Have my car on 3/4 full want to top it up but fuel gauge still showing 3/4 full

faulty fuel gauge sender unit, inside your fuel tank,bolted to the top and hangs down inside,uses a float and arm to give signal to fuel gauge.

Gas as in gasoline or gas as in LPG gas? If its petrol, the fuel sender may be out of adjustment. (Usually means a new sender) If its LPG, the fuel stations are set to automatically shut off at 80% full. The gas tank will never actually be full to the brim but the gauge should be showing full if the gas nozzle keeps shutting off. Have a look on the LPG tank itself, they have a little gauge on them which shows how full it is. I have 2 tanks on my car , when "full" or when the pump shuts down, the tank gauges show just under full but the gauge on the cars dash shows full (2005 BA Ford)

Many cars do this to discourage gas theft. If the car fuel gauge only shows 1/4 full, a potential thief may not bother to steal 2 or 3 gallons of gas. This is very normal.

If the gauge is stuck on full and you are sure the tank isn't full, you have a sensor sending wrong information to the dash gauge. Most cars now have a single fuel pump and sending unit that can be replaced by a mechanic.

Fuel sender unit could be faulty and showing a full tank all the time. Basically the gauge is a volt meter and the float arm adjusts the voltage going to the fuel gauge when it moves up and down. If it's stuck for some reason it'll show full when it's at the top.

A local garage or auto centre can fix this easily or if you're good with spanners, it's usually located under the rear passenger seat where you can find the access to the top of the tank.

needs change the sensor in the fuel tank

hi it can really only be two things, first is the fuel sender on the pump inside the tank it could be stuffed or sticking causing a false reading, second it can also be a faulty fuel gauge on the dash.

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

cost more to repair than whats worth ,gauge works ok?? then leave well alone

Its A Sure Sign That One Of Your Evap Fuel Lines Could Be Clogged.

You don;t say whether the fuel gauge is reading accurately, or whether it's always on empty. If the gauge is always showing empty, it's most likely a problem with the sending unit in the fuel tank. Check the resistance across the two terminals of the sending unit.

On a C900, the fuel sender level unit is separate from the fuel pump and located on the right side top of the tank, accessible by removing the trunk floor lift up panel (the pump is located on the left side top of the tank.
Possible problems are: electrical line off or broken, float not working (it's fairly delicate); or gauge not working.

Test the resistance of the sending unit with the lines disconnected, the resistance should be 2.6 to 3.8 ohms full tank, 63.3 to 67.5 ohms empty tank; and test for voltage across the top of the sender unit, if no voltage, that's likely the problem!

If the gauge is otherwise reading accurately (showing 'full' for a full tank, etc) then the problem is likely a gauge malfunction and can be simply replaced.

Good luck!

Saabreur