82 911SC high idle after fueling

[SCadaddle]

This has happened to me on a few occasions, most recently yesterday. Start the car as normal.....well, as normal as my CIS functions with fluctuations in idle at startup anyway....settled on a high 1100-1200 rpm cold idle then drive a few miles to get fuel. It's off the high idle by the time I get to the gas station and turn the engine off.

When fueling, I've got the pump handle turned around to better fit the curve of the fueling pipe to the tank. Add about 8 gallons to the 4 that were already in the car. Install the filler cap and then start the engine. It immediately idles at about 1600 rpm and continues to idle that high for a few minutes if I wind up at a stoplight after driving off before returning to about 900 rpm. Almost like relieving a vacuum condition in the fuel tank by removing the cap to fuel the car? I didn't listen or feel any tension on the cap when removing it to add fuel. Thoughts?