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Old 12-25-2009, 10:54 PM   #1 (permalink)
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Default Vacuum in Fuel Tank

Hi Guys,
I started to notice this problem when driving. My fuel gauge goes down from full to empty in a matter of 15 minutes. First I thought it was the level sender problem, but to my shock was the top of the fuel tank had collapsed, pushing the sender arm downwards. When I stopped the car, everything goes back to normal.

I discovered that there is some vacuum developed through the gas cap. If I close it tightly, then the problem will occur and my rpm is unstable. So from now on I leave the cap loose all the time, however at the expense of gas vapor escaping ang affects the fuel economy.

My mechanic told me that the EVAP purge valve is clogged. I hardly believe this, as I thought the valve's purpose is to release vapor from the tank, and not allowing any air sucking into it.

I'm reluctant to dismantle the whole fuel system at this point of time. The problem had permanently deformed the fuel tank.

Please.. any ideas is appreciated!
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Old 01-02-2010, 08:03 PM   #2 (permalink)
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The purpose of the valve is to allow vapor to be captured by the charcoal cannister instead of escaping into the atmosphere. Then as the engine runs it pulls the scavanged fuel out of the cannister and uses it to provide power. Listen to you mechanic he seems to have a correct understanding of the functioning of your Audi fuel system. If your tank collapses you have a serious problem with proper venting. Fresh air needs to be allowed in and fuel vapor needs to be captured when it expands. Fuel vapor is not something to be cavalier about for your own and others safety.

Last edited by MonroeS4; 01-02-2010 at 08:07 PM.
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Old 01-03-2010, 06:18 AM   #3 (permalink)
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Thanks Monroe S4. Unfortunately this is not an easy task for me i.e. I probably have to take down everything to the ground and inspect the connections. I couldn't find any source to refer to on the EVAP system for my A4 model 1.8 ADR (non-turbo).
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Old 01-03-2010, 10:55 PM   #4 (permalink)
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Not sure, but I think there should be two reinforced hoses to and from the pump to the regulator/injector. A third hose is the vent, which may be a weaker type. Perhaps you can view the area in front of the tank and perhaps identify these hoses. If you can locate the vent hose going to the vapor cannister and find a connection from hose to steel tubing. Try opening this joint and see if then you can determine where the clog is.
I'll see if I can find more info on the vent system for the both of us. There may also be a vent/valve in the trunk area. My '88 Audi 90 had one up behind the rear seat, that failed open and filled the trunk with fumes. On that car the tank is verticlly mounted hehind the rear seat.

Last edited by MonroeS4; 01-03-2010 at 11:03 PM. Reason: additional info
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Old 07-03-2010, 10:34 PM   #5 (permalink)
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Hey I found the root cause! Underneath the the driver's seat there's a tubing sucking air from the atmosphere into the charcoal canister. Right before the canister there's a valve (inside the wheel arc liner). I took it out, blew compressed air and a lot of dirt came out. That valve was plugged all this while!

I'm really relieved now but does anybody know the valve's purpose?
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Old 07-08-2010, 08:44 AM   #6 (permalink)
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Quote:
Originally Posted by maxiz1 View Post
Hey I found the root cause! Underneath the the driver's seat there's a tubing sucking air from the atmosphere into the charcoal canister. Right before the canister there's a valve (inside the wheel arc liner). I took it out, blew compressed air and a lot of dirt came out. That valve was plugged all this while!

I'm really relieved now but does anybody know the valve's purpose?
The fuel tank contents expand and contract with temperature change. As well fuel is removed while the engine is operating. To compensate for this there is a vent which allows excess pressure to exit via the charcoal cannister. There the fuel vapor is temporarily held for use later by the engine. Excess vacuum is prevented by a one-way valve which allows atmospheric air into the tank to keep it from collapsing. This is the valve you located and found to be clogged I believe. When you fill the tank as an example the fuel came from underground and is lets say 60 degrees. Now you drive down the highway on a 90 degree day, the asphalt is 100 degrees and the heat now causes the fuel in your tank to expand 5%. This causes the vapors into the cannister where it is absorbed by the charcoal. As you drive and consume fuel and later when it gets cooler the fuel contracts. Now the vent allows air back into the tank to maintain a normal pressure. Great fix locating the valve and line.
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Old 07-08-2010, 08:46 PM   #7 (permalink)
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Thanks MonroeS4. You're the best!
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Old 06-07-2011, 03:15 AM   #8 (permalink)
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Re: vacume in tank

--------------------------------------------------------------------------------

Quote:
Originally Posted by guy4ever
Hi Maxiz1
Please can you guys assist me.
I have the exact same problem.
I am having the very same problem with my car.
Please can you in detail since im not too mechanically minded
in locating the valve that needs to be cleaned also the charcoloal canister.
were about under the seat is it found and how do i get to it ?
I am contacting you from South Africa.
I own a Audi A4 1.8 1998 ..
my email address is moodley.mellin@transnet.net
mail me directly please

Regards
M
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Old 06-09-2011, 05:31 AM   #9 (permalink)
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can somebody please answer my question ..thanks
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Old 06-10-2011, 08:24 AM   #10 (permalink)
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Guy4ever, Maxiz1 said "Hey I found the root cause! Underneath the the driver's seat there's a tubing sucking air from the atmosphere into the charcoal canister. Right before the canister there's a valve (inside the wheel arc liner). I took it out, blew compressed air and a lot of dirt came out. That valve was plugged all this while!" Try sending him a private message to get more information. What year, model and engine do you have as they may be located in different places. I personally do not know where the valve is.
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