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,

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

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