The impulse manifold is a vital part of starships' impulse drive system.
In 2152, Trip Tucker detached a section of hull plating from Enterprise that a Romulan mine had clamped on to. Tucker noted that detaching this section of the hull would expose a good portion of the impulse manifold. (ENT: "Minefield")
In 2375, readying the Defiant for departure, Nog noted that the impulse manifold was purged and clear. (DS9: "The Changing Face of Evil")
Also in 2375, when the Enterprise-E was travelling through the Briar Patch, William Riker ordered full impulse. Geordi LaForge replied that the manifold can't handle full impulse in the Patch. (Star Trek: Insurrection)