DEAR CAR TALK:  My 1996 Buick Roadmaster is almost impossible to fill with gas. When attempting to fill the fuel tank, I must keep the nozzle at its lowest flow, and it still constantly clicks off.

I had the fuel pump replaced several months ago, and the vent line was clear.

Any thoughts on how to fix this? My wife refuses to drive it if it needs fuel. – Mark

RAY: Maybe you can tow an oil tanker behind you, Mark, and use that for fuel.

Nobody wants gasoline vapors escaping into the atmosphere, where they create smog. So your car, like all modern cars, has a fuel-tank ventilation system.

Here’s how it works: When you fill up your tank, you have to push air out of the tank to make room for the new gasoline. But that air contains gasoline vapors.

So there’s a vent valve in the tank that allows the air and vapors to be pushed out and stored temporarily in a charcoal canister.

Then, when you start the engine, that air and its vapors are sucked into the engine, where they’re combusted.

My guess is that your vent valve stopped working sometime during President Obama’s first term.

So now, every time you try to put gas in the tank, the air is coming back up the filler neck instead of into the canister.

That triggers the pressure switch in the gas pump, which shuts off the flow of fuel.

Your mechanic can test the vent valve for you. If that’s the culprit, it’ll cost you about $100 to have it replaced.

Or, if that’s too much, you can start bringing your Kindle with you to the gas station to pass the time. Good luck, Mark.

Got a question about cars? E-mail Car Talk’s Ray Magliozzi by visiting the Car Talk website, www.cartalk.com.