Most repair shops have a pressure gauge set up to test exhaust system back pressure because it is the easiest method of testing for a plugged exhaust. Evaporative emissions come from the fuels, oils, rubber, and plastic parts used on our automobiles. Most of the emissions come from the fuel and this has been controlled using a charcoal canister connected to the vent line from the fuel tank. The magic of the canister is that it can release the fumes when fresh air is pulled through the charcoal, so the canister can be used over and over again. Air is pulled through the canister by engine intake manifold vacuum and the hydrocarbon fumes are burned inside the engine instead of being released to the atmosphere.
Trouble develops when the air inlet to the canister becomes plugged.
This air inlet is also the vent for the fuel tank. When fuel flows out of the tank, air must be allowed to enter. When no air can enter, a vacuum develops and eventually the fuel pump cannot pull any more fuel out of the tank. I have talked to several car owners over the last few months with this problem. Sometimes it will restart after cranking for a while, but many times it must sit for a half hour or more before it will start.