I know there are float/sensor mechanisms in the tank that measure the amount of fuel in a tank. I also understand that when the level gets low the float may tend to rise/fall.
What i don’t understand is the situation in my 2000 Pontiac Bonneville. After 400 miles the gauge should be pretty close to E, but it isn’t. In fact, i have more gas at 400 miles than at 200. Here’s the kicker, only when i put the gear selector in neutral will the gauge actually fall–it doesn’t matter if i’m rolling down the road or sitting at a red light, neutral is the only option that makes the needle fall to E. Once it’s at E and i go in gear again, i get a full tank of gas for a few seconds, then it falls back to about 1/2 tank.
Any ideas what is causing this or why my gauge is somehow connected to the gear selector?
Anecdotal evidence, but probably pretty convincing:
There’s something wrong. I used to have a 2000 Bonneville, and the fuel gage worked absolutely perfectly. So, something’s funky. In most modern cars, your float isn’t directly connected to the gage – it’s all arbitrated through a computer. Ah, speaking of that, does your Bonny have the gallons-to-empty indicator? If you have the LCD display for all your options, it should have the indicator. Does it stay consistent?
No LCD, this model isn’t quite an SE, but it isn’t quite an SLE either. And the gauge’s behavior has been consisently wrong* for about 3 months now. It used to work perfectly.
The fact that it works when you shift into nuetral is really interesting. This leads me to think that you have a bad ground somewhere. When you have bad contacts and bad grounds, you can get things either not working at all or finding alternate paths to the chassis to ground themselves out. This is when really weird things happen, like in one car I have, whenever you honked the horn all four blinker lights would come on. Since the path to ground for the horn wasn’t very good, it was finding another route, through the blinker circuitry.
Squink’s link shows one of many possible causes for this, and the bad contacts may not be at the fuel tank, but could be anywhere in the circuit. If you’ve got a mechanic in your area that specializes in electrical problems you might want to take it there. These things can be hard to troubleshoot.
It won’t stop me from spewing information, though! No!
Awright, awright. Fuel tank sensors are all the same, even the ones that pretend to express it in tenths of a gallon. It’s an arm with a float on the end, either plastic foam or a little metal can full of air. Both kinds get dirty (which makes the float heavier,) and both kinds can get heavier on their own (foam will absorb fuel, and the cans get leaks.) Either way, they show less fuel than you have.
If you’re lucky, the floor of your trunk has a little bolt-on hatch that leads into the roof of the tank; that leads to the tank sensor.
Enlist a friend to stand by with a fire extinguisher while you risk your life on this part. It should be a straightforward repair from here, right? Sometimes, it is. You did fill the tank airspace with inert gas, didn’t you? And the ground strap around your ankle goes to ground? Oy, I’m such a worry wart. I just want to see you after the job. ;j