Select the chapter or view the game index.
Gears of War Walkthrough Act 2 - Powder Keg
Go over to the gas station. Old man will tell Marcus he didn't think Marcus would make it so he didn't fill the car up. Head over to the wheel. Turn the wheel to start fueling the junker.
The old man will tell Marcus to come in for some ammo. Several Locust, including a Boomer will attack. Take them out.
When they're dead a cut scene will begin.