One of the most annoying driving problems is a faulty fuel gauge. While it won’t have any effect on the car itself, you'll always be wondering in the back of your mind whether or not you have enough gas. Fortunately, there are a few simple fixes other than constantly keeping your gas tank filled to the top.
Checking The Gas Float And Sending Unit
The simplest explanation of a faulty fuel gauge is that the float in the gas tank has gotten stuck in a certain position. This mechanism is very basic - the float in the gas tank will adjust a variable resistor as it rises and falls depending on the level of fuel. This variable current is then sent to the instrument cluster where you get a reading on the gauge. When empty, the resistance should be around 110 ohms and around 2-4 ohms when full. To check this part, follow these steps.
- Lift up the carpeting that covers the parcel shelf on the driver's side.
- Under here will be a panel held down by 6 Phillips screws. This is the cover for the fuel sending unit. Be careful and make sure there are no naked flames around before opening this.
- Once you open this panel, you will be able to see the fuel sending unit.
- If you unclip the wiring cluster, you can check to see if the wiring is bad from this unit to the gauge. Connect an ohmmeter to the yellow wire and ground to see if there is a reading. If you get a reading, the wiring and the gauge are good and the problem is within the sending unit. (Shop gauges here.)
- Unclip the spring clips that attach the fuel lines to the sending unit and remove the lines. There is a possibility that a small amount of fuel will drip from these so be ready for this.
- Unscrew the 8 Philips screws that hold the sending unit in place and lift the sending unit off. This will expose the gas tank and its contents, so make sure you are in a well-ventilated area when you’re doing this.
- Reconnect the wiring cluster and press down and lift up the float a few times to try and free it from whatever has held it in place. Check the gauge as you’re doing this to see if there is any movement. The gauge may be a bit slow to react so be patient.
- If the gauge does move when you move the float lever up and down, this confirms the problem was with the float.
- If moving the float arm up and down didn't cause the gauge to move at all, you may need to replace the whole unit.
Replacing The Gauge
If, when testing the sending unit wiring, you discovered that the problem was with either the wiring or the gauge, follow these steps.
- You will first need to get the cover off of the steering column. This should be held on with 3 Philips head screws.
- Now you will need to remove the covering from the instrument panel. This is held on by 4 Philips head screws that are around where your legs would be when you are driving.
- Once all of the screws are removed, pull off the cover. There are a few plastic clips that hold this in place. Begin by working the two clips under the dashboard and then the one nearest the hood and pull it toward the steering wheel. The clips will make a “crack” sound when they come free but be careful not to pull too hard in case you break any of them.
- With the instrument cover removed. there will be 4 Philips head screws that hold the instrument cluster in place along with 3 wiring clusters. Remove all of these.
- You can now pull out the entire instrument cluster. Check the connections of the fuel gauge for any signs of corrosion as this may be what is causing the problem.
You can try to remove the corrosion and see if that fixes the problem but we would recommend changing the gauge. You can find all Miata parts here.