The 'Cold Stream' pack, which is currently in beta and playable for PC users, requires additional testing before it can go live.
If 20,000 users complete the campaign and earn the Stream Cross achievement by Sunday, July 10, Valve will release the 'Dead Air' campaign from the map pack early on July 22.
'Cold Steam' takes a different form from previous map packs such as 'The Passing' and 'The Sacrifice', as it is developed by the fan community and not by Valve itself.
'Cold Steam' will also feature the full 'Dead Air' campaign from the original Left 4 Dead.
"Normally on an internal project we would have the whole company take a part of each day and do nothing but play the upcoming game or DLC and submit bugs," read a statement from the firm.
"To add to the fun we often give out prizes for participating. Since Cold Stream is not just being developed internally but also with the community - we want the community to help pound on the campaign and look for bugs."
Left 4 Dead 2 is currently on sale in this week's Steam Summer Sale for £3.74 until 6pm BST today.