Overfilling the gas tanks?

[lw8000]

A couple times after filling up and driving in warmer weather (this time 83 degrees or so), I have noticed the smell of gas and can hear air hissing
from the gas cap, even a small trace of gas on the cap itself, after shutting off the engine. I think this is where the smell is coming from. It
also seems to happen more often when I have full tanks or just after topping off and driving a short distance. I've read here about how the venting
system works and now I'm wondering if I'm filling up to the point where these vents are being blocked. The tube from the filling point to the tanks
is pretty long and I would guess can hold a few gallons?, and when I fill I usually try to go as high as I can (usually try to add about 5-7 gallons
after the gurgling starts). Now I'm starting to wonder if I'm overfilling. Is this even possible?

I am pretty sure I have the correct cap, it is a vented one from the PO. My next step will be to try a different cap to see if it makes any
difference (I found the part numbers mentioned here).

Thanks!!
--
Chris S. -
77 Kingsley, 3.70 FD, mostly OEM -
S.E. Michigan

 

[Jim Hupy]

There are two venting systems on the GMC. One nearly parallels the filler
neck as well as the fuel line to the engine. It has the purposes of venting
the air displaced by filling of the tanks, as well as cancelling the filler
nozzle by burping fuel liquid back to the filler neck.
The other venting system uses hoses from fittings in the tanks to the
liquid/vapor separator, and then to the charcoal cannister, finally ending
up at the carb inlet.
If you fill the tanks to the point where liquid fuel enters the
separator, this system cannot function. It the filler neck vent is also
filled with liquid, then only the cap vent can function. If you open the
cap, and the weather is warm and the coach has been moving, you are going
to get a bath in gasoline when you open the cap.
What I do, is, when I hear gasoline gurgling back up the filler when I
am pumping gasoline, I usually stop filling at this point. I rarely get my
system over filled this way. If you persist in squeezing those last bits of
fuel in there (topping) be prepared for what you have described.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> A couple times after filling up and driving in warmer weather (this time
> 83 degrees or so), I have noticed the smell of gas and can hear air hissing
> from the gas cap, even a small trace of gas on the cap itself, after
> shutting off the engine. I think this is where the smell is coming from.
> It
> also seems to happen more often when I have full tanks or just after
> topping off and driving a short distance. I've read here about how the
> venting
> system works and now I'm wondering if I'm filling up to the point where
> these vents are being blocked. The tube from the filling point to the tanks
> is pretty long and I would guess can hold a few gallons?, and when I fill
> I usually try to go as high as I can (usually try to add about 5-7 gallons
> after the gurgling starts). Now I'm starting to wonder if I'm
> overfilling. Is this even possible?
>
> I am pretty sure I have the correct cap, it is a vented one from the PO.
> My next step will be to try a different cap to see if it makes any
> difference (I found the part numbers mentioned here).
>
> Thanks!!
> --
> Chris S. -
> 77 Kingsley, 3.70 FD, mostly OEM -
> S.E. Michigan
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[lw8000]

> There are two venting systems on the GMC. One nearly parallels the filler
> neck as well as the fuel line to the engine. It has the purposes of venting
> the air displaced by filling of the tanks, as well as cancelling the filler
> nozzle by burping fuel liquid back to the filler neck.
> The other venting system uses hoses from fittings in the tanks to the
> liquid/vapor separator, and then to the charcoal cannister, finally ending
> up at the carb inlet.
> If you fill the tanks to the point where liquid fuel enters the
> separator, this system cannot function. It the filler neck vent is also
> filled with liquid, then only the cap vent can function. If you open the
> cap, and the weather is warm and the coach has been moving, you are going
> to get a bath in gasoline when you open the cap.
> What I do, is, when I hear gasoline gurgling back up the filler when I
> am pumping gasoline, I usually stop filling at this point. I rarely get my
> system over filled this way. If you persist in squeezing those last bits of
> fuel in there (topping) be prepared for what you have described.
> Jim Hupy
> Salem, Or
> 78 GMC ROYALE 403

Thanks Jim, I like that idea of stopping just as you hear the gurgling noise. I never really thought about this until I was reading about the venting
system and how it actually works. With the various fuel issues we've had we've verified our system is good... so now I know and lesson learned!
--
Chris S. -
77 Kingsley, 3.70 FD, mostly OEM -
S.E. Michigan

 

[thomas phipps]

One time, I thought it would be a good idea to lower the passenger side of the coach while refueling. This because of the tank design.
After running gas out the separator, I came to the conclusion, flat is better. No more fuel loss.
Tom, MS II
--
1975 GMC Avion
KA4CSG

 

[josh r.]

The first time I filled my tanks I found out, nothing like 200 miles smelling of fresh gas.
--
1978 Royale 403

 

[Matt Colie]

> A couple times after filling up and driving in warmer weather (this time 83 degrees or so), I have noticed the smell of gas and can hear air
> hissing from the gas cap, even a small trace of gas on the cap itself, after shutting off the engine. I think this is where the smell is coming
> from. It also seems to happen more often when I have full tanks or just after topping off and driving a short distance. I've read here about how
> the venting system works and now I'm wondering if I'm filling up to the point where these vents are being blocked. The tube from the filling point
> to the tanks is pretty long and I would guess can hold a few gallons?, and when I fill I usually try to go as high as I can (usually try to add
> about 5-7 gallons after the gurgling starts). Now I'm starting to wonder if I'm overfilling. Is this even possible?
>
> I am pretty sure I have the correct cap, it is a vented one from the PO. My next step will be to try a different cap to see if it makes any
> difference (I found the part numbers mentioned here).
>
> Thanks!!

