A few days ago I got the "Gas Cap Loose - Check Gas Cap" error and SES dash light on my '00 Buick LeSabre (160k miles). The gas cap wasn't actually loose. No big deal, I thought, the cap is probably old and lost its seal and I'll replace it.

With my tank near empty today, I start filling it up at the pumps. Within the first couple gallons, I hear gurgling from the fill spout then *whoosh* gasoline comes spurting/overflowing out backwards. I kept trying to fill the tank veeerrry slowly, but it would gurgle and back up everytime. I'm not a big fan of wasting money by pouring gasoline all over the ground, so I stopped.

Obviously there is a vent plugged up someplace, that is probably causing the gas cap error too. Where do I look and how is this fixed?

its likely the vent valve plugged, try unplugging the line to the vent valve and see if you can fill the tank

Where is the vent valve and the line that goes to it?

was the car running at the time

Actually... yes. I hadn't considered that being a possible source of the problem.

its not expecting you to do that so the valve could have been closed and testing it. if you dont clear the light with a test tool it will have to time out and retest before the light goes out. may take a while because there are temp and fuel level spec it has to meet before it will test

I think I understand what you are saying about the valve closing... I'll try to fill again with the engine shut off.

The gas cap warning came on a few days ago before this spillage problem, so I think that might be a separate issue? If the overflow problem persists, is there a way to check for restrictions and clear that vent line somehow?

the vent valve has two jobs, it opens to vent the tank when appropriate, and closes to test the tank and lines for leaks. its often closed when the car is on and testing and the car would say its a loose gas cap if you have the cap off and filling the tank. i dont think you have any issues to worry about, you just cant fill the tank with the key on.

ALWAYS fuel with engine OFF.....try it and see if that helps....also remove the engine cover and get the engine up to operating temp....you will see a small black round canister right in the center of the front of the engine with an electrical connection and rubber hose..put your finger on it and see if you can feel it pulsating...if not replace

AGAIN NEVER fill the car while running..it sucks fuel into the canister and causes other issues....let us know

Quick update...

Engine off while fueling = no overflowing gasoline so that'* good.

Replaced gas cap = still have the "Check Gas Cap" warning and SES light. Only driven 1-2 times since then though and didn't have the code reset with scan tool (yet). I've been out of town and not driving the Buick the last few days, so I'll pull the (-) battery cable tomorrow to force a reset and see what happens with the code.

On a related note, is there any way to remove the plastic lanyard from the factory gas cap and affix to this new aftermarket one?