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| Re: [GMCnet] smog/vent? [message #74501 is a reply to message #74369] |
Sat, 20 February 2010 19:42   |
Terry Skinner
Messages: 379
Registered: January 2004
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This is a quote from Gene Fisher's info from Rick.
1. There are two vent systems from each tank. The one we are most concerned
with I call the "Fill Vent" system. It consists of a fairly large (3/8"
IIRC) hose exiting from the top center of each tank, from the pickup/gauge
unit, right next to the fuel outlet hose going to the engine. This hose MUST
be routed high and steady, with no significant dips, to the steel vent pipe
which runs along the side of the coach next to the filler pipe. (A P.O. has
replaced my tee fitting where the front tank fill vent joins the fore/aft
line with a kluge of brass fittings, but I doubt that's any better than OEM.)
In my coach, that steel pipe is routed just as high as possible through the
frame rails, and is, of course, unclogged. At the front of that pipe, a
large (7/16" or better) hose runs UP to the filler neck, again with no low
spots which might trap fuel.
It is this system which quickly vents the air and vapors while filling the
tanks, and is supposed to spit some gas into the filler neck when the tanks
are full to shut off the nozzle, before compressed air belches a slug of
gasoline back up the fill pipe and all over the ground. Most people with
fill problems (I believe) have a partly clogged or sagged fill vent line,
which obstructs or prevents the venting of the tanks. Remember, the fill
vent line should be level or constantly uphill all the way from the gas tank
to the filler neck. (Some folks have reported that lowering the right rear
suspension while fueling allows them to fill quicker, undoubtedly by
maintaining that uphill slope.)
2. The running vent system vents vapors after the gas cap is reinstalled and
sealed. It occasionally admits air into the tanks to replace withdrawn fuel,
but the vapor pressure of the gasoline ensures this only happens occasionally
in cold weather; the rest of the time the tank is full of almost pure
gasoline vapor. (This is why many fuel-injected cars can put an electric
fuel pump, complete with sparking brushes, right IN the tank, without blowing
up. That TWA 747 that blew up over the Atlantic had an "EMPTY" main fuel tank
which hadn't been filled in months, allowing air to enter with each altitude
change and become explosive.)
Anyhow, if you are finding gas dribbling from the gas cap on the road,
it's likely the regular vent system isn't working. That system connects
fittings on the sides of each tank (2 per tank IIRC) to small (5/16"?) hoses
which lead to the left rear wheel well, hopefully uphill all the way. There,
in the highest spot which is still OUTSIDE the passenger area, they tee
together and enter the fuel separator. The separator is that mysterious
little black box with a plastic (lighter than fuel) marble inside. If more
than a half-cup of liquid fuel makes it this far, the "marble" floats in it,
and blocks off the exit from the separator, preventing liquid gas from
continuing on down the line to the charcoal canister behind/inboard the right
front wheel, where it could contaminate the canister for days, and/or drip on
the ground near the engine causing undue excitement. =8-O The fuel
separator IS available from Cinnabar, but it is unbelievably expensive, so
it's worth trying to clean yours with carburetor cleaner (the marble must
rattle freely when shaken, and you should be able to blow through it when
upright, not when inverted).
Again, sagged, clogged, kinked hoses, a blocked separator, a clogged
charcoal canister (plural in California), even a steel tube blocked with rust
(which Arch reported after diligent sleuthing), can clog this system and
force fuel out the emergency pressure-relief in the cap. And though it
really isn't meant to vent the tanks quickly, this system does allow some
vapor out when fueling, and lets everything settle down so the tanks can be
filled. BTW, once you get 50-odd gallons in, I'd suggest resisting the urge
to top up completely unless you're going out on the highway immediately and
burn some off. The system needs a little "air space" (vapor space) in each
tank to function properly. When all works properly, this is about the time
the automatic nozzle clicks off. (I have gotten around 54 gallons in mine.)
And that's the vent system, to the best of my knowledge. It's basically
simple, but it is spread out over a large horizontal area in the GMC, and
it's hard to see whether a hose is running level, uphill, or downhill, when
lying on your back with road dirt falling in your eyes. Good
luck.HTH.Rick Staples
This is as close as I can find like mine. There are two vents on each
tank. One is in the gas gauge module and uses 3/8" hose and runs to
the filler neck. The other is off to the side, is soldered into the
tank and uses 5/16" hose and runs to the module in the left wheel
well. Just trying to find out if this is plumbed
corectly.............Terry
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--
Terry Skinner
253-686-2624
Roy. Washington
'76 GMC
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