Fuel pump questions

Shannen Durphey shannen at mcn.net
Sat Dec 13 18:39:43 GMT 1997



James Tidwell wrote:

> At 11:02 PM 12/11/97 -1000, you wrote:
> >
> >
> >James Tidwell wrote:
> >
> >> At 01:03 PM 12/10/97 -0800, you wrote:
> >> >Fuel splash tends to generate large amounts of vapor that must
> >> >go through the purge canister to eventually exit to the engine.
> >> >
> >> >Jim Boughton
> >> >jpb01 at ibm.net
> >> >

<snip>

> >----------
>  if the return is sometimes covered the return line pressure
> >> >> > goes up, the regulator can't bleed the pressure down, and the motor
> goes
> >> >> > very rich.
> >> >> >   Hope that better explains it   Bruce  nacelp at bright.net
> >> >> >
> >> >>
>
> >> > Question: Isn't fuel vapor pressure a function of temperature or some
> >> such thing? And if that is the case, then with a closed system (as in the
> >> case of a gas tank with the lid on) the vapor would reach a certain
> >> pressure and remain at that pressure as long as the temp remained the same.
> >> Perhaps minor variations with sloshing, but always returning to the
> >> appropriate pressure for a particular tem.
> >
> >  Yes, except that the fuel is heated by the engine.  If the fuel cap has no
> >pressure release built in, the tank can build much pressure.
> >
> >
> > I realize that, but until pressure relief occurs the system is closed and
> the temp-press relationship applies does it not? Also, in my experience
> many tanks seem to be below atmospheric pressure based on the inrush of air
> when removing the fuel cap.Also, some fuel vapor cannisters  act as a
> relief in that they are connected directly to carb float bowls (Not FI I
> know) never allowing pressure to build for whatever reason, while others
> have a solenoid valve closing off the system at that point until the PCM
> says open. My motorcycle gas tanks get a hell of a lot of sloshing,
> especially the dirt bike variety, and they never seem to notice.
>                                         Janes Tidwell

  It depends on the age of the vehicle. EPA requires OEM's to meet standing fuel
emission requirements, and I haven't noticed that there is any type of pressure
relief on newer, closed system cars. Of course, most of my experience is with GM.
The charcoal canister is supposed to be sized to the tank, and should contain all
of the vapors emitted by the fuel.  New caps have a vacuum relief, and if that
isn't working the tank will collapse after a few miles of driving. :)  Older
vehicles which have no vapor collection system, converting to efi can cause vapors
to blow out the fill pipe while driving,  or in some cases cause vapors to seep
out around the sending unit.  But there is probably a point at which
vapor/pressure relationship applies on these vehicles.

Are your motorcycles 2 stroke?  Running fuel/oil mix?  I think that oil in the mix
tends to limit the tendency to create vapors.  Makes sense to me, but doesn't mean
it's right. :)

Shannen




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