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Topic: G/Tank Pressure Buildup

in Forum: C4 Fuel, Emission Control, and Exhaust Systems

G/Tank Pressure Buildup (1/11)
 10/5/14 1:12pm
cco
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Joined: 8/16/2013
Posts: 191

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|
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Re: G/Tank Pressure Buildup (2/11)
 10/8/14 5:59pm
396c4
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, - Canada

Vette(s):
96 Vette with extensive list of mods, many one-off and custom.


Joined: 12/7/2011
Posts: 216

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.
Re: G/Tank Pressure Buildup (3/11)
 10/9/14 12:36pm
cco
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Excellent appraisal, 396C4 -- Thank you.

C.

Re: G/Tank Pressure Buildup (4/11)
 10/22/14 7:21pm
knnhon
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new poret richey, FL - USA

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1991 Coupe-L-98-auto, 1994 White Conv., l-98 6 spd.


Joined: 10/21/2014
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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 ?
Re: G/Tank Pressure Buildup (5/11)
 10/23/14 7:22pm
396c4
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, - Canada

Vette(s):
96 Vette with extensive list of mods, many one-off and custom.


Joined: 12/7/2011
Posts: 216

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.

If your purge valve fails, OBD II gives you a code but I don't know about your OBD I. My 96 has quite an audible vent when it's hot and the tank is low when I fuel up so I think what you have is normal.
Re: G/Tank Pressure Buildup (6/11)
 4/13/16 9:31pm
JonJon
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Verona, KY - USA

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'86 Coupe Silver/Grey Manual Trans


Joined: 12/27/2015
Posts: 21

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.
 
Teach me if I'm wrong, but could the '94 a returnless system to reduce evap emissions due to the warm fuel in the tank?
 


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


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Re: G/Tank Pressure Buildup (7/11)
 4/20/16 10:04am
Tesla220
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Posts: 64

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. 



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Re: G/Tank Pressure Buildup (8/11)
 4/20/16 2:30pm
cco
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Mine vents every time I turn the cap ..... the longer distance I drive the more it will 'burp'.
 
Seems normal to me since the car runs like a champ.
 
I did overfill it once, and I mean fill, to check mileage, and paid the price when I cracked the cap open. Apparently the gas had warmed, then expanded, and it vented gasoline. I quickly closed the cap tight.
I didn't fill it quite so much afterwards and just relied on the displayed mileage. (yes it's suggested to not fill it past the first pump nozzle shutoff to give the gas space to expand.)
 
C.


|UPDATED|4/20/2016 12:30:10 PM (AZT)|/UPDATED|
Re: G/Tank Pressure Buildup (9/11)
 4/13/17 10:22am
aldocar
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Joined: 12/3/2016
Posts: 89

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!!!!


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Re: G/Tank Pressure Buildup (10/11)
 4/19/17 11:15am
Kettsvette
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Scottsdale, AZ - USA

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1988 Dark Red Metallic Convertible with sports seats, Delco/Bose radio system, Z52 suspension


Joined: 7/8/2014
Posts: 28

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.
 
 
I hope this helps.


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Re: G/Tank Pressure Buildup (11/11)
 4/26/17 6:37pm
aldocar
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Thanks it helped.

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