Car: 2005 Audi Cabrio S4
Mileage: ~55k

We've been fooling around with different sets of wheels and ride heights. While doing so, I figured I would leave the rear fender liner out to speed up height adjustment. BIG MISTAKE!

This past Sunday, wifey and I were driving home from the Dustoff show in Seekonk, MA (about an hour away). It POURED on the way home. We didn't think anything of it until we got near home and noticed the fuel gauge was suddenly reading FULL following our two days of long trips... not right! When we got home I ripped out the rear seat, and checked to make sure the senders (and not the cluster) was the issue by taking the fuel tank access panels off and unplugging the harness on both sides to see what the fuel level needle would do. I knew this would throw a couple codes but no matter, I have VAG-COM. Sure enough, they went down to zero when I pulled the plug. Exhausted from the day and tired of being curled up in the back of the S4 Cabrio (I'm 6'5") with the top up and door closed (still raining) I quit and went inside to read up on this stuff.

At this point we figured one of the fuel level senders was toast. No big deal. Not the first time we've had this issue, but first time with this car. So I went to the gas station the next day and tried to fill it up figuring we could just ignore the fuel gauge and use the odometer until I got around to fixing the issue. Upon hearing a HUGE LONG HISSS out of the gas tank as I removed the gas cap, I started getting worried. Sure enough, I could only put in a few gallons before the pump clicked off due to high pressure in the gas tank. Poop.

I read up on the evap system and how it works and came to the conclusion that if I can't fill the tank at the gas station, either somehow the leak detection pump (LDP) was faulty, the charcoal canister was clogged, or somewhere there was a blockage in the lines. Around the same time, I realized the common factor... the missing fender liner and plumes of water shooting up into the LDP.

I took out the LDP and inspected it. I blew through the paper filter which was surprisingly still operational, but found a bunch of water in the electrical plug.
I borrowed my wife's "heat gun" (blow dryer) and dried out the connection and reassembled. Not yet fully understanding the system, I started the car back up, ran it for about 10 minutes, and tried to see if more pressure had built up in the fuel tank. HISSSSS! Yup.

At this point I started taking everything apart suspecting that the water not only got into the LDP, but made its way down to the Charcoal canister. Yup. I found water in the line between the LDP and charcoal canister, in the cylindrical filter attached to the charcoal canister (only some B6 have this apparently) at both top and bottom. I physically blew through all these and got as much water out as I could (lightheaded much). I removed the charcoal canister disconnecting the two lines on the passenger side of things (the big one from the fuel tank and the little one that allegedly leads up to the N80 in the front of the car. I started the car up, ran it for another 10 minutes and this time there was no pressure in the gas tank. GREAT SUCCESS!

Following the steps outlined in this awesome thread (thanks guys!), I deleted the carbon canister, replacing it with a T-connection. Last night, we went to the gas station and filled up. This time it let us go 4 gallons before clicking. Inconclusive but plausible since we've lost track of how much gas was in there. Since we were only in the car a short time, I now know that I should've opened the metal fuel filler door and waited for all the pressure to escape through the LDP assuming that it is functioning properly (I have yet to test this and wifey is out and about today). I can happily say that she just texted she drove about 35 miles and when she checked the gas cap at the rest stop, there was no hiss. progress!

So what I'm going to try next.

0.) Metal fuel door test (gonna have her push the metal door open all the way while I listen for the hiss on the drivers side from the LDP.
1.) Electrical test of the LDP using a Digital MultiMeter (DMM) to ensure correct operation
2.) Electrical and flow test of the N80 Valve (but I think it should be fine if there is no longer a large build up of vacuum in the tank (no hissing upon opening gas cap)
3.) Open up gas tank from access ports under rear seat and take a look at both senders to see whats going on there.
4.) Evap tests 070 & 071 in Vagcom. I only read today that the car has to be warmed up and maybe even moving during these tests.

Unfortunately, I think I already know why the sender(s) aren't working properly. Yesterday when I was under the car, I noticed the passenger side part of the gas tank is BADLY COLLAPSED!!!! My theory: I'm guessing the water got into the LDP, made its way down to the charcoal canister providing just enough water to block the fuel tank vent line causing a large negative pressure in the gas tank and voila, the tank implodes. What I don't get is, isn't the LDP pump supposed to only open during filling at the station, or does it cycle while the car is running at times as well?

By the way, absolutely no codes of any kind were thrown due to this!!!???

Thanks for any and all help! I'm still learning so feel free to correct anything you see I've stated incorrectly. Unlike many on these sites, I'm humble and can actually take constructive criticism.


Since I didn't take many pics yesterday when I did the majority of this, I decided to contribute by posting a pic of the car and cut the the canister into smaller pieces cause I was wondering what the flow is like through there... and it helped with the rage!!! ;-)

The car...


The canister :-D
Seems like most of the flow happens at the bottom of the canister under a thin carpet like substance, above which is the charcoal pellets, above which is a foam insert . Looks like the canister is split into 3-4 chambers. I had lots of condensation in here!

 

what kind of lip is in the front?

 

Front lip is ABT.