I wanted to add my experience/fix here.

My mom's car (2000 Elantra, 195k miles) has had this problem for a few years now. For about a year, I'd been able to remove that line from the charcoal canister connected to the liquid vapor canister, dump all the pellets out, and allow her to fill-up relatively trouble-free. The pump would still shutoff every $5-6. These past 6-7 months though, it got to the point where it shut off literally every few seconds and made it absolutely impossible to fill the tank up, even with the line on the liquid vapor separator completely removed. The filler neck would spit gas out all over you and down the side of the car as well. When you can no longer fill-up even with the charcoal canister to liquid vapor separator hose removed, pellets have clogged the lines between the fill vent valve and the liquid vapor canister, and nothing you do on the charcoal canister side will fix the problem at this point.

It's obvious/already stated here the main culprit is the charcoal canister. We already know there is a "membrane" that bursts when gas gets into the canister and swells the pellets, allowing the pellets into the vent lines and clogging them up.

I wanted to see this "membrane" and find out exactly why it fails so I cut the charcoal canister apart to have a look-see. Here's what it looks like cut apart, about what we'd expect, just a container full of charcoal pellets. Interestingly there are two separate chambers for each line:



Because of the two chambers, I cut the top into two sections to be able to seal the chamber back before putting the top back on. Anyway, as you can see, here is our mythical "membrane" - just a friggin piece of thin foam. No wonder it fails! A really surprisingly terrible material choice by Hyundai. Of course we aren't SUPPOSED to overfill the tank, but surely Hyundai knew it would happen and should have used mesh instead:



You can see this side hasn't failed (yet), but is close to it:



So, my curiosity quenched, I permanently fixed the canister (the way Huundai should have built it in the first place) with mesh. I just used the mesh from cheapo mesh sink drains:





I used two part "plastic weld" to put it all back together (better than new):



Now, clearly you don't have to go through all this trouble. As somebody said already in the thread, you can just use mesh over the outside holes on the canister and achieve the same effect. Again, I just had to see the exact cause for my own curiosity.

Unfortunately, even after this, I still wasn't able to fill the tank up. I figured the line between the fill vent valve and the liquid vapor separator was full of pellets and perhaps even the fill vent valve itself had pellets. I dropped the tank, and sure enough I was right. Here's the fill vent valve, full of pellets, as you can see:



I didn't take a picture but the entire hose between the fill vent valve and liquid vapor separator was full of pellets too. Unfortunately there's no good way to completely clean out this line or the fill vent valve without dropping the tank, removing the line and the valve, and manually dumping the pellets out. I also removed the liquid vapor separator and by being ale to turn it all around in various directions managed to dump a whole bunch more pellets out of it (surprising, as I figured it would be empty from my dumping it and tapping on it all the time in the past).

I put it all back together, went to the station, and filled it right up completely and quickly. I am confident it's now permanently fixed.

It sucks I had to go through all this trouble due to Hyundai choosing to use foam instead of mesh. It only cost me about $10 total but dropping the tank definitely isn't fun and all in all this took me a good 4-5 hours of my time/labor (though granted some of that spent on cutting the canister up and repairing it, again you can just mesh the outside holes). I know, however, that my mom will appreciate the permanent and complete fix. Good luck to the rest of you guys out there with this issue.

 

Thanks SOOO much for this information. I have had this problem on my 2004 Elantra Gt only in winter - once it gets cold, I have to start dribbling in the fuel. However, tonight, as I was dribbling in fuel, I saw what I thought was snow-melt coming off the mud-flap - turned out to be gas!!! The schematic is very helpful. I guess it's into the barn for a fun New Year's project! alfagirl56

 

Thanks for the great info on this issue. I've been dealing with this problem on my 2003 Elantra for the last few weeks. Thanks to info on this thread and others, found my charcoal canister had indeed degraded. I've replaced the canister (very simple) and been dumping pellets out of the vent line coming off the tank at every point that has a removable rubber hose. The problem has gotten slightly better (1/4 gallon goes in at a time now), but I have a bad feeling that the fill vent valve is crammed full of the pellets. Like previous posts say, it looks like the only way to access the fill vent valve is by dropping the tank, which is beyond my mechanical skills.

Does anyone know if the pellets will eventually work their way out back downstream toward the liquid/vapor separator? I'm planning to keep dumping these lines after every fill up in the hopes that this is what will happen now that the root cause has been replaced (new canister).

 

James: Brilliant! Now I know what to look for on my Accent. It's getting worse. I think that putting mesh over the outlets would work, but they may clog up with pellets too. Looking at other posts about the problem, it seems that even the Hyundai dealers don't know how to fix it. They replace the canister, but as you have shown, all the lines and other items need to be cleaned out. Now I have to decide if I am going to tackle the project, or pay to have it done. Thanks for posting this and all of your hard work.

