Fuel Tank Return and stuff

[A4 TSCHUSS]

I was bored so I took my car apart some to try and figure out how all the crap worked. First off that was a big mistake, I took the cap out of the tank on the driver side and about 1.5 gallons of fuel came flying out and dumped down on the floor over the top of the tank. After the fuel stopped flowing out like a waterfall onto my garage floor I then couldn't get the o-ring seal back in place to put the cap back on because it was acting like it was to big to fit back in place, so I had to fight with it for a good 10-15 minutes to finally get it to fit.

Anyway, I just wanted to know how all this crap works.

First, here are pics of the tank







Ok, so now I go and take the cap off on the driver side of tank and find all of this. Now what is all of this crap? There are lines running all inside the tank as you can see Yed off and all that other crap. I thought that the driver side was a return to the tank as well as the tiny hose on the passenger side that connects to the cap. I was then thinking though, how is it a return on the driver side if that hose goes onto the inlet of the fuel filter not on the outlet? So This driver actually flows fuel out of the tank to the feed of the filter as well? I am so lost with all of this crap and how it works. There is an electrical plug to the top of the cap which goes to that arm seen in the pic, so what is that for? There obviously is a level in the tank but is it to turn something on to flow fuel on that side of the tank or is that just a fuel level like on the passenger side? I tried to blow into the fitting on the top of the cap and I couldn't blow anything through it so that makes me think that there is some kind of flow that comes through that but going out of it instead of into it. Does someone truely know what this is and how it works?





I am planning on doing a return on my car soon using an AEB fuel rail so that is why I was looking at all of this.

[A4SoftWalker]

Good stuff... wish u lived near me ur "boredom" could be earning u money and making my car actually FAST.

[A4 TSCHUSS]

Anybody?

[dirtybrd]

All that crap in the tank is to keep the sides evenly filled. I thought I explained this to you.

[F16HTON]

It has been explained many times, on the drivers side you have the fuel level sensor and a vacuum assisted fuel leveling system.

[A4 TSCHUSS]

Yes but what is that line that comes out of the cap on the drivers side? It clearly is going to the inlet side of the fuel filter so how is that leveling anything? It isnt going into the other side of the tank. Those lines inside the tank could be used for leveling, but that one that runs across the top of the tank from drivers side down to the fuel filter inlet on passenger side makes no sense to me, unless that is pulling a vacuum by being hooked to the inlet of the filter and the fuel pump pushing fuel through the filter pulls that fuel as well which then causes something inside the tank to vacuum to move fuel around. But if that is the case then how would all that work if you were to run a "normal" return system since that line from drivers side will no longer be hooked to the fuel filter since there will be one inlet and one outlet on the filter (from the fuel pump to the filter and out of filter to the fuel rail).

So the actual return to the tank would be the dinky ass line coming out of the outlet side of the fitler and going up to the cap on the passenger side from what I think.

[dirtybrd]

That little line is one of the returns the other bigger one goes to the drivers side. Inside the drivers side of the tank there is a small pump that the fuel actuates the make fuel go from one side to the other. How does it "know" when the fuel is equal on both sides I have no idea. Just some in-tank magic! Stefan 20vturbo has his just like mine, one return on the drivers side.

[A4 TSCHUSS]

Yes, but think about this for a second. The line coming from the drivers side tank cap plugs in on the INLET side of the fuel filter (the side closest to the rear of the car) and there is an arrow on the filter pointing FORWARD where the line plugs in, so how is fuel going BACK to the tank through a line that is plugged in on the INLET side of the filter and also has an arrow poing FORWARD stating the flow is toward the front of the car. Then like I said, I tried blowing in the cap connection on driver side with the line unplugged with everyting I had and couldn't do anything, so if fuel was simply going back into the tank through that connection, then I should have been able to blow through that and push the fuel into the tank or make bubbles. That is why I am saying that line is flowing fuel OUT of the tank on that side not IN to the tank.

I honestly think that nobody knows exactly how all that works and is making assumption.

Explain the details above to me if fuel is going back into the tank through that line on the driver side.

[dirtybrd]

How can it? It doesn't connect to anything in the tank. Look at the underside of the pass. side lid, it just has a small 2-3" line on it that goes no where. Maybe theres a check valve in the filter?

[F16HTON]

The feed and return are both on the same sid eof the filter, the line that comes off of the front side (towards the front of the car) is the breather line.

[dirtybrd]

He knows that, he confused on the small line on the fuel pump lid. It hooks to the inlet side of the filter and goes to the fuel pump lid but doesn't connect to anything inside the tank.

[A4 TSCHUSS]

How can it? It doesn't connect to anything in the tank. Look at the underside of the pass. side lid, it just has a small 2-3" line on it that goes no where. Maybe theres a check valve in the filter?

