Fuel Gauge Stuck on Full 98 Pontiac Montana

The gas guage is likely a bad ground to the gas tank. Check the wiring harness where it comes from the body and goes into the gas tank. Likely there is broken wire there. The harness usually has 2 or 3 wires one may be neutral for both the pump and the fuel gauge. or 2 wires each. An Ohm (resistance) tester will show a particular amount of resistance on 2 wires indicating that the fuel guage is working properly as the resistance will be different depending on how much gas is in the tank. You should be able to buy a cheap digital tester for $10.00
It is likely that these wires are damaged close to the tank or in that harness area that is exposed to the elements, perhaps even at the spot where it goes back into the body or where the harness joins to the tank harness connector. Look for broken or corroded wires here. Try running a piece of wire from the body to the gas tank and see if it makes the fuel guage work indicating a bad ground to the tank.

Most likely it a bad fuel level sensor on you fuel pump. You will have to replace the fuel pump as that is the only way to replace the sensor for you fuel gauge. The sensor is built into the fuel pump. The contacts on the sensor become corroded over time and cause the sensor to malfunction causing your fuel gauge to read full all the time. I have a 2001 Montana extended version that had the same problem. In order to know when I needed to fill the tank, I would completely fill it each time I put fuel in and reset my trip odometer. I would go 400 miles on the tank before I would refuel until we were able to replace the fuel pump.

Most likely problem is a faulty sending unit. Bad news... it's inside the gas tank.

It needs a tank meter I'ts in the fuel tank

Under the front seats you have wires going to your seat belt anchors.These wires are atached to a sensor witch activate the air bags if the anchors have force pull on them.The wires have plugs under the seat.Disconnect the plugs and spray with electical cleaner and replace.Make sure the ignition is switched off befor you attemt doing.90%of the time this will work.good luck.

need to be reset by a garage sry

when the fuel pump was installed the gas tank sending unit was either jammed when put back in the tank, is defective, or the sending unit was no reconnected.

replace fuel pump/Fuel gage sending unit (All in one) also wire pigtail to asmbly Pontiac Montana's are prone to wire falures and shorts $300+ if done in drive way alot more if you have a shop do it!
P.S. REPLACE ASMBLY WITH AS LITTLE FUEL IN TANK AS YOU CAN...!

sending unit or float in gas tank, check if any broken wires or well plugged in and then check the float for the gas.

you need a replacement fuel tank sending unit, the resistor part of the unit is faulty.. good luck!