EVAP canister help

[Marteney, Steven J.]

So the tank is typically higher than atmospheric pressure because of fuel
vapor.  Therefore, the "sucking" sound when removing the gas cap is actually
air/vapor "blowing" out of the tank.  Correct?  This would explain the
warning on gas caps that says remove slowly to avoid fuel spray, or
something like that.  So in order to test, you would introduce a slight
pressure into the tank with a pump and monitor bleed-off with a gauge?
Sounds like I should start by simply trying a new gas cap.

Thanks, Steve 

-----Original Message-----
From: Shannen Durphey [mailto:shannen at grolen.com]
Sent: Thursday, June 21, 2001 1:37 AM
To: gmecm at diy-efi.org
Subject: Re: EVAP canister help


Sorry, missed the beginning of the thread.

If the car has a plastic fuel cap and purge canister, it's got a check valve
built into the cap.  The valve is supposed to allow fresh air into the tank
but
keep pressure from escaping.  If the valve doesn't allow air into the tank,
the
tank will collapse as fuel is removed and atmospheric pressure crushes it
from
the outside.  I've seen it, and it isn't pretty.  Some caps seem to "stick"
and
allow a small vacuum to develop.  I may have read about caps being designed
to
prevent atmospheric air from entering too soon to prevent premature pressure
buildup.  Then again, I may not have read that.  But running the car long
enough
to begin to warm the fuel returning to the tank should definitely begin to
build
pressure in the tank in the form of warm fuel vapor.  On a hot day, C4 Vette
tanks can build enough pressure to be felt as the filler cap is removed.  If
no
pressure is felt, especially after the car has been sitting in the warm sun
for
a period of time, the cap is probably faulty.

Shannen



romans, mark wrote:
> 
> Anybody have an 84 that can answer that?
> 
> ----- Original Message -----
> From: "Louis Baiani" <nepti at nbnet.nb.ca>
> To: <gmecm at diy-efi.org>
> Sent: Wednesday, June 20, 2001 8:43 PM
> Subject: Re: EVAP canister help
> 
> > Would an 84 have alot of pressure?
> >
> > Louis
> >
> > "romans, mark" wrote:
> >
> > > My 89 vet has pressure.  Every GM car I have ever looked at (Late
model)
> > > has pressure.
> > > Mark
> > > ----- Original Message -----
> > > From: "Marteney, Steven J." <smarteney at xlvision.com>
> > > To: <gmecm at diy-efi.org>
> > > Sent: Wednesday, June 20, 2001 1:08 PM
> > > Subject: EVAP canister help
> > >
> > > > My last two gas station visits have shown that when I remove the gas
> cap
> > > > from my 89 Formula 350, '165 absolutely NO "sucking" sound is made.
> It
> > > has
> > > > done it before but it was only a quick fizz.  Does the tank run
under
> a
> > > > vacuum, therefore the sucking sounds when you crack the cap?  Does
it
> take
> > > > time for this condition to develop?  (In other words, start car,
quick
> > > drive
> > > > to gas station, shut-off, typically no suck sound?)  Likely a bad
gas
> cap?
> > > > Bad vac line?  Bad canister?
> > > >
> > > > I know my 87 T/A 305CCC was quite obnoxious when the gas cap was
> removed.
> > > > I'm wondering if this is the cause of my laxluster gas mileage even
> BEFORE
> > > > the motor was rebuilt.  Any ideas on how to test?
> > > >
> > > > Steve
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