The fuel sloshes back & forth as you accelerate.
The fuel sending unit is sticking, or the wiring to the gauge is loose. These are the two most common causes of an erratic gauge.
Most vehicles have one fuse that protects all the gauges so if the fuel gauge is the only one not working, the fuse is probably okay. Most causes for a inop fuel gauge is a faulty fuel gauge sending unit in the fuel tank.
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Fuel gauge fluctuations can be caused by several factors, including a faulty fuel sender unit, which may not accurately measure the fuel level due to wear or damage. Additionally, irregularities in the vehicle's electrical system, such as a weak battery or poor ground connections, can affect the gauge's readings. Changes in fuel sloshing around in the tank, especially during acceleration or braking, can also cause temporary fluctuations in the gauge. Lastly, issues with the gauge itself, such as a malfunctioning instrument cluster, may contribute to inconsistent readings.
Fuel Pump
Defected fuel pump sending unit. The #1 cause is that the sulfur in todays gas coats the fuel leveling sensor and causes the fuel gauge to act up. It is a common problem that is starting to take place.
A bad o2 sensor can cause this problem in a car. A bad fuel pump, fuel filter or fuel line will also cause hesitation during acceleration.
Try to find a fuel pressure gauge. The fuel pump sounds suspicious from your description.
When you turn a corner the guage fluctuates.
The gas gauge measures the amount of fuel left in a vehicle's gas tank, not velocity, mass, or acceleration. It indicates how much fuel is remaining and helps drivers estimate how much longer they can drive before needing to refuel.
The economy gauge on the dash of a 1989 Mercedes-Benz 190E indicates the vehicle's fuel efficiency in relation to the driver's acceleration. When the needle is in the "green" zone, it suggests that the car is operating efficiently and using less fuel. Conversely, if the needle moves into the "red" zone, it indicates that the engine is consuming more fuel, typically due to aggressive acceleration or high speeds. Drivers can use this gauge to adjust their driving habits for better fuel economy.
The normal causes of an inoperative fuel gauge is the sending unit in the tank. Since the truck is 15+years old you might as well replace the fuel pump when you have the fuel tank removed.