'99 Blazer. After about 1.5-2 gallons of gas at the most when refilling gas start to come out of the goose neck at the cap. Seems like its all staying in the goose neck and after 15mins or so it seems to drain into the tank. I've heard from 2 different people its an evap issue. I can not locate/completely understand what im looking at underneath. I see what I believe to be the overflow can and some sort of valve or relay on the rear cross member that has fuel lines running to it. I also know there is a vent on the pump itself supposedly.

The pump itself works, however the gauge does not, planed on replacing it sometime down the road when it either craps out or i got tired of doing the mileage in my head....

can anyone tell me where to start?

 

if you cant get fluid in then there is pressure in the tank, check the vent at opposite end of tank , maybe clogged.
either way you are going to need to drop the tank.

 

...don't necessarily have to drop the tank...
...I had that same thing happen to me this past winter when one of the vent lines from the gas tank to the evap canister got plugged with ice (I'm still trying to figure out how that happened...maybe there is a connection between that and my intermittent P0440 code...I'm going to take the canister down and carefully inspect it for cracks)...
...don't know how different a 99 Blazer and an 01 Jimmy are in terms of the evap system, but my canister is a black plastic box mounted right by the rear bumper on the driver's side. There are three rigid plastic lines that 'plug in' to it with a sort of quick-disconnects. In my case, the bottom one of those lines, or more accurately the 'nipple' on the canister that the line connects to, had gotten clogged with ice (the canister had an accumulation of water in it that froze). When you disconnect those lines from the canister, you should clearly hear the accumulated pressure vent out of the tank through one of them, which would suggest the canister or one of the 'nipples' the vent lines connect to is clogged...if not, then one or more of those lines themselves need to be checked...THEN you might need to drop the gas tank...
...the gas tank needs to vent through that canister or air/vapor pressure builds up in the tank and will not allow you to put gas into it...that's what is happening to you...

 

My 2001 Blazer had a similar problem a few years back. I replaced the vent solenoid and never had any more problems since. It is located in front of the spare tire.

Blazeme

 

...update...

...took down the canister, cleaned it off and looked it over; no cracks or other damage. But figured out the 'mystery' of how water got into the canister...Evap canister has two large 'nipples' and one smaller one. Top one (large) is marked TANK, middle one (smaller) is marked PURGE and lower one (large) is marked AIR. AIR line (the one that got plugged with ice) runs to the vent valve (above spare tire). Vent valve has a solenoid valve (normally open) that snaps onto a plastic box; box has a piece of foam inside and has openings on top and bottom (thus tank vents through the foam and out the openings in the box...top openings are larger than bottom openings). AIR line is a downhill run from vent valve to evap canister; storm with sufficiently heavy rain and high wind blew water up under the vehicle...some of it entered the vent valve, ran down the AIR line, and collected in the bottom of the evap canister...storm was followed by a sharp drop in temperature; collected water froze in AIR line nipple and connector...tank could not vent until I disconnected AIR line, removed ice (whereupon release of accumulated pressure in fuel tank which had been preventing adding fuel to tank could clearly be heard) and reconnected AIR line...
...Found a flattened O-ring in the PURGE line connector and replaced; could have been causing the P0440 code (minor leak); actually replaced the O-rings on all three connectors...cleared the code and passed inspection...'Suzy Q' (Mrs. named it) is legal, back on the road...and does that ever feel good ...
...btw, the discussion of the P0440 code and the evap system on this forum is very good..