Posted 03 October 2010 - 06:29 AM
I once bought a truck that would indicate it was low on fuel.
The gauge made me think it needed to be filled up, when all the while, it was tricking me!
I suspect your Fuel Gauge is indicating incorrectly, and you "think" you need to fill up, and when the overflow occurs, you "think" your tank is unable to vent.
Because you have a 1991, I highly suspect your Fuel Pump SENDING UNIT, the variable resistor which is attached to a swing arm with a float on the end, is defective.
My 1994 LS400 has access to this pump and sending unit, by removing the rear seat cushion first, followed by the rear seat back.
Next you will see an access panel, with screws holding it onto the sheet metal where the seat back was. Removing it will allow access to the pump and sending unit.
Because I think your tank is VERY full, I would first drive it at least 200 miles, and then gain access.
You can purchase a subscription to www.alldatadiy.com, for a 1 year period, and have access to all kinds of good service manual info.
My 1994 LS400 has a solenoid valve, that is located on the driver's side, inner fender, down low. A single vacuum hose connects to this valve, and this rubber hose then attaches to the front top edge of the Intake Manifold. Only when the Computer commands this valve, does gasoline vapors from the tank, enter the Intake Manifold.
A blocked vent line, under the car on the driver's side would NOT cause the engine to die at idle. You should look for other causes of poor idle. It's likely a coincidence that filling up, and dying at idle occur.
Gas Caps are bi-directional and SHOULD allow air to enter the tank, when displacement is necessary; ie,,, you're using gas out of the tank, air needs to enter.
Does your tank suck air in when you open the cap? Replace it.
It would be nice to know your tank is not the problem, rather your Sending Unit, the item that tells your Fuel Gauge what to indicate.
When I worked on Audi's, it WAS the tank.........! I don't have nice things to say about Audis.
Best of luck,