anewbill

Like many of us, I have been chasing a fuel odor issue in my trunk and interior for a while now. So far, I have replaced the fuel filler neck with the updated part, replaced the gas cap, and cleaned the carpet. All of these items have definitely helped, but I can still smell some fuel inside the cabin in the mornings. The smell seams to be stronger when the tank is full.

I have seen posts referencing a fuel tank pressure test that is recommended to diagnose leaks in the system. I finally got a round to trying this test over the weekend. I drilled a hole in a spare gas cap and inserted a brass air fitting in the hole to make a tank-pressurizing tool. After checking the cap for leaks, I then hooked my homemade fuel-tank-pressurizing tool up to my air supply and fuel tank with a pressure gauge. When I tried to pressurize the fuel tank with a few pounds of pressure via the filler neck, the system would not hold pressure. I tried to find the source of the leak by squirting soapy water on all of the tank hoses that I could access. Unfortunately, I could not find the leak. Then I thought the air might be escaping from some sort of pressure valve in the system.

Does anyone know if the system is completely sealed, or if I need to plug a valve somewhere to perform a pressure test? Thanks in advance for any help.

Bill Gregory

Andy,

Another action which I did helped the fuel smell, and that is to replace the activated charcoal cannister. It's located in the left rear wheel well behind a cover, just in front of the tire. The part number has been superceded to a 993 part number, 993.201.221.01. In addition, I find if, in the garage, I leave the windows open a crack, that for the most part any gas smell is no longer an issue.

JasonAndreas

I don't think there is any way to pass USA evaporative emissions regulations without a sealed fuel tank? In fact, any MY1996+ is required as part of OBD2 to test for a leak and throw a CEL if there is one.

The suggestion to pressurize the fuel tank to 3 PSI comes from Porsche as part of TSB 2007-9401 "Fuel Filler Pipe". I don't know what was in use in 1994 but nowadays any of the ultrasonic leak detectors require a difference in pressure to "hear" the leak.

anewbill,
Have you checked the suction hose and fuel filter gasket on the bottom of the fuel tank? You have to jack the front right side of the car up and undo the fuel pump cover to get to it (all you need is an M10 socket). If you've checked everything up top it's worth a look and only takes a few minutes.

Dunasso

Bill is right about the charcoal canister. On most vehicles with this system, it is usually recomended that the canister be replaced around 35,000.

Duncan

anewbill

Thanks for the replies. Don't worry Indycam. I only used 1-4 PSI to do my test, but I will be very careful with compressed air and gas in the future. Very interesting, analogy to a 747 center tank.

I have not checked the suction hose and fuel filter gasket or the charcoal canister yet. I will plan to investigate…

A question remains…
So, it sounds like there is possibly a bypass valve that prevents the system from sealing completely? If so, how can a difference in pressure to "hear" the leak be achieved? Should I be able to pressurize the tank to 3 PSI and hold that pressure if the system is performing correctly?

Thanks again for the help!

cobalt

I have a very sensitive snif tester and have still not been able to find the minor leak that is causing the smell in my trunk.

BTW the 747 exploded because of poor shielding on old wiring harnesses that ran through the tank. A short created the explosion.

cobalt

Its all about one thing $$$$$$$.

JasonAndreas

Where is the tank vented to atmosphere? The only fresh air inlet I know of would be on the carbon canister(?), the fuel filler area is completely sealed by the cap. When you drive around for a while and then open the gas cap, the hiss you hear is the tank pressure equalizing with atmospheric pressure. If your tank wasn't sealed would that still happen? Porsche recommended 3 PSI so if there is an over-pressure relief valve as part of the carbon canister it should be able to hold at least 3 PSI.

This is exactly what almost every OBD2 system does when testing the evaporative emissions control system for vapor leaks, they pressurize the fuel tank (some use vacuum).

Andy Roe

I sometime smell the fuel odour when the tank is fuel is running low. I'm sure that I've not got a leak though, as sometimes when I open the fuel filler cap, you can hear air rushing into the tank. Is this right or do I have a problem?

Thanks!

bhensarl

The tank can't possibly be completely sealed or it would certainly implode as the fuel was pumped out. But I think Jason's right too, because it would seem to be somewhat sealed. Maybe there's a one-way valve, that helps to equalilze the pressure when it gets too extreme. Just spitballing here.
Brian