help diagnosing fuel consumption issue with vag-com

B6/7 S4/RS4
#1

My '04 S4 Avant 6MT won’t get better mileage than 11.5 mpg. Even with conservative driving. So clearly something is up.

I have a new air filter, fuel filter, spark plugs and pcv and still the problem continues. I’m wondering if I need to look at the MAF or front O2 sensor.

I have access to a vag-com and I’d like to troubleshoot this problem if possible. I have yet to do any data logging with it, but I have read the ross-tech how-to. Can someone with experience give me some tips on how to run some actionable logs, and then how to interpret the readings?

My guess is I’ll need to do two sets of logs per sensor. One for the maf and one for the o2. Is that correct?

I’m thinking for the MAF log: MAF, timing pull and rpm?

And for the O2 sensor: air fuel ratio, timing pull and rpm?

Car has JHM chip and piggie pipes.

Thanks

#2

Hey there. Is that based on actual calculations, or is that the trip computer? i.e. what are you getting from a tank of gas from full to almost empty…180 miles?

Also a good idea is to log groups 003 020 021 use turbo mode for starters so guys can see how the car/tune are interacting. Flat road, 3rd gear, 1500-7000 rpm FULL THROTTLE. Do it twice in the same direction on a flat road.

Then you can get into the fuel stuff once that is done. AFR should be pretty solid for making power and should likely SAVE you some fuel vs. stock tune as the stock tune dumps fuel at the car, so that’s just weird to hear you’re tuned and running like this.

#3

I’m calculating the mpg with real world data. Last time the fuel light came on I went to the gas station I filled 14.07 gallons and the odometer read 160.6. Fuel light just came on again, and it read 160 again. :frowning:

I’ll do a log of what you recommended on saturday.

thanks

#4

your way off the mark.

First lets get more info. What rims do you have on the car (this can be a big issue) when did you last have your wheel bearings checked.

I would first check for codes. Then check block 032. report that info but DON"T clear codes before you do.

Also check the oil and see if it looks like there is fuel in there. If the car is down on compression it might be running inefficient.

First thing I would look at is the obvious. What are your driving habbits. how far do you drive in one day and what kind of driving are you doing.

Lets get the full picture this will help more.

#5

Stock wheels and tires, no obvious hum from wheel bearings. No engine related codes at all, and the car feels strong on power.

I’m getting 11.5mpg with a pretty conservative driving style. The best I managed was on a road trip (80mph 6th gear all the way, and I got just under 17mpg). I am experiencing higher than usual oil consumption, but I’m hoping it’s just my valve cover gaskets which are leaking.

I’m getting a compression test and valve cover gaskets replaced next week. I just changed the oil, how would I know if there was fuel in the oil? I still have the old oil.

I’ll log block 032 tomorrow and report back.

#6

Get the oil tested. You’re supposed to get a sample midstream when draining it, so if you have it in a container now, I’m not sure how accurate the test results will be. Although I’m sure if you shake the container good, it will help. After all, you’re just looking for fuel in your oil.

#7

Ok, scanned for codes today nothing. I looked at block 032 too, and I’m pretty sure something is off with fuel trim but I don’t know enough about it.

Adaptation (Idle) Bank 1 Sensor 1: 1.1
Adaptation (Partial) Bank 1 Sensor 1: 3.1
Adaptation (Idle) Bank 2 Sensor 1: 1.5
Adaptation (Partial) Bank 2 Sensor 1: 1.6

#8

I don’t know what normal numbers are supposed to look like for the fuel trim but those numbers look fine to me.
It looks like the system is adding 1-3% more fuel to reach its intended goal. Not a whole lot. (But I’m no mechanic so wait for a more proficient person to respond).

You do seem to be a little off the mark though (I’d say by about 20% (±3%) according to my experience). When you say conservative driving, you mean rarely ever going above 3k-3.5k rpms?

Dumb I know but you’ve checked your tire pressure right?

#9

agreed…that’s crap consumption

ALTHOUGH…it’s not really unheard of. Most guys just accept it.

Here’s the results from a poll I did a while back on B6/7 S4 fuel consumption, using the DIS trip computer calculated long term averages (which are about 10% optimistic).

The MT6 average is about 16-17 MPG. That’s probably 15 MPG real world numbers, so at 11 MPG, you’re way off the average.

However, we had 28 cars in the 13 or worse category (which would be real world 11-12) out of a total of 288 so 10% of the cars are in the exact same boat.

http://i375.photobucket.com/albums/oo193/sakimano/Fuels.jpg

#10

I guess I as misreading my fuel trim, looks okay then.

If fuel trim is normal is it safe to say that the front o2 sensor and maf are ok? Or should I log those just to be sure?

#11

Good to see you made it over here Kem.

#12

Still trying to diagnose my bad fuel consumption. I did some more than usual highway driving on the last tank and managed to get a whopping 13.5 mpg. It seems the car won’t do better than 17mpg on the highway with conservative driving.

Just went to 034 to get my valve cover gaskets replaced the valve covers resurfaced (slightly warped). They did a compression test and a thorough tech inspection. No oil leaks, compression was excellent and axles, bearings everything spun as it should.

I’ve replaced air filter, fuel filter, spark plugs and PCV. My fuel trim via vag-com is well within spec so MAF or front O2 sensor seem unlikely. What else could be the problem?

Plugged cat? recently installed piggie pipes should have helped with mileage but it stayed the same.
Fuel pressure regulator? What’s the sign of a bad FPR?
Leaky injector? If an injector is malfunctioning is there any way to diagnose?
Any other ideas?

Appreciate any help with this

#13

fpr is in the filter iirc

try logging block 003 as you drive around/idling and post up the results, load and rpm vs timing goal is what I’d be interested in. I had a vac leak on my S4 at one point, a tiny one that wouldnt affect idle and didnt throw a code, but it was killing my power (like 80rpm/sec) and fuel economy. It showed up in the logs because my idle timing goal was ~1* ATDC (retarded). Fixed it and the car ran great.