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Old 08-24-2008, 01:23 PM   #1
samluke
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Default Fuel Tank Pressurized Vent System Schematic

I have been having a problem with what appears to be over pressurization of the fuel tank.

After a race, if if release the filler cap, there is a lot of pressure and it takes a long time to vent. I have had the trunk floor bow up and difficulty shifting after a while. The shift linkage has been rubbing on the fuel tank shield even though there is lots of clearance.

I am wondering if the excess pressure is causing the tank to ballon and cause the trunk floor and shifting issues.

Does anyone have a schematic of how the tank vent circuit works. I have the manuals and Clarks Garage etc but its not clear. I checked threads but couldn't find anything

I have checked the vacuum lines and the temp valve from the motor back to the control valve and everything seems fine. I blew the lines back to the tank and they seem open.

There's a fuel tank expansion tank and a Porsche test procedure, but it doesn't look like an active device.

Anyone had similar problems and figured out the cause?
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Old 08-24-2008, 01:36 PM   #2
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I would also check that its not the transmission hitting the tank instead of the other way around.
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Old 08-24-2008, 01:52 PM   #3
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Sounds like a problem similar to what happened w/ my 968. After a PCA autocross I noticed that my fuel gauge would slowly rise while driving and appeared full after roughly 20 mins of the car running. When removing the gas cap I heard lots of pressure being released like you describe. Tried the same test as yours but all valves and vacuum lines were clear. Turns out that one of the check valves had broken in the open position and allowed fuel to get sucked into the vacuum canister up front. The valves are supposed to be closed while driving and open occasionally to vent the pressure. I had to replace both the broken valve and vacuum canister so now it's fixed.
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Old 08-24-2008, 03:18 PM   #4
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I agree that my transmission is moving a lot and I am planning on changing to a solid mount, but the tranny hangs and the gap is pretty good. Its only after 20mins or so that I have problems. If it was just a mount problem you would think it would be there all the time.

PorscheG96, do you recall which valve it was? Was it the valve assembly in the engine bay or a shutoff valve near the carbon cannister

I may just replace the whole assembly, I should be able to get an assembly from a parts car, pretty easily.
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Last edited by samluke; 08-24-2008 at 05:18 PM.
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Old 08-24-2008, 07:23 PM   #5
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do you still have the charcoal canister or did someone plug it-ds wheel well.
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Old 08-24-2008, 10:54 PM   #6
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So far its exactly as it left the factory, in this regard. Should I bypass the cannister? If I blow through it, it seems to be open enough.
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Old 08-25-2008, 07:41 PM   #7
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I had exactly the same problem with fuel tank pressure. I removed all of the tank vent stuff and the problem has gone away.
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Old 08-25-2008, 08:19 PM   #8
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I am inclined to do exactly the same. I tested Item J tonight the "shutoff valve" I can blow back through it, which doesnt seem right, so that could conceivably pressurize the tank with boost air?

goldminer how did you configure the system without the vent controls?

I am thinking sealing the connection on the bottom of the throttle body, I would probably solder it up.

Eliminate the other vac line from the intake manifold. and block off the other line.

But what did you do with the lines coming off the tank? I don't like the idea of tank fumes venting into the engine bay.

Thanks in advance.
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Old 08-25-2008, 08:41 PM   #9
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Pull all that crap out all the way back to the tank, install a 1 way check valve in the line at the tank, and you will be good to go. Cap the throttle body vacuum port. Have had mine out for years.
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Old 08-25-2008, 09:02 PM   #10
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I capped the line to the throttle body, pulled the lines off the thermo switch and capped the other line. I then removed all of the plumbing in the engine bay plus the charcoal cannister. I left the steel line to the gas tank intact and uncapped in the engine bay. I've never smelt fumes in there.
On hot days at the track my engine would miss under full boost. Every time this happened the gas tank would be pressurised so I suspect that I had a leaking valve somewhere that was allowing the "blown" air to get back to the tank and interfering with the air/fuel mixture. I pulled everything out and the miss and the tank pressurisation both dissapeared.
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Old 08-25-2008, 09:10 PM   #11
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So which is the vent and which is the return line on the tank sender? left or right?

Whats to stop gas coming out of the tank if you overfill? the vent line on the tank is lower that the filler.

What about fuel sloshing around?
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Old 08-25-2008, 09:35 PM   #12
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There's also a fuel expansion/overflow tank in the C-pillar. I think one of those lines would be going there.
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Old 08-25-2008, 09:43 PM   #13
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Exactly, that would look to be a better place to vent, but that's within the passenger compartment, so there's risk of fumes in the car.

If I could figure out which line is which, I would think just joining the lines together and then vent through the carbon cannister to atmosphere (if it vents there?) would solve the vent problem and minimize the risk of fuel spillage.
I wish I could find a good diagram of where the lines go, otherwise I'll just have to figure it out myself.

goldminer, how did you do it?
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Old 08-25-2008, 09:55 PM   #14
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The schematic in the manual is hard to follow. The Carbon cannister appears to have a "flushing air line" item 6. Maybe I can just open this up to atmospere? and on my car there are 3 lines that join together before going to the sutoff valve.
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Old 08-25-2008, 10:33 PM   #15
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From memory line 6 is open to atmosphere under the drivers floor. I took the canister out and left the steel line that goes from the engine bay back to the gas tank intact and uncapped. That would mean that any fumes would enter the engine bay but I've never smelt anything there, even through an Aussie summer. The belts and braces approach would be to break the line at position 4. I was going to do this but I ran out of time that day and it hasn't been a problem.
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Old 08-25-2008, 10:33 PM
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