Why doesn't it fill with gas?

Modern cars have thing called evap canister. It is carbon filter installed on fuel tank vent. Problem is thank many people top of tanks to above it limits and gasoline instead fumes reach that filter when tank is overfilled. Once gasoline enters filter it became solid and prevents fumes from escaping tank via filter.
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First, is this a 4.0L or 5.0L engine? Without a description as to what is meant by "unstable" it is impossible to address. However, as to the "gas" smell there is a couple things that come to mind. I am assuming that you mean a raw gasoline smell and not an exhaust gas smell. If you mean an exhaust gas smell and it is a 5.0L engine you could very well have a cracked exhaust manifold and that most likely on the right side of the engine. If you have a gasoline smell then consider these things. First, if memory serves me correctly (but it's been nearly 20 years ago) I seem to remember a recall on Mountaineers and Explorers of those years for the concern you have. I think the recall had to do with replacing the fuel tank filler neck. But we need to know more details. Is the smell noticed from the inside of the vehicle or from the outside? Is the smell more from the front or the rear of the vehicle? Does it occur more when the tank has been filled? Does it occur more on hot days. Are you "topping-off" the tank when you fill? Is the "check engine" light on. If so what are the DTC's stored in the PCM? First step is to have the entire fuel system inspected for leaks or damaged lines. This includes the fill tube and seals to the tank, the tank itself, the fuel lines to and from the tank, the fuel rail at the engine and its associated fuel injectors, all the evaporative emission lines that lead to the carbon canister. If all that looks good then consider these things. If the fuel tank is "topped-off" when filling the tank this can cause a condition where raw fuel can be forced through the evaporative emission lines into the carbon canister. The carbon canister's purpose is to trap excessive fuel vapor due to thermal expansion of the fuel. The canister should be located in one of the inner front fenders. I think on the left side. These carbon canisters over time can become saturated with fuel vapor and need replacement. I recommend not topping-off the tank when filling. Especially on hot days. When fuel is pumped from the tank in the ground at the gas station the fuel is cold. But when it is pumped into the tank it begins to heat-up causing a great deal of thermal expansion and as a result vaporization. This is where the carbon canister comes in to trap those vapors and stored to be burned later when the vehicle is driven. If the smell is from the rear area from the outside of the vehicle then carefully inspect the fuel fill tube to the tank. If the smell is mostly from the outside and mostly by the front of the vehicle and no raw fuel leaks are present then it is most certainly from the carbon canister. Over time I have learned that the fuel odor emitted from a saturated carbon canister has a unique smell that is somewhat different from a raw fuel smell. If the smell is noticed inside the vehicle then careful inspection of the fuel system under the hood needs to be done. If nothing is found I would suspect the carbon canister. However, all these things I mention can cause a fuel smell both inside and outside the vehicle depending on the ambient conditions and the degree to which the smell is being generated. As an ASE and Ford certified driveability technician for many years I realize the fuel smell concern on Explorer and Mountaineer vehicles of your year can be a difficult concern to diagnose and pinpoint. As such this type of concern is best addressed by an experienced technician. So if no glaring fuel leaks are found I would suspect the evaporative emissions system and a saturated carbon canister. If the system is still working properly and if you don't overfill the tank the problem may correct itself in time as the carbon canister is purged of the excessive fuel vapor it has accumulated. But on the bright side it will keep you from smoking while you drive :)

You may have leakings fuel line on top of gas tank when filling up or line is leaking where they connect to fuel filter under body maybe behind rear wheel wells , or may have a leaking furl injector or even your carbon gas canister filtr need chaging or ovver filling can send fuel up to that canister which recirculates gas fumes back to fuel system check your gas cap maybe leaking or when gas attended over filling it runs down into drain tube

Joe

Vent tube runs parallel with filler tube, blow it out with compressed air.

Hello topromeo.

In modern vehicles there is a system in place to capture and burn the vapors when filling the fuel tank. This consists of a carbon canister, hoses and valves.
The vapors when filling the tank are routed through the carbon canister and stored there until the car is driven.

Under certain conditions these valves need to close to test the system integrity.
In your case there is a valve that has stuck closed and not allowing vapors to vent through.

This is called a canister vent valve and is located under the car on the carbon canister.
It should be available at your local Buick dealer.

If you require further advise on this subject please ask me here.

Thank you for using FixYa.

KL

Iown 02 hyundai accent gl 1.6l. Had the problem twice. I used to over fill the tank until I had to spend close to $550.00 USD. The problem is when we overfill the tank. the carbon canister gets clogged and it damages a sensor in the assembly called "DMTL". Thia assy is located under the rear driver side of the car to the left and rear of the fuel tank. Once I replaced the carbon canister, sensor and filter the problem went away. if you let it go for a long time you eventually start smelling gas inside the car after fill ups. Good luck.

Going to keep it simple since it started up again and is running, Chances are the fuel filter has come to the end of it's life and needs to be replaced. After saying this you may get away with filling the gas tank a few times and adding fuel injector cleaner to the gas since the new style fuel pump with the filter is also know as self cleaning filters and is good for life till the electric pump fails it self, which has an average life of 150,000 miles. You can get more life out of the pump if you run more then half a tank of gas always, the gas it self is what keeps the electric pump cool and less gas means hotter running fuel pump. 
GM has stopped putting in-line filters in some models since 2004. Other manufacturers have been doing it longer. The fuel sender assembly (pump/regualtor/sock/level sensor) inside the tank has a container on the bottom of it about the size of 1/2 coffee can. All the fuel pick up components are inside of this container. It has a small intake port in the very bottom that filters the bigger debris so that the fuel that is filling the container is fairly clean. The fuel fills the container and a sock(filter) is used at the end of the fuel pumps pick-up tube. Smaller debris that makes it to the sock is then filtered by the sock before entering the fuel pump and simply falls to the bottom of the container.

Todays pumps are much more robust then pumps of old also. We still see more failures of the old style pumps with in-line fuel filters then we do of the newer style. GM calls this a self cleaning system. Any debris that does not go into the canister of the fuel pump assembly falls to the bottom of the tank and any debris that the sock stops falls to the bottom of the canister when the fuel pump is shut off. We are talking very small amounts of debris here too.

Good luck and hope this helps and like mentioned, try running the HHR with more then half a tank of gas and adding fuel injector cleaner for now. 

Every gas tank has a vent tube, so when you fill the tank, air can escape, while the fuel is being pumped into the tank. Some vent tubes are metal, some rubber. Yours is either collapsed or plugged up. Look at the fill tube in the firebird and follow it from the gas cap to the tank, there should be a 2nd smaller tube that is your vent tube. Check to see if it's collapsed, pinched, or bent. This is definitely your problem. Check out this web site for pictures of different fill and vent tubes to get an idea of what to look for. Good luck let me know how you make out. Curt
http://www.carparts.com/REPLACEMENT-GAS-TANK-FILLER-NECK/GP_2005706_N__10618.car

This is the same issues I'm having. Is there any answer to this?

The fule tank vent tube is blocked/clogged. It's the little hole next to the filler hose that invites spiders that clog it up. Blow out the tube and it fixes the problem.