Vacuum build-up in gas tank (968)
04-30-2015, 10:12 AM
#16
Rennlist Member
Cloud: The suggest to check things out from the charcoal canister is a good place to start, easy to get to, just inside the inner fender on the driver's side front quarter. I posted pictures of it previously on the 968 forum.
This is one thing I do not miss about my 968, that pesky fuel vapor smell that I could never figure out.
This is one thing I do not miss about my 968, that pesky fuel vapor smell that I could never figure out.
04-30-2015, 12:05 PM
#17
Based on your description of the whooshing sound and the louder fuel pump, I think that JustinL is correct in suggesting that you have a clogged fuel vent line. It is critical that you find the issue and correct it, as it is possible for the plastic fuel tank to implode in an extreme scenario (I have seen this a couple of times); at the very least it would result in a lack of power as the fuel pump struggles to draw fuel from the unvented tank.
The culprit of the above is usually a bit of activated charcoal from the canister that has migrated into a vent line. The #1 cause of this is "topping off" the fuel tank after the gas station pump has clicked off, thus filling the fuel vapor recovery system with liquid fuel and breaking up the charcoal in the canister. However, it is possible for this to randomly happen in an older fuel vent system as well.
I cannot point you to any specific vent line that would cause this, but the best plan of attack is to expose the vent lines at the top of the fuel tank and at the fuel filler neck area, disconnect them one by one and attempt to blast them with compressed air to see if something frees up. Work your way towards the front of the car; the charcoal canister is located behind the left front wheel.
It would be wise to have some extra hose and clamps on hand in case you break some old, brittle fuel vapor hose or encounter any one time use crimp clamps.
The culprit of the above is usually a bit of activated charcoal from the canister that has migrated into a vent line. The #1 cause of this is "topping off" the fuel tank after the gas station pump has clicked off, thus filling the fuel vapor recovery system with liquid fuel and breaking up the charcoal in the canister. However, it is possible for this to randomly happen in an older fuel vent system as well.
I cannot point you to any specific vent line that would cause this, but the best plan of attack is to expose the vent lines at the top of the fuel tank and at the fuel filler neck area, disconnect them one by one and attempt to blast them with compressed air to see if something frees up. Work your way towards the front of the car; the charcoal canister is located behind the left front wheel.
It would be wise to have some extra hose and clamps on hand in case you break some old, brittle fuel vapor hose or encounter any one time use crimp clamps.
Last season I had this same issue - pressurized tank. I found that the vapor diaphragm under the hood (UFO thing next to the driver's shock tower) was ruptured. I replaced it and the problem went away. Then noticed this season that I had a pressurized tank again... and the diaphragm is NOT the problem anymore.
I wound up yanking all of it out and just running the vent line back to the intake. So far no more "PSSHHHHH" when cracking the fuel cap.
04-30-2015, 02:06 PM
#18
Burning Brakes
Thread Starter
i agree that a lot of my symptoms point to a clog in the return line to the charcoal canister, creating a build-up of gasoline vapor in the tank. There's one thing that doesn't make sense to me, though - if the whooshing I heard (which, as I said, went on for what seemed like an eternity, but was in reality probably a good 5-10 seconds) was escaping gasoline vapor, why didn't I get an overpowering faceful of fuel vapor smell when I opened the gas gap? There was no odor whatsoever.
04-30-2015, 04:23 PM
#19
Drifting
I'd still bet on it being a vacuum, not a high pressure. You just came off the track, so you were pulling a lot of fuel out of the tank at a pretty high rate. You would also definitely get a blast of gasoline smelling air coming out of the tank. Either way, a clogged line could cause a buildup in either vacuum or pressure. The length of time the whooshing happened is also a clue as a pressure build up would probably be a faster 'pshht' compared to a tank relaxing into its natural shape.
04-30-2015, 05:21 PM
#20
Burning Brakes
Thread Starter
Justin,
I'm leaning more and more to agreeing with you, that it was a vacuum, not excessive pressure build-up in the tank. The absence of a blast of gas fumes in my face seems to be a pretty major clue.
However, the problem seems to have gone into remission. I drove the car to work this morning, and removed the cap when I got in - nothing. I drove it the 2.5 miles to the track at around 1:15PM, when it was about 80 degrees (only a few degrees cooler than it was this past Saturday when I got the whooshing) and again removed the cap. Silence. Then I did my 25-minute session. I'm a solid intermediate driver - I'm getting where I'm carrying decent speed everywhere, and have on many occasions passed what should be much faster cars. I pulled in, removed the cap, and still not a hint of a hiss. So, maybe whatever what blocking one of the lines has worked its way loose. This wouldn't be the first time something magically cured itself on this car (I wish I could say the same about my mushy brake pedal). I'll keep checking it, but at this point, it seems to be safe to flog, I mean drive.
I'm leaning more and more to agreeing with you, that it was a vacuum, not excessive pressure build-up in the tank. The absence of a blast of gas fumes in my face seems to be a pretty major clue.
However, the problem seems to have gone into remission. I drove the car to work this morning, and removed the cap when I got in - nothing. I drove it the 2.5 miles to the track at around 1:15PM, when it was about 80 degrees (only a few degrees cooler than it was this past Saturday when I got the whooshing) and again removed the cap. Silence. Then I did my 25-minute session. I'm a solid intermediate driver - I'm getting where I'm carrying decent speed everywhere, and have on many occasions passed what should be much faster cars. I pulled in, removed the cap, and still not a hint of a hiss. So, maybe whatever what blocking one of the lines has worked its way loose. This wouldn't be the first time something magically cured itself on this car (I wish I could say the same about my mushy brake pedal). I'll keep checking it, but at this point, it seems to be safe to flog, I mean drive.