A leaky intake manifold interferes with normal engine operation. Once it is determined that your intake manifold is leaking there are only a few options for fixing it. The methods of fixing it are a little different in modern cars than they are in older vehicles. Modern cars have computers and sensors that need correct signals in order to keep the engine running properly. On vehicles without electronic fuel control systems you can sometimes just ignore small manifold leaks.
If your intake manifold is cracked, warped or broken it may be easiest to just replace it. If you have enough confidence in your skills you can replace it yourself and save yourself quite a bit of money, but plan on not having a working vehicle for a while. If you pay someone to replace it make sure that you use a trustworthy mechanic.
Your intake manifold has gaskets on two sides that can leak. On one side the fuel delivery system attaches to the manifold, and on the other the manifold attaches to the engine head(s). Both of these can leak, and if you're going to replace one gasket, you might as well replace them both. This is the type of job that requires a lot of attention to get things just perfect, so use a good mechanic if you're not comfortable doing the job yourself.
Intake manifolds can warp or bend under certain conditions. Once a manifold warps or bends it will not mate properly to the surfaces that it attaches to and can leak. To create a nice flat surface again the manifold needs to be machined. Take your manifold to a machine shop to get it resurfaced. This job involves removing the manifold gaskets so be prepared to replace those.
Heat stress can cause an intake manifold to crack, particularly aluminum ones. Cracks in the manifold can be welded. If you need to weld the manifold, it's a good idea to remove the manifold from the vehicle entirely. This allows you to get a good clean weld and look for any other cracks.
High temperature epoxies exist that can seal cracks in manifolds. This is a viable option for people with little money or resources. After finding the leak, make sure that the surface is very clean, and apply a good amount of well mixed epoxy. Keep in mind that epoxy across a joint will cause problems if you need to remove the manifold.
