Gas spilling when filling up

When I fill up my stock 25 gallon tank at the pump, the pump stops/clicks as normal when full and shortly thereafter I can hear gas spilling. Looking underneath, I can't quite pin point where the spill comes from other than the general tank area. Other than that, vehicle works great -- fuel gauge reads full and no other fuel delivery issues.

Crawling underneath the rear tank, I can see what looks like a vent hose that runs parallel to the fuel filler neck.


I also can't quite see if I have a 3 or 4 outlet sending unit but I can feel 3 metal lines that run alongside the passenger frame rail -- one big one and 2 smaller ones. I don't have an EVAP canister or provisions on my Quadrajet for a vapor canister purge vacuum valve - probably due to my 3/4 ton Suburban being above the 6,000 GVWR rating. Not sure if this is relevant but figured I'd share it.


A few questions for you folks:
- Thoughts on what I can do to not spill what seems like $3 worth of fuel each time I fill up?
- I'm thinking about upgrading to a larger tank (31g or 40g) since the big block drinks like a sailor (6-9mpg). Any "gotchas" or "while you're there"s I should know?
- When parked in my garage, there's a strong gas smell that my wife kindly reminds me about. Anything I can do to help with that? I've seen some DIY versions of charcoal canisters that look interesting but am not opposed to buying and installing an OE vacuum canister. I just don't know how to hook it up to my carb (T into the PCV hose?).
Lastly

There should be evidence of staining from the gas leaking each time.
Although really hard to pinpoint while filling up. Or see up in there.
The sender looks new(er) in that picture though. Is the return hooked up? How many ports on your fuel pump? Not really pertinent to the leak but possible sources of leak.
Icbw but I thought the first several years of squares were single line, no return line, vented cap? I think that’s what my 77 half ton was/is. But my fuel system and engine have been replaced/aftermarket saddle tank on one side. So not sure what was there originally.
Fwiw, the gas smell may just be the gas cap not sealing. It doesn’t take much.
Our boat, O ring on the filler cap cracked. Could smell gas fumes in the middle of the lake but never leaked a drop.
The 77 had gas fumes in the garage, switched gas caps side to side as they fit better on opposite sides (mismatched caps and fillers), no more fumes, although the seal on one cap is now in pieces and there’s a little smell again.
Reminds me, I need new gas caps for the 77…..can’t believe that rubber didn’t make it to 50 years old…..lol

Mine was doing that due to some really old fuel lines that had split on the vent side of things... I'd start by looking at all the lines which probably means dropping the tank unfortunately.

I was fortunate enough that all the affected lines were out in the open. Unfortunate in the fact that the first one to pop wanted to pee right onto the exhaust. The moment I started up the truck it pulled enough vacuum on the tank to relieve the pressure and stop the leak until the next fill up.

You might be able to CAREFULLY slightly pressurize the fuel tank. The exhaust from a shop vac, air compressor, or something like that. You would want to do it outsize and be cautious of static sparks near the top of the filler tube. Then with air entering the tank, you could look around under the rig for the sound of air swooshing out. I would not park a rig in a garage that has any kind of fuel leak or fuel vapors escaping the rig. Its unhealthy to breath, and its a fire/explosion risk.

Grit dog said:

The sender looks new(er) in that picture though. Is the return hooked up? How many ports on your fuel pump? Not really pertinent to the leak but possible sources of leak.

Fwiw, the gas smell may just be the gas cap not sealing. It doesn’t take much.

Click to expand...

Fuel pump has 3 lines, one from the tank, one to the carb, and a return line. Also, I just replaced the gas cap due to getting distracted while filling up and forgetting to put the cap back on so it's probably not that.

SlickGTP said:

Mine was doing that due to some really old fuel lines that had split on the vent side of things... I'd start by looking at all the lines which probably means dropping the tank unfortunately.

Click to expand...

Edelbrock said:

I would not park a rig in a garage that has any kind of fuel leak or fuel vapors escaping the rig. Its unhealthy to breath, and its a fire/explosion risk.

Click to expand...

I have no issues with either fuel leaking when the car sits in the garage or with it starting up (2 pumps and it turns over immediately when cold) so I figured there was enough pressure in the system to keep the bowl from draining back. Still, it didn't cross my mind that there may be an active break/tear in the fuel system and that could be both the cause of the gas spill and the gas smell.

For the garage smell, it's mostly only an issue when the tank is freshly full and I attributed it to parking it soon after filling it up (and therefore still had some gas residue from the spill). Otherwise, it's just your normal hot classic car smell most days which isn't bad.

Sounds like upgrading the tank to 30/40 gallons soon is called for since I'm going to want to get a better look at the fuel lines and connections anyways.

Just a thought, if it’s using that much gas, and you are deleting the spare tire any way for a 40 gallon tank, you could go nuts and make it a three tank system, or keep the saddle and make it a dual tank. It is just wasted space if you don’t. Sounds like that saddle tank has to come down either way. I agree, it’s probably bad fuel lines someplace.

Plenty of us can help you with this mod if you want. Pollack valve is about $90

@texdave
It’s gotta be one to the fuel pump, one return, one vent.

Broken85 said:

Just a thought, if it’s using that much gas, and you are deleting the spare tire any way for a 40 gallon tank, you could go nuts and make it a three tank system, or keep the saddle and make it a dual tank. It is just wasted space if you don’t. Sounds like that saddle tank has to come down either way. I agree, it’s probably bad fuel lines someplace.

Plenty of us can help you with this mod if you want. Pollack valve is about $90

Click to expand...

It's a Suburban so no saddle tank and no spare tire delete, it's a rear tank mount.