Cadillac Repair: 92 Cadillac DeVille--Gas Tank problem, fuel tank

Question
I've got an old cousin with this car, she's had it since 93 or 94 when she bought it used.  She's so old now so I sometimes drive it more recently taking her to appointments and stuff.  I've noticed, the car only holds 6.5 gallons of fuel or so.  The digital meter says there are 15 gallons left, when it starts running out of fuel, with minor hesitation, and soon it hesitates more and loses speed quickly (on the highway anyway).  Then you fill up the tank, and just today, while encountering this for the second time, it only takes 6.68 gallons to top off!  Then drive away from the station back onto the highway, and the car runs fine, no more hesitation or anything!  This is the second time the car has done this since I've been driving it for her.  It seems the tank only holds less than 7 gallons, but she insists the tank is an 18 gallon tank!  I have no idea what could be going on here...  Would you have any guess?  I'd appreciate anyone's help!  She always takes the car to the original dealer where she bought it, to have any kind of service done, even oil changes.  I hate to see her take it again, when she just got it back two weeks ago from the dealer, and have to pay out more money for something we don't even know (other than the fuel tank?).  

Thanks,

paul

Answer
Hi Paul,

     My question would be have you looked under the car at the gas tank to see if it might be collapsed. I have heard of rare occasions that a vacuum will get pulled on the tank because the vent valve is restricted while the fuel pump pushes out the gas and causes it to collapse. I checked in the shop manual and your cousin is correct...the tank is an 18 gallon capacity. I can't think of any other reason that physically you can only add 6.68 gallons to the tank and it causes it to run again. Bill