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 melee. 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, and sometimes when a Survivor finds one he or she will yell, "Molotov here!".

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.

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 finale and The Passing finale, 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 this is neither recommended nor wise because they are valuable and necessary to complete the chapter. Be aware that the Spitter can ignite these gas cans to prevent them from being poured and to damage Survivors.

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 firetrap weapons, serving only as chapter objectives and to alter character models.

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 tank can be significantly less dangerous to players than thrown weapons. Due to lag or confusion, Molotov cocktails 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.
 * The gas tank can 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 Finale-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 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.)
 * 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 melee attack using the gas can. In practice this means that gas cans can be carried much further than some players imagine.
 * 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 player or team.
 * Gas cans can be used to get the "Burn the Witch" and "Towering Inferno" achievements, even though both achievements ostensibly require the use of a Molotov cocktail.

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 charging from in massive groups. Once there 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.
 * 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 a Tank is known to spawn or traverse and back-pedal away from it.  When the Tank 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.
 * 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 incapacitated and within its danger radius. A flaming gas can not only inflicts 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 couple 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 tanks 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 back-pedal 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.
 * If you're playing 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 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).
 * If a player is carrying a gas can (Especially in Scavenge or the Dead Center finale) and he/she is 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).