XtremeVette

I have been trying to figure this out.....and think....mine is vented? Even though I told the guy give me a NON VENTING gas cap...cause I was told our c4 vents have a pressurized system and require a NON VENTING gas cap!.....but now I don't know...I went to 3 auto part places and all three on the computer brought up venting gas cap....

so iam confused..lol
anybody know....and know where I can get one?

Jeff :cool:

GlockLT4

I may be wrong, but I thought all caps are venting... when you go get your car inspected, they have to test the pressure at which it will let vapor vent out.

jackdaroofer

I believe the system is vented at the cap! Water in the filller area wiil be sucked into the tank :rolleyes:

:seeya

FrankLT4CE

My guess is that your is vented. My 96 with OBD II is non vented. I bought one from my Chevy dealer for less than $6.00 so I recommend you find out how much one from a dealer is. FrankLT4CE

FlyboyZR1

Get a locking cap from MidAmerica. :yesnod:

XtremeVette

so the question remains


hmmmm still wondering?


my biggest concern is the fact.....that I have a mysteries gas smell once I run my car for like 20 minutes.....the smell gets really bad in the interior.....

can't find any darn leaks though and my charcoal canister is fine....any other recommendations.....I have checked the lines....and return lines....the rubber hoses on those lines and the charcoal canister....Iam going nuts.....


jeff

marrepka

Ok, so now I'm confused... If it's a vented gas cap why does it go "phsssssssssss" when I fill up on a warm day? would'nt a vented system release all that pressure and not go "phsssssssssss"?

XtremeVette

see...exactly...this is what Iam saying!..... Mine use to do that too...the pssssssssssssss.....but now...its more like a pzzzz.

so hmmmm i dunno....can't find anywhere in my manual either.....

jeff :cool:

GlockLT4

My guess would be that there is a pressure valve that opens up when the pressure reaches a certain psi.

XtremeVette

tttt

fundriving

Xtreme, Cap will allow for air (and water if the filler area gets too full) to be drawn into tank. Normally cap allows for pressure build up inside tank which explains the pfffft we all here. As for the fuel smell, I had one that bugged me recently because the odor was so strong yet I could not find a wet spot of fuel.... found the problem in the fuel vapor return lines. They are separate from the pressurized fuel lines. Vapor lines on my '86 run under the driver an go around the rear just inside the LR wheel. I found a piece of rubber hose that joins the 2 vent lines at this point by the rear wheel tunnel. This explained why the smell was so strong, it is right behind the driver... Anyway, a little bit of fuel hose later. No smell. Hope this might help....

JonM

The cap on a C-4 is a Pressure-Vacuum cap. A ratchet tightening device on the cap reduces the chances of incorrect instalation, which would prevent sealing of fuel vapors. The cap has a large "O" ring in it the seats on the neck. Pressure-Vacuum cap is part of the emission/fuel delevery system. You should use the same type cap when replacing.

XtremeVette

so Jon....is this what a non-venting cap is ?> or are all the auto part places correct in saying....i need a venting gas cap.,...I have checked on mine and the others that are vented...all come with that o ring...

and fundriving....you seem to be right on the head with the return line...iam thinking too this is my problem.....it is most noticeable on my drivers side...

jeff


[Modified by XtremeVette, 1:06 PM 8/11/2002]

JonM

Think about it, you cannot have vacuum or pressure if the damn cap has a hole in it. I think it was in 1970-vented caps were outlawed. The emissions standards replaced the vented cap, with the charcoal canister to catch vapors.

5abivt

my 93 is not. AFter pressing the car hard i notice when i remove the cap there is a lott of gas buildup.

Barrier

I believe the cap design allows air in to take the place of the fuel, ( no collapsed tank) yet does not allow air nor vapor out. As the fuel heats the vapor would cause pressure to be heard when you open for refuelling.
Barrier

XtremeVette

i see..and understand ya.....but what ya think about the like 6 caps the dummy's brought up to me that were all vented caps?....strange huh....


i dunnno.....weird.....

