The repair kit is a tool used for recovering lost durability on any component found in the player's Vehicle. It can only be used to repair 3 full points of durability, and will vanish when it cannot be used anymore.
The repair kit will simply add 1 full point of durability for every use. Since the game hides the decimal values, it may seem like you can use the repair kit more than you should be able to. The fact is, you're repairing the remaining decimal values that are not shown. For example, if your engine has 2.87 durability remaining, the game will show you 3. Using the repair kit will recover the remaining .13 durability, and will also subtract that much from the repair kit's total.