Gas tank question
 

Can a dented gas tank throw off the gas gauge? I would fill the gas tank in my 69 firebird and it would show full on the gauge, but I would shut it off after driving it and then it would show 1/2 tank. Any ideas what this could be? The tank has a dent it it too.

How big? Are we talking about small hail ding type dent, or does it look like Big Foot Kicked it?

The sender could be bad. Did it work before the "dent" ? how big of a "dent" I think its doubtful a dent would alter the reading. Tank sender grounded well?

The dent was in it when I bought it. I had the sending unit taken out and tested which it worked along with the gauge.as for a dent yeah it looks like Bigfoot kicked it in.

Hmm, well, if it reads the level, and the dent is huge and in the bottom, the fluid would read higher as there is less space inside.

If anyone doubt's, take a soda bottle or water bottole or milk jug. Turn it on its side. (Make sure air can escape)Create a large dent in the bottom. Watch the fluid level rise.

Wonder if I someone tried to jack it up there

A good body shop should be able to pull that dent out (mostly) but new tanks are cheap enough. If it was out to check the sender, it would have been easy to replace it or get the dent out then.

Does it go back to full (or a little under) when you turn the key back on again or did I not understand the question correctly?

it fluctuates. I would fill the tank then drive for a bit and it would still say full. I would park it and shut it off. Then when I go to drive it again the gauge would change and it would say half tank or sometimes even quarter of a tank. I would of fixed the dent before when I had the sending unit out and testing it but the sending unit was tested with the gauge and a meter. The sending unit works fine so I think but the gauge is still not reading correct.

I would clean the sending unit ground wire and where it connects to the body. Then unplug the unit in the trunk area and use an ohmmeter to see what the readings are (0-90). If you move the sending wires around and you get no reading change, there may be a pinched wire (or mouse chewed), bad connection or gauge itself.
Float may be sinking or sticking on sending unit.

I will check it out. Thanks for the help.

do the 68's work on resistance the same way the earlier cars do? If so, I'd think less gas = less resistance = lower reading.

As for how much it really effects things, I've dented up the 22.5 gallon gas tank on my jeep despite skid plates and it has not made a noticeable difference in capacity - and I run it VERY low on a regular basis (no flaming me please, at 12MPG, I stretch my fill ups out as much as possible).