Gas gauge inaccurate

1987 XJ 4.0

When my gas tank is completely full, the gauge is on the full mark, never above, like I have seen in my other XJs. When the gauge reads 1/8 of a tank, the low fuel warning light comes on as expected. I can easily drive 75 miles with the light on. If I fill it up when the light comes on, I find there is still about 6 gallons in the tank. I am asking, based on your experience, what you feel the probabllity is that this is due to the sender or due to the fuel gauge.

Where im at that retarded light goes on all the time because im always going up or down hills, also pushing the gas peddle will make it go on sometimes. I can have a quarter tank on flat ground and go up a hill and the light goes on. All mine did that and i have driven awhile a few times with that light on and never ran out. I just think there an estimate and have no accuracy.

As I said in the original post, the gauge is inaccurate, that is my problem. The only reason I mentioned the low fuel light was to make the point that it agreed with the inaccurate gauge reading.

They are known to be inacurate. In some cases by an 1/8 tank or more. If you really wanted to do something with it you would have to pull the pump and inspect the bobber thingy I do believe.

Well, couple of things. The sending unit float could have become saturated, so it isn't rising enough. Replace the float and see what happens.

The float arm could have been bent at some time in the past, and bending it a bit "down" should have the effect of it providing higher readings.

From the FSM 90, resistance values: 0 ohms = empty; 44 ohms = 1/2 tank; 88 ohms = full. Buy a couple 44 ohm resistors at Radio Shack, test the gauge using known resistance values to see what the gauge reads.

Good luck.

MoFo said:

1987 XJ 4.0

When my gas tank is completely full, the gauge is on the full mark, never above, like I have seen in my other XJs. When the gauge reads 1/8 of a tank, the low fuel warning light comes on as expected. I can easily drive 75 miles with the light on. If I fill it up when the light comes on, I find there is still about 6 gallons in the tank. I am asking, based on your experience, what you feel the probabllity is that this is due to the sender or due to the fuel gauge.

Click to expand...

Sounds like it is working properly(the light should come on with about 5 gallons left).

RCP,
Where did you find the 5 gallon threshold for the low fuel warning? I did not see any such reference in the FSM. I know the tank does not hold 40 gallons. In both my other XJs the light comes on when the gauge reaches exactly 1/8 of a tank. Once again, my problem, quesiton or complaint is not with the low fuel warning light. The mention of the light was only to support my question of the inaccurate gas gauge reading. This is why I chose the title for this thread. Forget about the light, if that makes it easier for you to think logicaly.

you can pull the unit from the tank and actually adjust the float arm sender for the fuel gauge. There is a little screw on it you can loosen to adjust it back and forth depending on which way you need to adjust the gauge.

The light is controlled by a module in the instrument cluster that reads the gauge resistance, and is set to come on when the sender resistance shows 1/8 of a tank. If it comes on at that level, then the light and gauge are probably accurate, and the problem is that the sending unit in the tank is out of adjustment.

My beater had the same problem, and once I got the tank dropped and pulled out the sender unit, I saw where the breadboard had slipped from its original position (marks on the breadboard from the original screw positions showed that it had rotated a little). I loosened the screws, shifted the breadboard back into position, and it has worked wonderfully ever since. Might also be a problem with the float becoming saturated, or if you have landed on a rock maybe you dented the bottom of the tank and the float won't go down anymore, any number of possibilities really

Good thing to do is know approx mileage you can get out of a tank, then it does not matter how incorrect your fuel gauge is.

4 things are certain in life, death, taxes, lies from politicians, and lies from gas gauges.

None of my 4 work right, they are all broken in slightly different ways. 96 reads a half tank lower than it actually is (due to a 97-up tank and sender I installed), 91 reads nowhere near linear and completely inverted (due to the '88 sender I installed), 98 reads properly but randomly drops to 0 due to a gunked up sender, also trips DTC P0463 (no CEL from this one fortunately.) M54A2 reads *mostly* correctly but is screwy toward the bottom end, so I pop the cap and see how full the tank is myself.

Carry a full 5 gallon jerry can in the back and know about how many miles you get to a tank, the 5 gallon will save your ass once or twice and you'll be glad you have it.