PeteL

Google.com:

Fuel Evaporative Systems: We all know gas evaporates very quickly. But it doesn't just disappear, it goes into the air where is helps create smog. So if we can contain the gas in our gas tank, we can limit another 20% of our smog. So instead of an old gas tank that had a vented cap, we now have sealed gas caps. But not totally sealed, because if we contained all that pressure from expanding gas on a hot day we could damage our gas tank. Or on a cold day, the tank might deform inwards from the contracting gas and vacuum created while driving. So we have a vacuum and pressure relief valve built into the system, usually in the cap. But before we get to this point of venting off excess pressure that would release gas vapors, we have a hose that lets the vapors escape to a charcoal canister. Here they are stored, actually trapped in the tiny crevices of this rough carbon stuff that looks like a cross between tiny lava rocks and ashes from your fireplace. If we have a carburetor, we need to be able to route the fumes from the bowl vent out to the canister as well. And we need valves to shut this off or turn it on. But we can't store this gas in the canister forever, so we have hoses and sometimes valves or solenoids that let the fumes into the intake manifold so they can be safely burned. (We call this purging, and it usually happens during closed loop cruising, but some systems do purge at idle while in closed loop.) We purge during closed loop so we can control the air-fuel ratio because we don't know exactly how much fumes we will get. To do the purging, we might connect the canister to ported vacuum on older systems. Or we can have a vacuum valve or solenoid connecting the canister to manifold vacuum. The solenoid can turn the purging on and off exactly when needed. Or it can give us more precise control so the computer can even ground the solenoid at different percentage rates (variable duty cycle) and get different amounts of flow if it wants to.


EVAPORATIVE CONTROLS
Gasoline evaporates quite easily. In the past these evaporative emissions were vented into the atmosphere. 20% of all HC emissions from the automobile are from the gas tank. In 1970 legislation was passed, prohibiting venting of gas tank fumes into the atmosphere. An evaporative control system was developed to eliminate this source of pollution. The function of the fuel evaporative control system is to trap and store evaporative emissions from the gas tank and carburetor. A charcoal canister is used to trap the fuel vapors. The fuel vapors adhere to the charcoal, until the engine is started, and engine vacuum can be used to draw the vapors into the engine, so that they can be burned along with the fuel/air mixture. This system requires the use of a sealed gas tank filler cap. This cap is so important to the operation of the system, that a test of the cap is now being integrated into many state emission inspection programs. Pre-1970 cars released fuel vapors into the atmosphere through the use of a vented gas cap. Today with the use of sealed caps, redesigned gas tanks are used. The tank has to have the space for the vapors to collect so that they can then be vented to the charcoal canister. A purge valve is used to control the vapor flow into the engine. The purge valve is operated by engine vacuum. One common problem with this system is that the purge valve goes bad and engine vacuum draws fuel directly into the intake system. This enriches the fuel mixture and will foul the spark plugs. Most charcoal canisters have a filter that should be replaced periodically. This system should be checked when fuel mileage drops.

EVAPORATIVE EMISSION CONTROL SYSTEM


This system controls the escape of hydrocarbons [in the form of fuel vapour] from the fuel tank. To achieve this, the fuel tank is sealed to the atmosphere by means of a non-venting fuel filler cap, which although non-venting, but some are equipped with a relief valve to relieve any pressure or vacuum build-up in the fuel tank. A hose, connected at one end to a venting pipe located on the top of the fuel tank or fuel tank filler neck, and connected at the other end to a charcoal canister draws the fuel vapours from the fuel tank to the charcoal canister. The canister Utilises activated charcoal granules to absorb and store the vapours until the vapours are drawn out and burned by the engine during normal operation. When the engine is operating at above idle speed, the purge control valve [located in the top of the canister] is opened by a vacuum signal from the throttle body. This action allows air to be drawn through the filter, at the base of the canister, and through the charcoal granules, purging the vapours from the charcoal granules and drawing the vapours into the inlet manifold to be burned in the combustion process.
| Trade Tip | ** Because the air, which is drawn through the filter via the hole in the base of the canister, is usually heavily contaminated with road dust, the filter may become blocked or restricted. This component is normally totally overlooked by all automotive service technicians when it comes to finding a performance or fuel problem.
Certain vehicles incorporate a bimetallic vacuum switching valve [BVSV] in the canister purge hose preventing canister purging action until a specific engine coolant temperature has been attained. Thereafter, purging action is controlled by the vacuum signal operating the canister at engine speeds greater then idle.

rodj

With reference to the gas smell I had excessive pressure build up whenever the car was run for more than half an hour.I found steel line from the tank to that join by the wheel full of rust.The vapour was unable to vent to the canister so as mentioned here the excess pressure vented from the cap. Since replacing the vent line , no more pressure build up and no more smell. If you cant blow back form the canister to the tank it is fromally blocked.

XtremeVette

Ok...well just got back from parts supplier to GM and my GM dealer.....the parts supplier has two caps in stock for the 1990 corvette....one is a pressurized vented cap for $25 and one is a non pressurized vented cap for $17...and GM...LOL....they have no clue what the car takes....unreal....lol..

so....which one.....any ideas?....I have read all the great information listed in this post....but still am curious as to why they even make two different ones.....donŐt make much sense....

jeff