Gas can

The gas can, also known as fuel or petrol cans, can be found throughout the game. They can be picked up with the Use key (default: "E" for PC; "X" for Xbox 360) and release flames similar to a Molotov cocktail when shot. A player cannot shoot while holding a gas can but can still shove. There is no inventory space for this weapon, so the only method of transporting it is to pick it up and carry it. These cans are usually found around Crescendo Events and finales. They are very effective when set up and ignited to create walls of flame providing a strong defense against large horde attacks. Strangely, gas cans share the Molotov's glass shattering sound file when they are ignited.

On account of their efficacy when defending against large horde attacks and for survival maps, players will often attempt to stockpile them for later use. This can spell disaster for a team because if cans are shot by mistake, the team will engulfed in flame and must flee its camping spot to become unexpectedly exposed and vulnerable. Even on lower difficulties, where fire damage is usually negligible, the fact that all of the cans ignited together in the same spot means that all of the instances of fire are stacked in the same area, and that anyone standing in that area receives many times more damage from the fire than they would normally.

The most useful and generic tactic with gas cans is to spread them in an arc in front of a defended position, shoot at one can and to rely on its flames to ignite the unlit ones nearby, thus causing a large chain reaction and a wall of flame that helps with horde attacks and finales.

In Left 4 Dead 2, the new mode, Scavenge Mode, as well as the Dead Center and The Passing finales, uses gas cans as a non-offensive purpose. In these contexts, yellow gas cans are scattered across the level (normal red gas cans can also be used if carried over from the preceding chapter to the finale, but their use to fill the generator in The Passing finale has been reported to cause game stability problems) and must be collected and carried back to a generator (or car) to refuel it. Yellow gas cans can still be shot, but in most scenarios this is neither recommended nor wise because they are valuable and necessary to complete the chapter. Be aware that a Spitter can ignite these gas cans if they have been picked up and dropped to prevent them from being poured and to damage Survivors. Spitters cannot ignite normal gas cans under any circumstances.

Finally, there is also a new green gas can containing diesel fuel that is only seen in Hard Rain. These cans are unusable as melee and fire-trap weapons, serving only as chapter objectives and being attachments to character models. These cans are retrieved to refuel the Lagniappe, the boat that Virgil uses to rescue the Survivors from the Swamp Fever and Hard Rain campaigns.

Tactics

 * The gas can has a broader area of effect than the Molotov cocktail, making it more effective for large choke points and wide-open spaces.
 * If placed before a showdown or event, the gas can is able to be significantly less dangerous to players than thrown weapons. Due to lag or confusion, Molotovs and pipe bombs can land in close proximity to Survivors, making them a liability by inflicting damage on the team and sometimes forcing them to abandon their preferred choke point. By contrast, the gas can provides a safe weapon that can be pre-positioned and set off at the right moment from a safe distance.
 * However, if the survivors wait too long before attempting to trigger the gas cans, their line of sight may be blocked by other infected that must be killed to allow the bullets to reach the can. If the survivors are trying to ignite a Tank, this delay may be more than enough for the Tank to run past the can's area of effect.
 * The gas can is able to be carried from chapter to chapter if it is brought into the safe room at the end of a stanza. Consequently, a gas tank found near the end of a chapter might be more valuable when retained for use in the next chapter rather than expending it at its original point of location (campaign and single player only, not Versus).
 * If the Survivors carry gas cans to a finale, which requires the use of the special yellow gas cans, the stock red gas cans from earlier on in the campaign can also fill up generators and the stock car using the Fire or Use key just like the yellow cans, meaning you can go 15/15 with only 11 yellow cans, or you can even fill it up to 19/15 (be warned, however, that using red gas cans in this way can sometimes result in the counter resetting meaning that the team has to start the task all over again). This can be very helpful on Expert Difficulty, where traveling to get gas cans from far away is dangerous.
 * While it is often better to drop the gas can for hordes, small groups and single Common Infected can be killed or batted away with a shove using the gas can. In practice, this means that gas cans can be carried much further than some players imagine.
 * Players can't use any ranged attacks while carrying a can, however, so if a teammate is attacked by a Special Infected, dropping the can will be required before the player can react, allowing more time for the infected to harm teammates.
 * The fire from the gas can does not normally burn in water, but it has been known to do so occasionally.
 * Gas cans cannot be damaged while being carried, so players should not worry about friendly-fire risks in these circumstances; however, as soon as a gas can is dropped, it returns to a damageable condition and can be accidentally ignited by a stray bullet, with consequent risk to the Survivor team.
 * Gas cans that are on fire before being picked up will explode in Survivors' hands after a period, however. But if the Survivor drops the can, the fire will somehow extinguish itself and the can will no longer combust from it.
 * Gas cans can be used to get the "Burn the Witch" and "Towering Inferno" achievements, even though both achievements ostensibly ask for a Molotov.
 * If you explode the Gas can next to the Witch, she won't attack you while she's on fire.
 * If a second Gas can is ignited by the first Gas can, the Witch won't attack you as well.
 * There's a trick to set her on fire without counting as startling her. To do this, place the can near to her, shoot it and quickly go idle before the fire ignites her (Note: Do not place it too close to the Witch or it will set her on fire instantly before you have time to go idle). If done correctly she will not be startled and will simply die without any risks to survivors. This trick can be very useful in Expert where she can one-hit kill survivors. It's recommended to bind a key (Bind   go_away_from_keyboard   ) since using the menu key won't give enough time. This trick works as well for the Molotov.
 * Smokers' tongues are immune to gasoline fire damage so occasionally a Smoker standing off at a distance behind the fire will snag a Survivor and drag them through the defensive flame wall. When this happens it will be difficult to initiate a rescue and the victim will invariably suffer health damage from both choking and the fire. Survivors can counter this risk by keeping behind hard cover when they know a Smoker is around.

