Gas gauge fluctuate/vibrate like a hummingbird, only when keys on. its a 98 chevy pickup. how do i fix it

if it bad need to replace it cleaning it not going to work

there are two things that can be wrong here.
1-wiring or the sensor itself may be bad.
2-you have a bad head gasket(not blown all the way)but starting to go therefore the fluctuation on the gauge itself.
does it go down when you give it gas? does it uses coolant with out leaking? let me know

98 chev pickup s10 put gas in the gas tank drove it about 2days and the check engen light came on had it check mech said it was the gas cap replace the gas cap the light back on now what help

all later model chevy vehicles are plagued with this problem it,s the fuel level sensor mounted on the fuel pump module inside fuel tank unfortunately on some models it,s only available with purchase of whole new fuel pump module that can run as much as $500.00 dollars for just the module not including labor to drop fuel tank and replace I encourage you not to waste time and money with aftermarket less expensive pump module assemblies not to knock any parts people but our shop just installed the 3rd napa fuel pump module in a 98 blazer because the new pump will not read correct amount of fuel finally it was replaced by a pump module assemblie from dealership at cost of $458.00 for pump it now reads correct hope this helps if you need more detailed instructions on how to replace repost at fix ya thanks

i have a 98 cav with the same problem, it's not really worth fixing if you know how much it is off by,

I'm not a mechanic but i had the same problem for 2 years with my 98. I fixed it yesterday. You have to repair or replace the gauge. Start by pulling the trim piece off the gauges.(it just pulls off, no screws.) Then there will be four screws that holds the instrument panel in place. Remove the instrument panel. on the back of the panel there will be four more screws, this will give you access to the gauge. twist and pull the gauge out and replace or clean the contact plugs. I had to fill the gas tank up to make sure i got the indicator needle in the right place. (to make sure the tank was full when the gauge said full) This worked for me, hope it works for you. GOOD LUCK this only takes about 45 min. from start to finsh

I think that the problem mainly lies in the Fuel Sending Unit. Check that its Fuel Level Sensor is all right and is the latest model. I am telling this because if it is not the new one, it can cause corrosion in the sensor and cause shaking and fluctuation of the Gas and Fuel Gauge. I think you can get it.
Thank you Sir, have a nice day.

sound like a problem with your sending unit inside the gas tank with fuel pump or gauge in dash defective but with GM usually it is the sending unit

try replacing the vehicle speed sensor in the rear of transmission

as far as the temp gauge goes...i have a 91 1500, i had the same problem. i replaced the temp sensor and it fixed the problem.

as far as the bad gas mileage... if you haven't already done so, perform a tune up; plugs, wires etc.