RKA

I'm trying to understand a common cause of a fuel pump shut off when refilling at a gas station. The issue is with a 2003 3.0. STFA, I found a bunch of anecodotal evidence (or worse, silly advice about going to a diff gas station), mostly useless, but I did find this post below by Bob. I'm curious to know what the outcome was. Anybody recall the outcome? Has this pump shut off issue ever been resolved/acknowledged by Audi?

It seems like most of the useful content in the archives points to a pressure build up in the system when refueling. So my question is what lines and parts are responsible for releasing the air in the tank as the fuel is dispensed, and where does that air flow (through what channels and parts)? It just just vent right into the atmosphere, it has to go through some method of filtration so fuel vapors aren't released into the atmosphere. I think Bob was on the right track about something being clogged resulting in a pressure build up. I'm just trying to narrow down the primary cause and gain a better understanding, so a friend can take that information to the dealer and get them to actually DO something about this.<ul><li><a href="https://forums.audiworld.com/a4gen2/msgs/732149.phtml">https://forums.audiworld.com/a4gen2/msgs/732149.phtml</a</li></ul>

ModifiedA4

apparently this is caused by topping off the tank past when the pump shuts off.

someone on the b5 S4 forum in norcal had this happen recently.

RKA

I'm piecing together bits of the puzzle here. Here's what I think happens...

1. Insert pump nozzle into tank, causing flap in filler neck to open, which subequently allows air from tank to escape as fuel is dispensed.

2. Vapors pass through carbon canister (same thing as Evap cannister I assume). A line is run to the engine bay where there is an evap purge valve.

3. I believe the purge valve is a solenoid. I would guess a pressure threshold in the evap system causes this valve to open, sending fuel vapors into the intake stream?

I do know the purge valve will set off a DTC if it's not working correctly. So odds are there is an obstruction at or near the evap canister thats causing the issue. Can overfilling the tank with fuel cause that obstruction? Does it just take a while for the liquid fuel to vaporize and pass through the evap system?...or does this issue necessitate replacement of the evap cannister?

I could be on the wrong track here. There isn't a lot of concrete information in the archives to work from, but I haven't looked outside the B6 forum yet. Looking for details on the evap system in other forums will probably shed some more light.

ModifiedA4

there was another post like this i believe recently, though i cant remember which forum...thats the post where someone said it was caused by topping off.<ul><li><a href="https://forums.audiworld.com/s4/msgs/1961615.phtml">https://forums.audiworld.com/s4/msgs/1961615.phtml</a</li></ul>

RKA

The canister was clogged prior to filling. Fuel couldn't drop down into the tank fast enough, and ended up backflowing out onto the ground, and into the canister. What was the canister clogged with? Could be fuel from a previous fill up (overfilled), or could be something else entirely?

ModifiedA4

sounds plausible...if you figure it out, post the answer

Bob Petruska

Bottom line it was the charcol canister.

Now this is easy to determine to make sure it is not the fuel tank. You read my post(link).

The vent system works a follows.

1. The pump nozzle swings open the cover in the fuel filler tube filling hole. The cover pushes a venting switch which allows the fumes(pressure) flow down into the carbon canister. Use your finger to push the cover all the way open to feel the switch opening. Pray that this switch is not bad as it's not replaceable and the tank will need to be replaced.....horror in doing that. See below.

2. The fumes flow through the carbon granules absorbing the raw fuel, cleaned air venting out the canister, through the LDP pump. The LDP(leak detection pump) turns into a vent when the car is turned off.

3. The fumes from the granules are pulled back into the engine by vacuum to be burned off when one starts the engine.

The easiest way to determine if you have a blocked canister is to remove the large tank to canister hose on the passenger side of the canister after you remove the circular plate covering it. Then go fill your car. If the tank fills easily then most likely the canister is blocked. You can also have a defective LDP pump which will not vent. To determine if that is the problem. You need to reconnect the large vent hose just removed, drive the car until fuel is low, and remove the small vent hose on the canister driver's side(this is not the small hose along side the large hose on the passenger side). The large hose is an easy disconnect(push or pull ring, pull out hose). This small hose needs to have the clamp cut off and replaced with a normal hose clamp when done testing. If you can't fill the tank this time with the small hose off, then the canister is defective, if you can fill the tank with the small hose off then the LDP is bad. Change the canister first as this is the most likely problem.

If under warranty the dealer may want to change the fuel tank.....I would sell my car if they did that. You need to remove the following to replace the tank.


- rear seat
- trunk panels
- exhaust system
- rear differential
- rear axles
- sub frame
- drive shaft
- tank fuel pumps
- transfer pump
- sending unit
- all internal tank hoses, lines, fittings.

Hope they put it all back together correctly...not!

My dealer wanted to replace the tank as corporate Audi Tech Hot Line told them this is the problem..defective tank. I proved to them with my experiment it wasn't!

How does the canister go bad!

Topping off the tank at fillup. Especially on a cold night, then parking on incline, during the next day when the temps go high causes the fuel to expand. There is another sensor in the tank that releases pressure and fuel into the canister if the tank pressure ever builds too high while the car just sits there. Small quantity of fuel block the canister. I removed the bad canister and blew though the canister and could not detect an blockage, air flowed very freely but not freely enough as the replaced canister solved my problem. Thus the blockage is so subtle that only removing the hoses proves the problem.

Let me know if you need more help or info?

Bob Petruska

It pulls vapors when engine is running only. Now you may have seen some people leave the car running when filling up and that the fumes would be pulled into the engine and the tank filling should go normally without stopping on cars with filling problems. Well Audi is smart. There is an electrical reed switch in the filler neck triggered by a magnet in the fuel cap that controls when the purge valve selenoid can function. Cap removed, disabled purge valve action. System works well.

RKA

Makes perfect sense!

RKA

The link below provides more details on evap systems. Though not specifically related to Audi's, the info provided is good background info on the components within the system and how they work.<ul><li><a href="https://forums.audiworld.com/s4/msgs/2006543.phtml">https://forums.audiworld.com/s4/msgs/2006543.phtml</a</li></ul>