G/Tank Pressure Buildup

Any experience with either  a pressure buildup or vacuum in the gas tank of your C4?

I get a slight pressure buildup or vacuum on my '94 depending on ambient temperatures, and assume that it's normal.

It's also been suggested that some gas caps are both vented and non-vented. Is this correct or should all the caps be vented  to help alleviate this pressure or vacuum/

Thanks.

C.

|UPDATED|10/5/2014 11:12:04 AM (AZT)|/UPDATED|

I'm no expert on this one but here's what I know....The system is essentially sealed within pressure limits to improve emission losses, but the system can vent to a storage reservoir (under the right rear quarter panel) which gets purged via a solenoid valve when certain engine conditions are met. The cap is vented, it gets checked for proper operation when you do an e-test. However, the system is only open to the environment under limited conditions such as preventing dangerous over pressure but it also protects from excess vacuum as the tank drains. Pressure happens as tank heats up from warm gasoline recirculating back from the engine (return line bypass) which in turn increases the partial pressure of the gasoline that creates the pressure you notice when you remove the cap. Normally, vacuum is not an emissions issue so air gets admitted to compensate for fuel loss but it is also offset by evaporation (fumes) in the tank. There is also a roll-over check which prevents fuel spill out the vent line ( if you wind up scratching the shiny side) and I think it sets the vacuum pressure while the fill cap limits the excess pressure.

Thanks, my '91 [I'm in SUNNY fl., just about blows off the cap when i remove it when i've driven a few miles. My '94 never does. So, i should inspect the solenoid to purge trank? Also, GREAT looking wheels. What brand/size/back space are they ?

My wheels? Billet Specialties "Fast Lanes",  ZR-1 sizes (9-1/2" F & 11"R) with all the back spacing they could give me but the half shafts have been custom made and shortened to get them under the fenders.

My '86 vents for 5-10sec when the tank is at/below 1/4 tank and I've tried a couple different gas caps also.

|UPDATED|4/13/2016 7:31:32 PM (AZT)|/UPDATED|

Some manufacturers actually do that....MOPAR... but that can lead to vapor lock at times, hard to start after heat soaking the engine and extended crank after FF replacement. GM uses (on most cars) a return system as you know which IMHO is great at using every bit of fuel in your tank as the return dumps into a cup in the tank where the pick up sock and F-Pump are, keeping the  pump sock submerged in fuel at all times till you are completely dry. 

Mine vents every time I turn the cap ..... the longer distance I drive the more it will 'burp'.

|UPDATED|4/20/2016 12:30:10 PM (AZT)|/UPDATED|

I was just reading over this topic what I have is that the gas cap is hard to remove it seems like it is sucked in.  The car runs great.  Yesterday I purchased a venting gas cap and after a ride of about 40 miles it was easy to remove but vented longer than the cap that came with car.  It was way easier to remove the cap.  Most likely my system is not working as it should but is it ok to use a vented cap or will it blow something up?

Thanks Al!!!!

Corvette Magazine addressed this issue in its April 2015 issue.  I have uploaded a copy of their response to a reader with a similar question and issue.