Chris,

Even though your coach is vastly newer than mine, it could still have a problem I did have.

Behind the panel at the driver's left elbow (where the fill and vent piping come up from below) the fill vent line there is rubber. And if it is like
most of the rest of the rubber in the fuel system, it is shot. That line is 1/2 and it is an amazing PITA to replace. But I did and the gas smell
stopped There are other fuel system mods that are interesting and so are easy. If you PM me a phone nr. and good times to call, we can talk about
them. Or, you could wander over here and I can show you some.

As to over filling, now that I got the vapor separator and carbon canisters working right, I can fill so I can see the level in the fill neck and not
get gassed. And, I can do most of the fill at on the 9.9GPM stop on the fill nozzle. This makes a full fuel stop 10 minutes instead of half an hour
(or more).

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

 

[jim kanomata1]

​Never remove cap on a warm day rapidly.
Treat it like a radiator cap and do it slowly.
I have had fuel in my eyes once and almost few times. BE CAREFUL​

> The first time I filled my tanks I found out, nothing like 200 miles
> smelling of fresh gas.
> --
> 1978 Royale 403
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502

 

[lw8000]

> Chris,
>
> Even though your coach is vastly newer than mine, it could still have a problem I did have.
>
> Behind the panel at the driver's left elbow (where the fill and vent piping come up from below) the fill vent line there is rubber. And if it is
> like most of the rest of the rubber in the fuel system, it is shot. That line is 1/2 and it is an amazing PITA to replace. But I did and the gas
> smell stopped There are other fuel system mods that are interesting and so are easy. If you PM me a phone nr. and good times to call, we can talk
> about them. Or, you could wander over here and I can show you some.
>
> As to over filling, now that I got the vapor separator and carbon canisters working right, I can fill so I can see the level in the fill neck and
> not get gassed. And, I can do most of the fill at on the 9.9GPM stop on the fill nozzle. This makes a full fuel stop 10 minutes instead of half an
> hour (or more).
>
> Matt

Matt, thanks for the info. I believe I know exactly the vent line you are talking about, basically inside the wall behind the plastic panel. We've
been in there before, but I did not pay enough attention to realize that was a rubber section in there. I'll get in there and have another look at
it. I can definitely see your point about that being a PITA as there's not much room there. We have a trip coming up so I will probably mess with
this later this fall if I have to change anything out. And thanks for the offer on looking at the other parts of the system. I will probably start
with this and see where it goes. We've had the entire fuel system "checked" by a couple of places, including at the Co-Op. I don't remember the
charcoal canister being discussed, so again something I should probably investigate more during our off-season.

Thanks again and Best,
--
Chris S. -
77 Kingsley, 3.70 FD, mostly OEM -
S.E. Michigan

 

[robert mueller]

G'day,

While you're in there you might want to consider this:

http://www.gmcmhphotos.com/photos/aa-miscellaneous-photos/p51372-fuel-tank-vent-mod.html

Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808


-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Chris S.
Sent: Friday, July 21, 2017 2:01 AM
To: gmclist
Subject: Re: [GMCnet] Overfilling the gas tanks?

Matt, thanks for the info. I believe I know exactly the vent line you are talking about, basically inside the wall behind the
plastic panel. We've been in there before, but I did not pay enough attention to realize that was a rubber section in there. I'll
get in there and have another look at it. I can definitely see your point about that being a PITA as there's not much room there.
We have a trip coming up so I will probably mess with this later this fall if I have to change anything out. And thanks for the
offer on looking at the other parts of the system. I will probably start with this and see where it goes. We've had the entire
fuel system "checked" by a couple of places, including at the Co-Op. I don't remember the charcoal canister being discussed, so
again something I should probably investigate more during our off-season.

Thanks again and Best,
--
Chris S. -
77 Kingsley, 3.70 FD, mostly OEM -

 

[lw8000]

> Chris,
>
> Even though your coach is vastly newer than mine, it could still have a problem I did have.
>
> Behind the panel at the driver's left elbow (where the fill and vent piping come up from below) the fill vent line there is rubber. And if it is
> like most of the rest of the rubber in the fuel system, it is shot. That line is 1/2 and it is an amazing PITA to replace. But I did and the gas
> smell stopped There are other fuel system mods that are interesting and so are easy. If you PM me a phone nr. and good times to call, we can talk
> about them. Or, you could wander over here and I can show you some.
>
> As to over filling, now that I got the vapor separator and carbon canisters working right, I can fill so I can see the level in the fill neck and
> not get gassed. And, I can do most of the fill at on the 9.9GPM stop on the fill nozzle. This makes a full fuel stop 10 minutes instead of half an
> hour (or more).
>
> Matt

Matt,

I finally found at least one source of the gas vapor, that we could smell inside the coach. We had an open air filter on the carburetor in the engine
box (it was a low profile one that the last place that worked on the coach put on because we have a high rise aluminum intake manifold). I recently
replaced that with a closed air filter housing with a snorkel and routed the fresh air intake portion of it with a piece of 4" aluminum dryer vent
over right next to the wheel well, just behind the alternator. My purpose was to pull in slightly cooler air to the carb because of the vapor lock
issues we've had. After putting on this new enclosed housing, the gas vapor smell in the coach is completely gone.

I did look at the vent pipe you mentioned, and it appears ours turns to metal (from rubber), in the engine box area and is metal through the floor up
to where the cap is.

Thanks!
--
Chris S. -
77 Kingsley, 3.70 FD, mostly OEM -
S.E. Michigan