Gas Tank Pressure

JonA · 03-14-2008, 06:14 PM

Anyone know the max pressure that you should find in the gas tank. Is there some type of relief valve and what does it relieve at?
I just noticed recently that a lot of air releases when I remove the gas cap, it seems like more that any of my other vehicles. I don't remember the 924s ever doing this before and suspect that something has happened.
On the plus side I guess my tank doesn't have the dreaded cracks on the top cause it's passing the pressure test......

Jon

rudidelange · 03-15-2008, 02:57 AM

A tank must not be pressurized. I have the same problem and removed the one way valve in a Volkswagen fuel cap (fits on the 944), now the pressure in the tank is released through the fuel cap.
I noticed that there is a vent hose coming from the top of the tank to the side of the tank, bottom of the car, which I assume allows pressure to escape to the atmosphere. I think that this pipe might be blocked / collapsed or even a valve in the base of this vent pipe not working.
Prom previous post, it does appear to be a problem on the earlier models.

John_AZ · 03-15-2008, 06:18 AM

In Arizona, the motor vehicle emissions test for my 924S checks the gas cap. The valve cannot be removed or the car will fail the test. I have a small amount of pressure released occasionally when I put fuel in.

John_AZ
1988 924S + 1987 924S

rudidelange · 03-15-2008, 08:02 AM

wow, you live in a developed country, I see occasionally cars driving here with a rag and a piece of plastic closing the gas filler neck. Our cars are only checked once for roadworthy, when a person buys the car, emissions are never checked.

Razorback1980 · Razorback1980's Garage

Definitely check caps in Texas as well. I experience what John does

matthewb · 03-17-2008, 11:36 AM

I am pretty sure your pressure is supposed to be releived through the carbon canister. I often have pressure release through my gas cap when I remove it on my other cars. but I think the pressure is supposed to go through the carbon canister where the carbon can soak up the gas vapors before the air passes out of the car. then the canister is supposed to get sucked out under high vacuum conditions I think.

JonA · 03-17-2008, 05:15 PM

I did some more research and pulled the fuel filler assembly inside cover off. There are two valves in there on the vent line to the charcoal cannister. One is to vent the tank and prevent pressure buildup and the other is to prevent fuel from leaking when the car is inadvertently inverted. I'm gonna replace both cause I can't figure which is which.
When operating properly there should be no pressure build-up in the tank.

Jon

John_AZ · 03-18-2008, 06:22 AM

Thanks for the information. I needed to look at the valves:

Number 16 is the bleeder valve-Pelican $30.75 and Number 20 is the shut off valve-Pelican $42.
Let us know if the new parts solve the pressure buildup.

John_AZ

Tidybuoy · 03-18-2008, 11:30 PM

Quote:

Originally Posted by matthewb

I am pretty sure your pressure is supposed to be releived through the carbon canister. I often have pressure release through my gas cap when I remove it on my other cars. but I think the pressure is supposed to go through the carbon canister where the carbon can soak up the gas vapors before the air passes out of the car. then the canister is supposed to get sucked out under high vacuum conditions I think.

Exactly right. The carbon in the cannistor converts the vapor to liquid and is stored until it can be re-used (controled by a vacuum device near the brake booster).

There is also a plastic expansion tank (I believe behind the carpet on the right rear). It's possible that you have a kinked hose or something.

Vern

JonA · 03-19-2008, 06:26 PM

Parts are ordered, I'll let everyone know.
Thanks,
Jon

timbieber · timbieber's Garage

Jon,
did that fix your problem. I've got the same thing. Was so bad my 944 was bogging down like it was running out of fuel. had no idea what was going on but remembered when i filled up last the pressure escape was bad. so i pulled the cap off and only screwed it on a couple turns and it's been dandy ever since.
Far before this i've always had a bad fuel smell in the cabin and have been searching all the posts on problems. Going to start digging into it now/this weekend. curious to know your outcome.

JonA · 02-21-2009, 06:11 PM

I did change both valves and I still occasionally get a little pressure relieving when I remove the gas cap. It's not be as bad as it was and I kinda think what I have may be normal. My Tacoma does the same thing. I don't have vapors in the cabin so I think it's safe.
Whenever the clutch goes and I have to drop the tranny I'm going to drop the tank and replace all the hoses.

Jon

timbieber · timbieber's Garage

i figured out my problem. Past owner completely removed the vent tank that's in the panel directly above all the filler/breather hoses. the main breather line that is suppose to run up into that tank is just hanging with a spark plug jammed in to block up the fumes! I will have to find this tank and also figure out the other hoses and vales that go with it for in/out lines etc.??

JonA · 02-22-2009, 03:00 PM

Wow. Good find. Some people shouldn't pick up a wrench, not with a dangerous attitude that would allow him to do something that stupid.

Jon

timbieber · timbieber's Garage

ya, pretty ridiculous! glad i found it though. I've got the breather line pulled as far as i can get it and reaching just outside the car pinched when closing the rear hatch/window. Seems to do the trick so far until i track down the vent tank and associated hoses, vales etc. and figure out hit all goes together.
Still looking for some early off-set wheels for the car as well... would love cup 1 or 2 17" if anyone knows anyone selling...