 

To 90%, the filling issue is caused by the sticky FUEL-SHUT-OFF VALVE, which is a one-way-valve (PN# 31160-2H500), and located in the filling opening of the gas tank.
its function is to prevent fuel spilling out of the filler opening, when the fuel vent valve on top of the tank closes to prevent overfilling, causing a "shock wave type" backwash towards the filling opening. This part is inexpensive and easy to replace. When the tank is 1/2 or less, remove the rubber hose at the tank, put your finger into the tank, pull out a plastic tube, and insert the new one. Fill tank, to test that the issue is solved

 

Update on my 2002 Accent slow fill problem: It seems that there could be several causes to slow filling: 1) Carbon in vent lines caused by failure of foam filter material in carbon canister. Obviously an incompatable material in the carbon canister. I checked the vent lines and they are all clear. 2) Faulty (sticky) fuel shut off valve (check valve). I tried this, and am waiting for the part to arrive. The part is under $6 at the dealer. The rubber fill hose was stuck to the metal of the tank. I used a tool made for removing stuck radiator hoses with out damaging them. Well, guess what. It poked a hole in the fill hose and that costs $45. (Outrageous). The fuel shut off valve does not come out easily and is so long that the tank must be removed to gain enough clearance. This is easier than it sounds. First, remove the lower cushion of the back seat and pry off the black access panel. Remove all of the hose fittings (squeeze the blue plastic parts on the black plastic hose fittings and pull) and electrical connector. Remove the fill hose and vent hose on the back of the tank. Remove 2 nuts on the back of the tank straps and the tank drops down. I had to pry some on the valve to pull it out. Seems OK, but the new part is arriving tomorrow so I'll replace it. Parts man at dealer said that he has never ordered one before. If you need to order one, make sure to give the model and year, not the part number. Have the VIN number as Hyundai seems to make changes during a model year run. 3) "Sampling Pump" between the fill pipe and the rear bumper was suggested. Cost:$400+ It is about the size of a small tomato paste can.
This will probably be the next diagnosis if I can find more info on how to test it. 4) The filler pipe seems very small, and probably does not help. In the UK, it seems that gas stations put smaller nozzles on their pumps so that Hyundai owners could fill their tanks. Note: Fuel Filter is under the access panel on top of the gas tank. Easy to change, but a bit of a pain to get to.

 

I changed the check valve on the gas tank of the Accent, and it improved the situation, but I don't think it is as good as it can get. I removed the gas filler hose on the tank, and the check valve is so long that it can not be removed without removing the tank. There are 2 nuts that hold the tank strap on. I removed them and the back of the tank dropped down. It then slipped back and broke a fitting on the pump/sender assembly. BIG TIP: Remove the hoses and electrical connector FIRST. The hoses just pull off if you squeese the 2 blue (on mine) blocks on the connector and pull. When I removed the check valve, it seemed a little sticky while the new one operated smoothly. I bought a used pump/sender assembly from a wrecking yard. It had been sitting on a shelf for a long time, and the pump looked like it had some corrosion on it, so I took the best looking parts from the one I had and the one I bought and put it back in the car. This assembly was used until April 2003, so if you buy one, check it first. The later one is obviously different and has only 2 hose connectors, where as mine has 3 connectors. The Hyundai dealer here always asks for the VIN number to make sure on any parts they sell. I put everything back together, and the battery powered impact wrench my wife gave me for Christmas really came in handy. At least now, I can slowly fill the tank, but I can't use the latch on a fill nozzle- it trys to fill too fast. It also seems to me that the fill pipe on the Accent is very small, and maybe the tank just can't vent fast enough to let it fill. My other cars have larger fill pipes. I'll keep everyone updated. Does anyone have any other info?

 

So here's my variation on the many ways one can approach this: removal of just the carbon pellets themselves.

What I did:
1. Removed the canister and cleaned stray pellets and other debris from hoses.
2. Used 0.75" bit and hack saw to cut canister open over both chambers.
3. Completely dumped out all pellets; removed clogged 1" circular filter and all cotton debris, as well as plastic spacer and (2) springs. Left white breather filter in place.
4. Cut plastic piece from storage bin lid; shaped using heat gun.
5. Re-sealed with epoxy resin, holding in place with zip ties for overnight curing.
6. Asked the gods to offer eternal rewards to inventor of epoxy resin (optional).
7. Re-installed exactly as before.

The results:
- Re-fuels the same as any other car. Not a single click-off until tank is full. Took ~15-20 minutes previously.
- ECU code disappeared and has not come back.
- No loss in fuel economy. Vapors can be smelled during re-fueling, but I actually like the smell.

Theories:
Anyone have any predictions? I'm wondering how this might play out in the long run. In any case, so far -- so good. Couldn't be better.

--
2003 Hyundai Sonata 4-cyl 222,248 miles

 

Fixing the “hard to fill gas tank” problem, without removing the gas tank. Sonata 2004, 2.4L 4cyl. Like so many others I too had the slow fill problem. I fixed the carbon canister and then the air-filter and still no joy. I removed the gas tank, pulled the “Fill Vent Valve (ORVR )” and shook out the carbon pellets and that was the problem. Back together and all is well. I will tell you that pulling the gas tank is a pain and was unnecessary. I came up with a way to avoid pulling the tank. First a review of what others have done and my own work. Note my info about the carbon canister is specific to the Sonata. It seems there are different canisters for different models and my info on the canister is Sonata specific.

