This tip applies to all current and previous fuel-injected GM vehicles. When replacing an engine due to internal damage, extreme care must be taken when transferring the intake manifold to the new engine long block.
After removing the intake manifold from the engine, carefully inspect all cylinder head intake ports to see if the valve heads are still present and not bent. Usually, when the valve heads are missing or sufficiently bent, internal debris will be present in varying degrees in the intake port of the cylinder head. If debris is present, the intake manifold should be replaced.
The replacement is required due to the complex inlet runner and plenum design, making thorough component cleaning difficult and often impossible to verify removal of all debris. Installation of a contaminated intake manifold can result in damage and potential failure of the new engine.
Source: Auto Service Professional