Where is the fuel tank located? where is the fuel injector located?

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 :)

What year ? Is it 4.3 V6 ? The 4.3 had injector units and regulators that were prone to leak .

Your problem may be one of the following:
First check or just replace the fuel filter. Usually located under the car in front of the rear axle, drivers side.
Next, the fuel pump may be worn out. Usually they get loud or stop working (running).
Next, check the fuel pressure regulator. Usually located on the front fuel rail, drivers side.
Is the vacuum line attached? Not leaking or rotted?
Also, do you smell a strong fuel odor when it won't start? May be one or more stuck-open injectors, or the coil pack is bad.
Checking these in the order listed will more than likely fix your problem.
The clicking noise indicates the battery can no longer run the starter, which also means the fuel pump won't run either.
It could be a fuel issue. How old is the fuel filter? Can you hear the pump run for a few seconds when you turn the key on? Try turning the key to ON without trying to start engine three times waiting for the pump to stop running each time. Then try to start the engine. If it starts, Its a form of Vapor lock caused by the extreme heat we have had lately, and a weak fuel pump.

If none of this works, try a new fuel filter first. From there have someone check the fuel pressure regulator and then the pump. If you are in need of a new pump, take my word for it and spend the money to get one from a Dealer, and not from your favorite parts store. Most parts store parts are made in China and they are JUNK.

Your problem may be one of the following:
First check or just replace the fuel filter. Usually located under the car in front of the rear axle, drivers side.
Next, the fuel pump may be worn out. Usually they just stop working (running).
Next, check the fuel pressure regulator. Usually located on the front fuel rail, drivers side.
Is the vacuum line attached? Not leaking or rotted?

You dont need another fuel tank. Running a vehicle out of gas allows any trash or goo to get in the fuel system. Trash mostly is trapped at the pump inlet screen and in the filter. GOO can make its way into the injectors. Throw in a bottle of injector cleaner. A sure indication of a bad fuel pump is an audible constant change in rpm's. It sounds like it's changing gears.

Gas leaks are serious. If it was in the evap system the engine light would be on. So you need to check or have checked the entire system. From the top of the fuel tank, the lines under the vehicle up to the engine(mostly steel) At the engine they turn to rubber or hard plastic. Also check the fuekl rail and injectors. There are two Orings per injector. And check the fuel regulator. I lost my van to a gas leak that ignited from a faulty spark plug wire.

The gauge no longer working is related to the pump, I would take it back to the shop can have then correct that.

Not sure what you mean by smelling gas, is it leaking on the ground or running rich?

If it is running richy, the fuel pressure needs to be checked, if it is too high it could cause it to run rich. Otherwise now that pressure is up with a new pump, older/leaking injectors could now be the issue not holding up under the full pressure.

One other thing you would try is reset the computer (disconnect battery for afew minutes) then drive at a steady speed for about 10 minutes.

Hope the helps.

The filter is inside the tank. could have a fuel leak, thats why it smells. I would ck it cold, turn the key so that the fuel system pressurizes, then follow the lines and look for a leak. ck any joints, and at the injectors.

have 1994 4.3 cpi and the problem I had sounds same.
It was a leaky pressure line in pletium . easy fix $45.00 part $15.00 gasket.
the leak was caused from the fuel lines rubbing together in pletium.
take the vortec cover off ,then remove air regulator in middle and look inside . I turned on power to key and gas spray came from drivers side of pletium looking in hole.fuel line side. if gas spray on passanger side then gas regulator faulty. takes 5 min to check
After fixing truck runs runs great. no more gas smell in cab.
also you can smell oil for gas

My experience in this makes me think that you have a rusted fuel line, at the top of the tank. This is a common problem. I remove the tanks and replace the fuel tank fuel line/bracket assembly quite a bit.

You more than probably have a gas leak somewhere in the fuel system. If you can smell gasoline, it is leaking somewhere. The lines run along the frame to the engine. The leak might be hard to see or hidden (on top of the fuel tank).

The leak is robbing fuel pressure from the engine, causing it to idle rough, so is probably on the pressure side of the system.

(Most fuel systems, except for "returnless" systems with Pulse width modulated fuel pumps (not very common), have a pressure line and a return line to the tank.