It is the Toyota intake manifold gasket that creates the seal between the intake of your engine and the cylinder heads. If for any reason this seal has a breach in it, the air and fuel mixture in the combustion chambers will not be correct and the engine will not be capable of producing the power and fuel mileage the Toyota owner expects it to produce. It is a very rare occurrence for an intake manifold gasket to fail. There are no oil or coolant galleys that can be breached like in the heads of the engine. The most common reason for the intake manifold gasket to be in need of replacing is when the Toyota engine is being disassembled for another reason; such as PCV service, valve replacement, etc. If you have a blown head gasket, then during the resembling of the upper portion of your engine, a new Toyota intake manifold gasket will need to be used. Before this is done, the old one has to be completely removed. If any of the old gasket material is left behind on the heads, block or manifold itself, the new seal will not be formed and a leak will occur. Unlike other gaskets that seal oil or coolant, the intake manifold gasket seals air. Because of this, there will be no external way to inspect the seal to see if it is functioning properly without help. The typical manner in which a leak in the intake manifold gasket is detected is with a vacuum gauge hooked up to the engine. A leak can be heard as a whistling sound if the breach is large enough, but that again is very rare for a Toyota owner to experience. A cheap way to check for an intake leak is to mix some liquid soap with water, and gently rub the mixture along the joint between the manifold and the head. If you see bubbles, there's a leak.