Fuel system issues...need help

[travis]

Sorry...not really a performance question per se, but I’m kind of lost here.

1999 C2500 suburban 2wd with the L31 350. I discovered a leak “somewhere” on the fuel tank...there is some weeping around the vent holes in the plastic guard under the tank. I haven’t went looking for the leak just yet (too f’ing cold and windy outside right now) so I’m not exactly sure where it is. Is epoxy a effective fix for a fuel tank?
I question whether epoxy will last long for this reason. Every since I bought this thing, there has always been a massive amount of vacuum in the tank when I pop the gas cap to add fuel. I replaced the gas cap with a new one thinking it was plugged, but no change. It seems to run the same whether the cap is tight or left loose (so it won’t build vacuum). I’ve owned plenty of fuel injected vehicles and none have ever had vacuum like this in the fuel tank. Aren’t these things supposed to be vented somehow. Wouldn’t this put a lot of strain on the fuel pump, or structurally on the tank itself?

I was looking at replacement fuel tanks, and figured I might as well replace the fuel pump while I’m in there as mine has always sounded loud and rough to me. This gets confusing...California vs federal emissions is easy enough (mine is federal), but what is this “GFM code” thing?? There is also a pretty dramatic price difference between pumps...any suggestions which is best?

Could having the wrong fuel pump be causing this vacuum issue? The CA emissions pump is significantly cheaper than the federal pump through local parts stores (but not so much online), so it makes me wonder if someone replaced the pump in the past with the cheaper version.

[barnym17]

It shouldn't buld a vacuum the fuel system has a purge valve that basically vents the tank to the intake other componets involved but the ecm controls it, fuel tank repair epoxy does work if you follow the directions religiously esp. about cleaning the repair area.

[travis]

I’m reading through a chiltons manual and I’m confused. The book doesn’t specify how the tank is vented...just that the cap vents under “extreme” vacuum or pressure. It states that the fuel vapors stored by the charcoal canister and drawn into the engine once certain conditions are met. The book makes it sound like air/vapors only travel one direction...from tank to charcoal canister, then from canister to engine when the pcm tells the purge solenoid to open. So how is the tank supposed so stay at or near ambient pressure? Where does the the air come from to replace the fuel pulled out by the fuel pump during engine operation?

[SupStk]

I'll pass along some advice that was offered to me by a buddy who runs a repair shop. When the pump went out on my shop truck ('02 K1500) he advised me to replace it with a genuine AC Delco. The original pump lasted 113k and the replacement now has 70k and still going strong.

[user-23911]

So how is the tank supposed so stay at or near ambient pressure? Where does the the air come from to replace the fuel pulled out by the fuel pump during engine operation?

The fuel evaporates into the tank.

Otherwise known as "vapour pressure".

[travis]

Did some troubleshooting...here’s what I’ve got. 21” of vacuum at the intake mounted purge solenoid mounted on the intake manifold, and 21” at the end of the purge tube at the charcoal canister. I’ve got about 1/2” vacuum on the tank side of the charcoal canister, and just sitting there idling there is a light vacuum in the fuel tank. Apparently when driving is when it is building big vacuum in the fuel tank. From research it seems that the fuel tank vent valve clogging up is fairly common. Question is now...where the heck is this vent valve? Mounted on top of the fuel tank? My service manual makes no mention of this valve at all

[rfast racing]

dorman shows the valve as 911-001 it is located on top of the fuel tank the best thing to put on the tank leak is seal all it comes in a yellow tube you can put it on even when leaking gas too stop a leak

[travis]

I’m just going to replace the tank...I’m going on a 4-5k mile road trip here in a few weeks...don’t need any issues popping up in the middle of nowhere (and where I am going is certainly going to be the middle of nowhere)

[stealth]

Plastic tank on this correct?

I would check fuel filler and feed/ return hose connections etc.

Lifting the bed is the best way to get to the fuel pump module

[travis]

Plastic tank on this correct?

I would check fuel filler and feed/ return hose connections etc.

Lifting the bed is the best way to get to the fuel pump module

Metal tank with a plastic guard around it. No bed to lift...it’s a suburban.

Didn’t get to it this weekend...too busy cleaning up Friday nights storm damage 🤬