The gas and the temperature gauges don`t work. they don`t move at all, it shows like the gas tank is empty and the temperature in the same position

Remove the gauge cluster and check for a bad ground. If it is bad you can resolder the trace on the back of the cluster back to the groung pin for the connector.

Buick gas tanks are shaped so when full they take a while before the gauge comes off full. After 1/2 then the gauge will move towards empty quicker. When on an incline it will show more gas than is actually in the tank, so when you're cornering, it's reading will be indicating the moving level. I wouldn't worry about it

If your fuel gauge is showing you in accurate readings this is normal to all vehicles that are not used to fill-up gas full regularly but sometimes the cause of the inacuracy is due to faulty float valve in the gas tank you need to replace it

Turn the ignition on and off several times before inspecting the gas tank because a blown fuse is the usual cause for fuel gauge malfunctioning. If the gauge remains motionless, check your fuse box and determine the fuse's location using your service manual.
Replace the fuse if the link is broken or damaged. Then, inspect the gauge's wire connections for cleanliness and make sure the wires are connected.
Turn the ignition switch to the "off" position. Attach one end of a jumper cable to the fuel-gauge terminal behind the gauge itself. Attach the other end to the ignition terminal.
Replace the sender unit in the gas tank if the gauge reads full and your car does not have a full tank. Replace the fuel gauge if the needle remains inoperative.

Hi, 1st , of course, check for a blown fuse for your instrument panel.
If your gas gauge is reading empty then it could be that the float in the gas tank has a leak and doesn't float anymore. It has sunk. This will cause a working gauge to show empty all of the time. The float that goes into the gas tank is about 1.5 inches long by about an inch wide. It should be a hollow brass cylinder.
this cylinder attaches to a swinging arm that when it moves in the tank, it shows a different resistance depending on where the float is. In other words the height of the float connected to the arm shows the different reading. Note: if you are going to replace the float, or the fuel tank sending unit, make sure the gasoline tank is as empty as possible.
Photo below of what a float looks like (this is probably not the exact one). You may only be able to purchase a complete "fuel sending unit", which includes the float. Probably the best way to get one is from a junk yard.


Hope this helps,
Happy New Year,
Mark

the gas is regulated from the switching valve you control off the dash. Check the valve to hear it switch
have a friend help you do this. it will be located under the truck between the two tanks. You may have a faultly valve or switch rattling inside changing the tanks setting also while driving which will be hard to tell

It sounds like the fuel sender is bad in the fuel tank.

the problem is caused by a defective gas gauge sending unit, this part is located inside the gas tank, you must remove the gas tank to replace it.

The gauge circuit is shunted on most cars, this is wierd, but needed to stabilize the readings... Your description sounds like normal function. Drivers would complain if the qauge were to be instant, going around corners, starting, stopping, all move fuel, and the gauge would indicate this, cauing driver alarm...Been this way for ever, although yours sounds like it is on the far side of the scale...if it reads correctly after a while, you could replace everything and you would have the same performance.

Well that tells me that someone installed the wrong fuel level sensor. Your going to have to drop the tank and pull the pump and float assymbly out. Then go to a parts store and get a sending unit that has the opposite resistance of that sending unit. Just wait until the tank is empty so its a lot lighter. You can call the parts store or go down and see if there is multiple sending units. The sending units work by resistance, when the tank is full the float moves up and turns a wheel in the sending unit, which either removes or increases the resistance causing the gauge to lower. The sending units either give full resistance at full or no resistance at full and then changes as the level goes down until it does the opposite at empty.