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Discussion Starter #1
I have a simple question and a more complicated question.

1. There are 3 lines that go from the back to the front of the car. One white, one green, one black. I know that these are for. Black is a fuel line coming off the fuel filter and to the fuel rail. White is the EVAP purge line going to the N80 valve. Green line goes to a vacuum port. My question is:

What exactly does the green vacuum line do? I know basically how the fuel tank works. It has valves in it that release vapor to the N80 through the charcoal canister. I know that the tank must maintain a specific pressure to work correctly so I'm assuming the EVAP purge keeps vacuum balanced as fuel is used and heat rises. I just don't understand what the green vacuum line controls. Can someone please explain that to me? (Maybe the leak detection pump?)

2. The more complicated question. Can someone explain to me how the valves in the fuel tank work? I have replaced the valve that was faulty do to the recall when these cars very first starting having "hiccups". It had never been done on mine, so I ended up doing it. Now I don't have fuel spewing out my fender well. However, when the car heats up the fuel pump starts going haywire, I can hear it and have felt it doing it when it happens - it surges and sounds like it sucking up air. I'm pretty certain it's too much vacuum in the tank as the fuel is drawn out and this starves the fuel pump (I have observed on VCDS a fuel trim scenario that supports this theory as it goes lean when it's happening and the computer pulses the injectors to increase fuel). I have replaced the fuel pump with a stock unit and now it has the DW high pressure pump installed so it's not the pump. Which leads me to believe there is a restriction somewhere. When I did the recall I replaced the charcoal canister and blew out the lines (I'm going to blow out the lines again just in case I didn't get it all and that's where the restriction is). Otherwise I have a feeling it's one of the other two valves I didn't replace during the recall. I didn't replace them cuz my understanding is they aren't the same as the one I replaced and I can't just get the other valves. Also the tank is no longer available or I'd just replace the whole thing. Can anyone explain exactly how the fuel tank in the R32 MK5 works? I'm especially curious about the valves in the tank. Maybe this is a question for the Humble Mechanic but I'm assuming someone around here has to know, like a master tech or just a real smart guy like Grogory on MK4s.

Thanks!
 

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Discussion Starter #2
Once again I'll answer my own questions. Yes the green line is a vacuum line to the LDP.

Those two valves in question have to do with the fuel tank filling process to let out vapors when refueling.

So now I have another question that maybe one of you (I doubt it though) can answer. What is the orange valve above the filler neck for?

Also, how does air get into the tank as fuel is drawn out to prevent vacuum? I see there is a filter on the LDP but if I try to blow into the filter it is closed. I can blow from the other direction and it comes out. Yes I've watched the video on EVAP so I know when the system is being pressurized by the LDP it sucks air in using vacuum line and reed contact with a valve inside to check for leaks. I just don't understand how air gets back into the tank to prevent vacuum.

There has to be someone on this worthless site that can explain to me how VW EVAP systems prevent vacuum from building inside the gas tank. There is one line off the LDP that goes to the charcoal canister, one line that goes to/from the charcoal canister to the orange valve on top of the filler neck, and another line that goes to the N80 valve from the charcoal canister. Typically there is an EVAP vent valve but I have assume this one is built into the LDP cuz I don't see any other valve that would let air in to the fuel tank. There is no condesant drain line or anything of the sort...
 

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Discussion Starter #3 (Edited)
So after more digging around I have determined that the orange valve is the gravity valve on this vehicle. What I find interesting is that it must also be a vent valve of some sort. At least to the point where it should remain open unless the car rolls over. The reason I say that is because it's the only way into and out of the tank's EVAP system. One line goes to the charcoal canister from the fuel filler neck which is also where all the valves inside the tank lead to. Then from there I suppose air comes in through the leak detection pump and back into the fuel tank to prevent excess vacuum from building up.

The issue is, when the car gets hot and its hot outside, the fuel pump starts to get all wonky and doesn't put out enough fuel. If I try to go fill the tank during this time it shuts the pump off even when not full. I know it's not a restriction as I have blown out all the EVAP lines and found them to be free and clear with no kinks. The LDP is new the charcoal canister is new. The vapor valve inside the tank is also new (recall done). It shuts off the fuel pump under normal conditions as it should (when it's not exhibiting this problem) which leads me to believe the OSVR - or whatever its called - valves are also functioning properly. This leaves me with the gravity valve. It is not replaceable to my knowledge. The gas tank is no longer available.

My guess is that for some reason when the car gets hot and starts building up a lot of pressure (or vacuum builds up excessively and pulls on the valve) the gravity valve fails in the closed position. Now air can't get in or vapors get out increasing the temps and pressure/vaccum in the tank to the point of over working the fuel pump (brand new DW pump). I suppose my only option is to gut out that valve and see if it fixes the issue. That will leave me in a dangerous spot if the vehicle ever rolls over.

Can anyone (haha) explain to me how exactly that gravity valve works and what negative repercussions [other than fire in a roll over accident] would occur by deactivating that valve?? I want to make sure it doesn't have anything to do with actually stopping the fuel pump from filling the car with gas I don't want to have to guess or have it all come flowing out when I try to gas it up. Wouldn't this valve getting stuck closed give me some sort of CEL. Like an incorrect flow EVAP code or a lean/rich condition form non functioning purge vapor draw?

At this point I'm waiting for the vdub gods to respond. This is a huge issue with this model of car and I personally believe it's one of the many reasons people get rid of them. Just search R32 vapor lock, or hiccups, or anyone who has a story about driving across country and getting to AZ, NV, NM and having the car start to act up. No one to my knowledge has come up with solution except to say it has to do with the PO26 recall. Nope, that gives you a rich condition ad spews fuel out the fender well. Been there done that. This is a fuel starvation issue from an overworked pump caused by either excess vacuum and/or pressure from a faulty valve somewhere.
 
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