Im not talking about the passenger side. On the passenger side there is that dinky ass line that comes out of the center of the filter on the front and goes back to the top of the cap on the passenger side, that would be the fuel return since it just has that little line that hangs in the tank like you are saying.

On the driver side there is that single line that goes from the cap to the inlet of the filter in the center of the rear of the filter.

[A4 TSCHUSS]

He knows that, he confused on the small line on the fuel pump lid. It hooks to the inlet side of the filter and goes to the fuel pump lid but doesn't connect to anything inside the tank.

No, that little line is on the front of the filter (outlet) not the rear.

[A4 TSCHUSS]

The feed and return are both on the same sid eof the filter, the line that comes off of the front side (towards the front of the car) is the breather line.

I wish someone had an old filter to cut open to see the inside. So you are saying the fuel goes in on the outter connect on the rear of the filter from the pump and the goes through the filter and out the outlet on the outer connect on the front of the filter and then also goes into the center of the filter and then back out the back of it? And that tiny line coming out of the front of filter and going back to passenger cap is nothing (a breather), so there wont be any breather when switching it to a "normal" return system?

[dirtybrd]

Both of those lines are fuel returns. Think about that dinky line on the pass. side. If you were cruising and not using hardly any fuel, do you think it'd handle all that fuel being returned especially with a HO pump. I bet that little line is just to keep the pump cool in low fuel situations and to help keep the basket full.

[A4 Moda]

Wow that just looks disturbing.

So the Passenger and driver side mouse-hole covers, do those go UNDER the seat? or are those portions under the trunk bed?

the pump is on the passenger side right? And you are asking what the driver side does. Or rather, how it works.

I remember for FWD, the tank is all encompassed in the Trunk bed. So with my stupid 6 cubic foot speaker box, I have to remove all my rear seats, including lower and upper seat backs. Then I have to lift out that 300 pounds box.

THEN! I can get to my fuel pump. But I had to cut my dynamat the first time around to get to the stinking pump.

~sigh~

Now it's whining again. I just changed the stupid fuel filter 1000 miles ago.

[A4 TSCHUSS]

I got an aeb fuel rail for free, so it will cost me another ~$80 probably to convert it since I already have the feed from the filter to the rail and will use my stock feed as the return, I just need like 1-2ft of hose from pump to a new filter and 2 or 3 fittings. It was recommended by Unitronics people also to have the rising rate running these kind of power levels and it will make my injectors act bigger too!

[A4 TSCHUSS]

Wow that just looks disturbing.

So the Passenger and driver side mouse-hole covers, do those go UNDER the seat? or are those portions under the trunk bed?

the pump is on the passenger side right? And you are asking what the driver side does. Or rather, how it works.

I remember for FWD, the tank is all encompassed in the Trunk bed. So with my stupid 6 cubic foot speaker box, I have to remove all my rear seats, including lower and upper seat backs. Then I have to lift out that 300 pounds box.

THEN! I can get to my fuel pump. But I had to cut my dynamat the first time around to get to the stinking pump. Just pop the seat bottom out and then take 3 screws out to take off some metal covers and the caps are below them.

~sigh~

Now it's whining again. I just changed the stupid fuel filter 1000 miles ago.

Those caps are under the rear seat. Pump on passenger side and that other contraption I posted on driver side.

[Kodymfk]

On the fuel baskets for both sides, does any one know the size or diameter to the outlets on the fuel hats. Or size of the push clip used to connect to them? Are they 5/16?

[Dake]

Major necro post here, but I'm trying to determine why my tank will no longer accept fuel (it just builds up in the neck, then what's in the neck trickles into the tank over the next ~10 minutes) and it's come down to my understanding of what all these lines do. Charcoal canister is eliminated, N80 is good, LDP is nonfunctional but it wasn't functioning for 3 years and I never had this problem until recently (I will replace it later). There is a clog or pressure buildup somewhere which is not allowing fuel to pass into the tank.

There's a whole lot of ambiguous wording in this thread, so could someone please define what the Red X and Blue X are in my photo. Also on my car there is another tank with a hose coming out of it where the Red O is, could someone tell me what it does too. By process of elimination I think that has something to do with collecting vapors and sending them to the charcoal canister. Thanks.

And really if someone could just give me a step-by-step of what happens when you place fuel in the car. On my 72 BMW you literally put fuel in the neck and it flows to the tank. On this A4 there's obviously lot more to it.

a4gastank2.JPG

[BumblebVR6]

I wish someone had an old filter to cut open to see the inside. So you are saying the fuel goes in on the outter connect on the rear of the filter from the pump and the goes through the filter and out the outlet on the outer connect on the front of the filter and then also goes into the center of the filter and then back out the back of it? And that tiny line coming out of the front of filter and going back to passenger cap is nothing (a breather), so there wont be any breather when switching it to a "normal" return system?

Sent from my iPhone using Tapatalk