Diagnosis: Slow fill. If when you first begin filling gas, you can get 1 or 2 tenths gal in before the cutoff it is UN-likely to be a stuck “Fuel Shutoff Valve” or the head of a Barbie doll stuck down the filler pipe. Also if the slow fill problem has been getting progressively worse over time this is a clear indication that the foam filters in the carbon canister have failed and now little carbon pellets are floating through the hoses where they ought not be. And the chances are near 100% they have clogged up the “Fill Vent Valve (ORVR )” and that is the primary issue as to why you can't fill the car with fuel.

1. Pull the Carbon canister. I marked the ½ inch ID hose going to the Canister Close Valve with a white marker so as to make reassembly easier. Same for the small hose going to the fuel tank pressure sensor. And a corresponding mark on the canister. If carbon pellets fall out when you pull the hoses off or when you shake the canister with the tubes down, then you know for sure the canister is defective. Get a new one. In my opinion all of them made from 96-05 are defective so buying one at a junk yard is begging for more trouble. Or if you want to repair yours I suggest cutting off the rear end without the 4 ports about an inch from the end. I emptied mine out and replaced the foam goo with green scrub pads that I had on hand. Would have used the white if I had it. Local supermarket item. Two 6x9 inch pads will work fine. By cutting an inch from the end you can use some thin strips of metal (I used Aluminum) 1x5 and pop riveted it back together. Then sealed it with a hot glue gun. It must be air tight. Any air leaks and the check engine light will come on. Just buy a canister unless you always wanted to know what was in there. Don't put it back on yet.

2. The “Fill Vent Valve (ORVR )” has carbon pellets in it and that is what is keeping you from filling the tank. While this is called a “Fill Vent Valve” it also has a float that sticks down in the tank about 6 inches and in normal operation when the tank fills, the float pushes up and cuts off the outgoing air. There is a large (1 cm) ball bearing in the top of the valve that pushes down and holds the air vent open. But when the float pushes up from the bottom and carbon pellets are present they hold the ball bearing up and that keeps the valve closed all the time. So you start fueling with the “Fill Vent Valve” already closed and of course it shuts you down. When I pulled my “Fill Vent Valve” out and shook it the ball rattled and a bunch of pellets fell out. Then when I disassembled it it was clean and had no problems. I mention this because it shows that it is easy to clean without dis-assembly. BTW there is no reason to buy one of these. They are easy to clean and have no parts other than the gasket that can go bad. My gasket was in excellent condition and was reused.

Update: There is also a small spring in the valve that holds the valve up (closed) if tilted on it's side or upside down. The weight (force) of the ball bearing is greater than the spring and opens the valve when the tank is not full. See the Fig 4 below to see how the carbon pellets fall under the ball bearing and keep it from pushing down and opening the valve.


3. I had considered using a vacuum to try and suck the carbon out but after thinking on it a bit I decided the ball bearing resting on the carbon would not allow the pellets to come out. That leaves two solutions. Remove the tank or make an access panel to get at the “Fill Vent Valve”. Please understand that there is already a plastic access panel to get to the fuel pump. It is in the center of the car at the back of the trunk. Open that up and you'll see the fuel tank is about 1.5 inches below the access panel.
The “Fill Vent Valve” is 7 inches to the left side of the fuel pump on the Sonata. Measure from the left side of the fuel pump 5 and 9 inches to the left and mark (Sharpie). You now have two marks 4 inches apart left of the fuel pump and on center line of the fuel pump. Drill 1/4 inch holes at those marks. Warning. Just drill through the trunk and not the gas tank

SPECIAL TOOL NEEDED. Harbor Freight Tools sheet metal nibbler. They sell 2. One is up to 22 gauge and the other is up to 18 gauge. The smaller number is thicker so get it. SKU 60507 under ten dollars.

With the nibbler cut between the holes and then cut forward 3 inches on each side and bend it up. Warning, sharp edges. Cover all edges with Duct-tape. The valve will be right there and you can remove it with a Phillips screwdriver (3 screws) pliers for the hose. Pull the valve out and shake it and see the pellets come out. Get an air blower and blow air through the hose in case any pesky pellets are sitting in the lines.. Put it all together and place a six inch square of sheet metal over your new access panel. Glue and screw or what ever makes you happy.

See the photos

Miscellaneous: The diagrams show an “air filter” after the canister-close-valve but don't show where it is located. It is on the side of the fuel filler tube and the liquid vapor separator just below the fuel inlet. To get to it remove the left rear wheel and then remove the plastic cover in the wheel well. The air filter is cheap and hard to cut open and repair. I'll include some pictures. Newer cars call for replacing it every 60k miles. Probably a good idea to replace it if you are doing everything else. Mine was very dirty and had a little resistance to air flow. Once you get to it there are three bolts holding it in (10mm heads).

“Fill Vent Valve” can be disassembled by prying the two tabs (under the gasket). If the tabs break it is not a problem as the three screws hold every thing in place. Don't leave the ball bearing out or you cant fill the tank.

 

Great job, very informative.. Thanks guys