Air buildup in gas tank - evap not venting?

[A4 Quattro]

Usually around this time of year, I have the problem where the gas clicks off every half gallon or so, though that only lasts for a couple weeks for me, and my car magically fixes itself. This time around, it's different. I filled up totally normally a few days ago. After driving maybe 20 miles and coming home, I noticed what sounded like liquid bubbling from the back of the car. Assuming it was the gas tank, I went over to the cap, and loosened it to release the pressure. There was so much pressure in the tank that the gas cap literally flew off into my hand as I started to loosen it. I've had little bits of air buildup before, but never like this. Consistently now, every time I drive (and overnight, when not driving), I have to purge the air from the system before driving the car. My gas tank is also inaccurate now, since the sensor is reading the gas plus the additional unvented fumes in the tank. So my gas level goes up as I drive (win/win, right?).

My searching lead me to one main conclusion, my evap system is not venting as it should. And since there is no leak in the system, I have no codes for it. Makes sense, so where do I go from there? I've read up on all the trouble filling gas threads, hoping someone would have experienced this before, but to no avail. I got the car up in the air the other day to take a look. The passenger side fender well makes a "clicking" noise when I purge the air through the gas cap, so I started here. No obvious filters, nothing that really appears to be clogged, so I moved on to the other side. I pulled the filter out from that side and cleaned it up a bit, nothing crazy in it. I let the car sit for a few hours, and checked. No air. I drove it later that night. No air. I drove again the next day. This time, it released enough air to shoot the cap off again. My next step is to pick up a new charcoal canister (maybe a cheap one from a junk yard), to see if that helps solve the problem, and see about cleaning mine out in the process. PS: is there a difference between the charcoal canister in the 3.0 vs the 1.8T? I can Google that, but if anyone already knows...

Any other suggestions/things to be checking? I was expecting having trouble filling properly, but not this.

[old guy]

Here's an old post that explains how the tank venting system works. Clicky click

It sounds like you either have a clogged charcoal canister or a stuck LDP. When you are driving the car the N80 evap purge valve should be purging the tank of any pressure build up. Once you turn the car off the LDP opens up and acts as a vent for the fuel tank. The tank pressure pushes through the charcoal cannister, gets scrubbed of any hydrocarbon fumes and exits through the LDP located underneath the drivers side rear wheel well liner.

Here's how you can determine if its the canister or LDP. Simply disconnect the hose connecting the canister to the LDP and see if you still get a pressure build up.

If you turn the engine off and there is no pressure when you open the gas cap you can still determine if it's the canister or LDP. With the engine turned off you can disconnect the line connecting the N80 evap purge valve to the gas tank. Use an air nozzle to apply pressure to the hose. Apply no more than 2 or 3 psi. You should be able to hear the air escaping through the LDP. If not, disconnect the hose from the canister to the LDP. If you then get air the problem is with the LDP. If you don't get any air the problem is a clogged canister.

If you have access to a smoke generator you can perform the same test and get a visual result.

[Gheorghelucian]

Have you solved your problem? I have the same issue. My car-1.8t Quattro (a6 c5). N80 valve working. Carbon canister not clogged (allows air to flow through it). I don't have LDP in drivers side wheel housing(is that only for us spec cars or do I have it in other place? My car is Quattro). I have some sort of check valve (4b0 201 246 a) on the line that comes from tank to charcoal canister. Removed and tried to check that. It does not let me suck or blow air on the end that comes to charcoal canister. It DOES let me blow and suck air on the end that goes to tank. The metallic tube that comes from tank does not let me suck or blow air (either with tank cap on or off)-afer I disconnected the 4b0 201 246 a check valve. What else should I check?

[old guy]

The A6 C5 1.8T wasn't available in the States so I am not familiar with how the evap recovery works on that particular model. I did look up the parts diagram and it doesn't look at all like the A4 B6 1.8T. There is no LDP listed. Clicky click®

Sorry!

[Gheorghelucian]

I solved my problem!!! There îs a notch in gas tank neck that should be pushed inside by the gas cap. For some reason, my gas cap was not pushing that notch. I replaced that valve.

[old guy]

I solved my problem!!! There îs a notch in gas tank neck that should be pushed inside by the gas cap. For some reason, my gas cap was not pushing that notch. I replaced that valve.

Glad to hear that you figured it out!

Thanks for the follow up