http://en.wikipedia.org/wiki/Nissan_Altima
Third generation Altima's have unfortunately been affected by a recall related to the intake manifold and the Power Valves, also known as the Butterfly Valve. The Technical Service Bulletin - Campaign Number P5202 - does not mention all affected vehicles for production years with the same intake manifold design. This has left some owners affected by the problem with no recourse or remedy for what in some cases has amounted to catastrophic engine failure.
The Power Valve is a series of four plates in the QR25DE 4-cylinder engine, one per cylinder. The plates open and close simultaneously to allow more air into the combustion chamber for increased power. Each plate is held in place by two screws. The screws have been known to come loose as well as the plates and are then ingested into the engine via the intake ports.
Nissan's remedy has been to remove the intake manifold, and if the screws are present, remove them and apply loctite upon reinstalling the screws. Nissan has in some cases elected to replace entire engines, however on a selective basis.