Crazy fuel consumption - what can it be?

[Ceveman73]

Guys,

Need desperate help here, this is driving me nuts!

Recently bought an RS6+ a few weeks ago, and after subjecting her to the works in terms of TLC and servicing (all fluids, all belts, pulleys, plugs etc) and an MRC health check, I've noticed that my fuel consumption has gone silly.

On a quiet run, including mostly motorways and A-roads, I am getting about 170 miles to a full tank. That equates to like 12-13mpg. Now this is normal driving, not going balls out everywhere, mostly cruising, and I'm stumped as to what it can be??

It was fine until I had all the work done, but since the servicing, it literally drinks fuel.

All I can think is that it's the plugs as I put in Denso Iridium IK22s instead of the NGKs, and I have 2 K&Ns instead of std air filters, but that wouldn't halve the consumption would it?


The MRC diag machine came up with no faults, so it cant be the MAF or anything either.

Totally stumped. Anyone??

Mart.

[bam_bam]

Tyre pressures? Sorry, that's all I got.

[ecain63]

Might be overfueling with the plugs not working correctly. I was advised that you should only ever use NKG plugs in the 6. Other than that, hole in the fuel tank? I know its happened before.

[Daveperc]

Fuel leak somewhere?

[Ceveman73]

All good points, thanks!

Just out of interest, am I right in thinking that I should be getting at least 350 miles from a tank on a long motorway run?

M.

[philroach]

350 on motorways is about right , short journeys , enjoying the miltek etc , sees me averaging 15mpg

[Daveperc]

You should get over 300 miles certainly - I reckon on filling up every 250 or so on normal mileage, and I tend not to run to the very bottom of the tank.

What is the DIS showing for MPG?? If that is also registering well below 20 then it would suggest that it's an overfuelling issue as I think it measures fuel used rather than tank level - not sure though. Having said that if you were over fuelling by 60%+ then I would have thought it would show in the exhaust and the performance.

Is the pattern of usage looking the same? Mine stays over half full for a long time, and then plummets once it passes the halfway mark - if yours is doing the reverse it might be that there is a leak in the tank.

I had an old Austin Princess once (don't all mock!!) which the seam round the tank went - it pissed out till half full, and then was fine - so I only ever filled to half tank and it was fine (till it blew up!)

Dave

[RichRS6]

Yep, 300 - 350 miles depending on how heavy Your Right foot is.

Have You spotted any leaks after it's been stood ?

Can't imagine the plugs would make any significant difference and certainly not doubling the fuel consumption..

[morginator]

Have you tried the sender unit in the fuel tank. Sometimes if I filled up to much it would only show half a tank on the dash, this happened quite a lot.
You may think you are empty and its half full.

[chesney]

Binding brakes or some form of resistance in the drivetrain?

I think MAF's don't tend to show up as faulty on a fault scan but can be read using vag-com to see if the readings under load are ok.

Does sound a bit like a leak as that is horrendous MPG!!! Like somebody pointed out if your DIS is showing better MPG then it might point to a leak.

I take the car feels fine and doesn't show any signs of loosing power? Overfuelling isn't good for the bores!

[Ceveman73]

Thanks for all the tips, great help!

A mate warned me that as K&Ns are oiled, the oil could come off and form a film on the MAF causing it to mis-read. Do we think this could be a possibility in such a short time? It's only done 200 miles since they were fitted.

So so far, possible culprits I think are likely are:

- Fuel leak

- Wrong plugs

- Dodgy MAF

- Dodgy sensor in tank

Any other likely causes?

Cheers,

M.

[TechNick]

Don't know if it’s advisable on the 6 (hopefully someone more knowledgeable can comment) but when I had a suspect MAF on a VAG 1.8T, I was able to test the theory by simply disconnecting the cable to the MAF and effectively running without for a test period...

Believe there's a (minimal) risk of messing up the cat(s) if you were to leave it permanently disconnected (assuming you’ve still got cats of course!), but for a short period it was fine.

Again, don’t know if it’s advisable to try this on the 6, but did prove very useful to me when I had a similar issue on that other car, in at least whittling down where to focus… I was then able to drill down into some of the readings via VAG-COM (and attempt to reset the trims, etc.) until ultimately determining a new MAF was needed (along with a breather tube, as it happens)…

[posterboyj]

get rid of the denso plugs quick

oil filters bad for maf, you can clean maf. with electrical contact cleaner

check for boost and vacuum leaks

[Cornishmoocher]

.... Or mooch has been robbing it at night and going for a little jolly.............. MOOOOHahahahahahahaaaaaaaaaaa

[300gnspitzer]

The fuel consumption on mine dropped on average about 2-3 mpg after a service
when I had K&N's fitted, perhaps thats why they aren't recommended.