Explain Gas Tank Pressurization To Me...?

[GasGuzzler]

The only time you'll get pressure in there is when the tank is filled half way or less when cool then the tank and the air and fuel inside gets significantly warmer causing a spike in pressure.

Most likely you're hearing vacuum. The tank is put under vacuum by the EVAP system which draws fuel vapors back through the charcoal canister before venting them to the atmosphere to reduce emissions much like the clunky crappy gas hoses in emissions counties.

This is the system that will turn on the SES light if you leave your gas cap loose (P0440 and P0442) or when the the canister and vent valve get plugged with dirt road dust (P0446).

HTH

[LS1GMCTruck]

The only time you'll get pressure in there is when the tank is filled half way or less when cool then the tank and the air and fuel inside gets significantly warmer causing a spike in pressure.

Most likely you're hearing vacuum. The tank is put under vacuum by the EVAP system which draws fuel vapors back through the charcoal canister before venting them to the atmosphere to reduce emissions much like the clunky crappy gas hoses in emissions counties.

This is the system that will turn on the SES light if you leave your gas cap loose (P0440 and P0442) or when the the canister and vent valve get plugged with dirt road dust (P0446).

HTH

All good info, thanks. Heres my situation. I have an LS1 installed in a 1996 GMC shortbed. All the EVAP from the LS1, and the truck, is gone...on purpose. I have the small EVAP vent tube coming off the fuel pump/fuel sender module simply plugged off at the tank. Coming off this same module is the usual, larger 5/8" vent hose, that simply ties into the larger main fuel fill hose. Do I have sufficient tank venting? I'm thinking I do, but would appreciate a second opinion. Thanks.

[Fastime09]

The fuel vapors are released into the carbon canister by the pressure built up in the tank from fuel & vapors, then the pcm closes the tank pressure solenoid and then opens the canister purge valve and pulls the vapors into the intake along with outside air thought the canister, but thier is never a vacuum on the tank, only pressure, I'm not sure if you would have enough venting, some of the 5/8 hoses have a roll over valve in side & just to vent while filling tank, check the hose real good..

[r.barn]

Get a $10 vented fuel cap ....... problem solved

[LS1GMCTruck]

Get a $10 vented fuel cap ....... problem solved

Good idea. Yeah, I already removed ALL my EVAP equipment & programmed it out of the computer, years ago. No issues at all there. Just inquiring about fuel tank pressurization/venting...so buying a vented gas cap sure sounds like a simple fix, to a questionable situation. Thanks again guys.

[DBRODS]

I ran a 3/8 line off of the sending unit up to where the gas filler is and used a rear end breather on it...no problems here

[LS1GMCTruck]

I ran a 3/8 line off of the sending unit up to where the gas filler is and used a rear end breather on it...no problems here

Hmmmm...another good idea. Do they sell rear end breathers at the local auto parts store, or...? Thanks

[GasGuzzler]

The fuel vapors are released into the carbon canister by the pressure built up in the tank from fuel & vapors, then the pcm closes the tank pressure solenoid and then opens the canister purge valve and pulls the vapors into the intake along with outside air thought the canister, but thier is never a vacuum on the tank, only pressure, I'm not sure if you would have enough venting, some of the 5/8 hoses have a roll over valve in side & just to vent while filling tank, check the hose real good..

Nope, the tank is subject to vacuum not pressure.

You need to vent the cap if all the EVAP has been killed.