erik_plus8

2018 S6 Avant, 47000 miles. With full, or close to full tank, when maneuvering at low speeds, there's a slight fuel smell in the cabin. Of course I suspect something leaking on top of the tank when fuel is sloshing around. Before I go ripping my back seat out, just wanted to check if someone else has experienced this and knows exactly where the problem is? Thanks in advance!

hopkinsd21

I have an S7. I look at S6 forums too as the cars are virtually the same, just different body shapes.

I have something similar too that I have been chasing for nearly a year now. I have a theory and we will be trying to solve next year. We have had the EVAP canister replaced, several hose near the HVAC Fresh Air Blowers, have left the car with a VERY reputable indy shop, to no solve.

The fuel smell is only when the HVAC system is running. When we recirculate the HVAC, then no smell.

From lots and lots and lots of reading, reading about other makes and models with similar issues, all point my theory. Would love anyone to poke holes in it.

My theory is, based on where the HVAC Fresh Air Intake is and there the downpipe are, and given I had to replace the upper O2 Sensor recently, the flex portion of the downpipe has failed. It is known that this can happen as these flex portions are tightly interwoven metal mesh providing support of when engines may have larger flexing. This can occur on small 4 cyl to typical large v8 and bigger motors. Because of where all these components sit, the fuel smell is happening here at perfect location and moment that I am getting the fuel smell there.

I cannot locate and find the fuel smell anywhere else, while the engine is running.

We want to replace the full exhaust anyways, so we will start with downpipes first, before I spend money on the full system, and if that solves it, Amazing.

Chasing this down has been a major pain. I have read some horror stories with the 4.0TT fuel smell, I am praying its not the latter of sone stories I had read.

Not sure if this helps and when we replace our Downpipes, I will follow back up.

Avantly

I have the same problem on a C7 S6, how much ethanol is in your fuel? If the commies are putting ethanol in your fuel, look up the Audi TSB regarding fuel smell vs. ethanol content. You can remove the ethanol from your fuel by pouring an appropriate amount of distilled water into the fuel container, then draining off the pure gasoline after the water bonds to the ethanol. Then you take the sludge and dump it into the ocean and send a selfie of yourself doing it to your local environmental protection agency office.

hopkinsd21

@Avantly do you know what TSB this is from Audi? I search Google so many ways and cannot find the TSB you are referencing.

Thank you

Avantly

I can't find that particular TSB with a google search either and I don't have access to my service laptop right now. In general you should read through all of the fuel system TSB's for the 4.0T, there are quite a few of them. Common failure points are the HPFP leaking fuel into the crank case, fatigued high pressure lines leaking externally, and some others. One specifically mentioned ethanol bearing fuel, I believe the concern was with fuel that had >10% ethanol, and was rather recent, maybe from 2021 or 2022. I'll see if I can find it later this afternoon.

erik_plus8

Thanks for this info. Please let us know when you tried that.
I get a bit curious by the "horror stories" - do you have any links or can give any pointers, please?

hopkinsd21

Thanks @Avantly. I would greatly appreciate it. I have read nearly 80 pages of NHSTA reports and TSBs with no success.

I have replaces the EVAP canister, both HPFPs and have gone through the fuel lines with a very reputable indy shop and no solve yet.

Look forward to what your find. Thank you so very much. This has been puzzling.

Reading across other Makes and models with 4.0 v8 TT, at this point either leaky fuel injectors which means removing the engine or downpipes. But open to read and learn more.

Mirgy

Hello all,

My thoughts are keep it simple.
I can’t talk about my C7 S6, but my S500 Mercedes had a similar problem.
It turned out to be the fuel pump collar ring, under the rear seat.

It had cracked and when the tank was full, would allow small amounts of petrol to leak and sit on top of the tank
which then generated the smell inside the cabin.

The smell was strongest when near the rear driver side door and so I suspected the evap canister as well.
Luckily, I have a clever and honest local mechanic who lifted the rear seat and showed me the problem within a few minutes.

If the tank on your car sits under the rear seat, it is a LOT less work to lift the bench and have a look at what is going on under there
rather than removing exhausts and engine bits.

In my case, the HVAC circulated the air inside the cabin and brought it forward to me, generating the smell, it had nothing to do with the engine bay or other compartments.

For reference, I live in Abu Dhabi. The HVAC is ALWAYS on and is often on recirc for a few minutes after you get back into a hot car.

Good luck.

Avantly

Mirgy, very interesting! Did you smell it all the time, or just intermittently? Did it depend on whether the tank was full or not, and did you have EVAP leak codes? I would expect that you'd have EVAP small or major leak detected codes with a cracked sealing ring.

hopkinsd21

Thank you @Mirgy . I will check this out. It sounds a little similar.

The fuel smell I have, when the car is sitting and idling (at a stop light, or ordering a Starbucks at a drive through) with the HVAC on, heat or AC. The fuel smell will come through the cabin. If HVAC is off, no smell in the cabin. If HVAC is on and recirculating, the fuel smell does not enter the cabin. No fuel smells in the back of the vehicle (trunk/boot)

If you stand outside the vehicle, you cannot smell the fuel.

This is why my theory is around the downpipes at the flex points. I am going to go back through all of the hoses and double check. I am all open to anyone's suggestions.

Car starts fine. Took a 200 mile trip, average 23mpg. Again as threads get long... already replaced the EVAP canister, both HPFPs, and have gone through hoses. Cannot find this fuel smell in the cabin.

Thanks for the feedback.