Survivor Tactics

 * Never stand near a gas can when setting it off; stay outside a minimum safe distance.
 * When faced with a Crescendo Event or a finale, take time to set up first, and this includes searching the area for gas cans, propane tanks, or other explosives. Try to place gas cans in choke points, or in areas where the Infected usually charge from in massive groups. Once a large group of Infected is in or near the target area, set it off. The flames will effectively thin out most, if not all, of the zombies in that rush.
 * When throwing a gas can up, punch it right after throwing it. (Mouse 1, then Mouse 2 quickly after) The can will fly a much greater height and can make it up ledges otherwise unreachable.
 * Since Common Infected always charge towards the team in short, straight lines, those behind the front ranks of a horde will continue to run blindly through a wall of flame thereby incinerating themselves (a burning Common Infected is no longer a threat and can be safely ignored).
 * Gas cans can make decent substitutes for a Molotov when facing a Tank. Set a can down in an area in front of where he is known to spawn or traverse and backpedal away from it. When one appears, shoot the can to ignite it and allow the Tank to set himself alight when he charges through the fire.
 * If gas cans are thrown by pressing the "fire" key while holding them, they bounce in random directions and make accurate placement impossible. A better method is to put them right where they are needed by standing in the desired position and dropping them by selecting any inventory item or pressing the Use key.
 * Gas cans are handy for covering your back in a Panic Event.
 * Be careful when using them: Before they burn out, the fire will spread about 4-6 feet, so being right next to it is not a good idea.
 * Never shoot a gas can if there is an incapacitated survivor within its danger radius. An exploded gas can will not only inflict further damage on the incapped player but makes it an awkward and unpleasant experience for teammates trying to effect a rescue and revival mission.
 * It takes a gas can a few seconds to ignite if spat on by a Spitter in Scavenge mode. During this time, the orange outline will flash a warning and a quick player can pick up the can to prevent ignition. This usually requires running through acid, so the Survivor needs to be sure they have enough health to secure the can and remove it from danger. If not, they may be incapacitated and the dropped can will ignite, leaving the player to suffer both acid and fire damage.
 * When setting up gas cans for a crescendo event or a finale, try to leave at least one escape route open. This comes in handy when, for example, a Tank unexpectedly shows up and the team can backpedal without having to get through a wall of fire.

Infected Tactics

 * Gas cans, in addition to propane tanks and oxygen tanks, give a clear signal of where the Survivors expect Infected to come from. You can use this to your advantage by spawning or moving to areas that are in the opposite directions of these weapons.
 * As a Smoker, one strategy is to position yourself on one side of the flames and drag a Survivor through it—especially if you can keep the Survivor in the flame (great if it's a set flame, such as some of the flames on The Subway), without being burned yourself.
 * While playing Scavenge or Versus (on The Atrium and The Port) as a Spitter, try to spit on the dropped cans. The acidic goo will ignite the fuel cans, costing Survivors a valuable point and extra time (however, this only works on gas cans which have been picked up first).
 * Cans that are together in a pile will ignite each other, further delaying the Survivors.
 * If a player is carrying a gas can (especially in Scavenge or the Dead Center finale) and they are on their own, pouncing them as a Hunter will often set the gas can up next to you and your victim, increasing the chances that it will be shot by their would-be rescuer in an attempt to kill you, and deal extra damage to the player (as well as creating problems for the person if and when they try to revive their teammate).
 * Additionally, if the can is ignited somehow and you aren't killed, the Hunter deals extra damage to pounced Survivors while on fire, further increasing damage dealt.