Does anyone know what creates the air pressure in the gas tank?
my take on it is the returning fuel carries a higher temp than the gas already in the tank. That elevated gas is near vapor state. But the returning gas molecules are larger than the in tank molecules, the gasses mix, therefore the returning gas cools off and loses it’s temperature, reducing it’s mass but warming up the in tank fuel.
that fuel in turn begins to have a larger presence in the tank. But the engine is using fuel, so some of the warm mixed fuel gets burned, and does not return.
the tank is exposed to some elevated air temperature as the car moves, from engine heat and exhaust heat, further elevating the in tank temperature.
so if you buy gas after driving for an hour, there’s a lot of pressure volume, but if you open the tank in the garage, there should be some residual pressure, but not a lot of air, just some.

ok why am i boring you?
occasionally, on my 86, there’s no tank pressure.
there is a gas tank pressure regulator up near charcoal canister, i replaced all the vacuum hoses, worked great for a month, now no pressure at all.
And yes , i replaced them one at a time.
why is there no pressure at times? My only go to is the vapor purge solenoid is intermittantly leaking.

 

Chris, there should be no pressure in the tank if the “normally open” vent valve is operating correctly…when the engine computer does its EVAP test it closes the vent valve, opens the purge valve and pulls the tank into a vacuum…it then closes the purge valve and then the computer uses the fuel tank pressure sensor (its voltage) to see how fast that vacuum decays…if to fast it will throw a DTC…sometimes with a stuck closed vent valve you may hear a hiss when removing the gas cap and filling the tank is difficult…it will keep shutting off the pump.

 

Hey Rob!
this is my C4. On the C5, i know there is a tank press sensor, but on the C4? I have changed the pump a couple times, and no sensor on the tank.
don’t remember seeing a sensor near the purge valve when i changed the canister.
thanks

 

Coupeguy2001. Thanks for the input. I have not yet checked the vapor canister pressure relief valve but I took my 97 C5 to the dealer to have them check that valve as well as the engine purge valve solenoid. It is my understanding that the relief valve only opens while you are filling the tank which allows pressure that builds up from the fill to escape into the atmosphere. It's not necessarily gas vapor that escapes during that time but more fresh air. Frankly, I don't see how some of the air couldn't be gas vapor but that is just the way my mind thinks about it. It is also my understanding, there is a vapor purge solenoid on the engine that opens and allows built up gas vapor from the canister while the engine is running to be emptied into the engine for burning in the cylinders. What kind of pissed me off is that the dealer didn't check these two valves during their supposed diagnosis of the tank pressure problem, before telling me that they wanted to take down the left side gas tank of my C5 to look around and replace the fuel pump for the fourth time. I told them I didn't want them to put in a fourth new fuel pump because the three that I already had put in make the same noise. It's obviously not a faulty fuel pump but something that is causing all these pumps to make noise. That just hasn't been figured out yet. Hence, my issue on the Corvette Forum. Forum members here have at least given me good ideas and places to look. The dealer didn't do their job and that is why I talked them into not charging me for their supposed diagnosis. They realized they goofed.

 

I ended up having to replace the metal lines that run from the tank to the engine compartment they were
corroded internally, there are two lines one that runs over the tank and down to the frame rail and one that runs
along the frame rail along with the brake lines on the driver side. The one over the tank I managed to just yank out
and I replaced it with a plastic line that i was able to snake over the tank.

 

There are a few things that can cause pressure in the gas tank. The faulty gas tank pressure relief valve can cause it. A faulty evap canister relief valve can cause it. A faulty (saturated) evap canister (from over filling the tank) can cause it. A faulty evap purge solenoid can cause it. I have a similar problem and I'm starting with the evap canister pressure relief valve and moving to the gas tank pressure relief valve and on from there. I haven't solved it yet but I'll let you know.

 

Was this replacement of the metal lines a fix for the noise that was coming from the fuel pump and/or gas tank?