My 99 mountaineer is unstable and smells like gas

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 :)