I am not aware of any aftermarket headers for the A4, could be wrong.

At the end of the day for the exhaust, depends on what you want to spend.

catback's - $500-$900
HiFlow Cat - ~$250

Use Youtube to figure out the sound you like specific to the brand. This site will also cover the feedback on sound and performance gains. It would be something to consider to get your car Dynoed to get a before and after HP after your mods.

Also, you could consider changing out your airbox for a short RAM intake or move to a CAI. I don't have experience on A4 RAM or CAI mods, the site search can answer that.

As you just got your audi, fix all the small stuff first and spend money on the big toys after.

I am buying my wife her first Audi this weekend. Got a killer price on a 2001 A6 Avant 2.8L Wagon. She's asking me to black out the rims, possible drop etc. Told her I will fix the small stuff first then toys later.

 

if it's needing alignment then there's your mpg drop. if the wheels are not straight, its parasitic drag as your trying to force the wheel to go straight.

also i agree with twopedalwarrior on the type of gas. due to the design of the engine, 91-93 is optimal for performance. if you run 87-89, the ecu monitors knock constantly, and once it detects knocking thats abnormal or outside the range, it starts to pull ignition timing. thus giving that sluggish feeling and loss of power/performance.

also 87-89 is not as "clean" as 91-93. there have been studies on the actual octane rating of the 87-89 and in the real world it actually varies from 84 and up.

 

Thanks for all the great advice, I am an avid ebayer with 80% of my non-essential buying as it will usually save me 25% or more off retail for the same item in stores. The only thing I was concerned about is cheap aftermarket parts claiming to be OEM and then they don't last long at all and I have to replace them once they brake in a few months.

Got my alignment done the day after I got my new tires, and that is how I found out about the control arms being shot, I am assuming this is another cause to my mpg going down although after the alignment it went lower than before the alignment strangely enough. I figure if I save about $1000 on my control arms from ebay, I will definitely put that toward an exhaust system.

Since I am in college and this is my first real car to be able to enjoy and customize to my liking, I am unaware of any local shops that offer the services I am looking for, not just to sell me expensive parts and services, but to be able to point me in the right direction for value/performance. I am in the Portland area which has a ton of local mechanics, but not any I am familiar with do performance upgrades and customizations.

My tank says about 80 miles left, so I will try filling up next time on mid or premium and see if I notice a change. The only thing that had me worried was my sister has a siAudir audi, I think 04 A4 1.8t if I am not mistaken, and she recently had a check engine light because her o2 sensor was reading that her fuel mixture was too rich which she said might have beeoctanecatane in the fuel or just a bad o2 sensor like what happened on my car. Should I wait until basically empty to fill so the gas doesn't mix, or does it not matter?

One last thing on the control arms if someone reads this that knows. If the MPN is the same, or they are interchangeable, should I just go ahead and get the parts online and save money now? I get the rest of my funds for the repair probably in a month and a half but if it will save me big I will buy the parts now and wait on the labor. I just don't want to miss out, the guy only has 1 left in stock of one of the parts I need.

 

Can't answer about the control arms. Maybe someone else can.

BUT you said maybe use midgrade again. Switch to 91 GRRR. You're going to need to properly flush your tank of the old gas. Your going to mix 91 with probably about 10 Liters (haha canadian thing) of your old grade gas. One more tank after that you can get a real idea on how the engine drives and mpg.

Anyways glad to help out, happy driving. Also, post some pix of your car.

 

I usually fill up with regular (from my last car) which is the 87-89 ocatane I believe, never paid that much attention so hopefully when I go to fill up tomorrow or Friday it will be posted easy to see if mid or premium is 91, or I will just go with premium regardless if I don't see it shown anywhere.

I haven't been on this forum long, maybe a month since I got my car so I don't know if others can see my car I added to my garage or not? I have a pic there if others are able to view it. Haven't really had the time or weather around here in Oregon to get a decent shot of it, but I am hoping to be able to do that sometime this spring if it ever stops raining.