The dark gray liquid commonly termed "diesel slobber" or "slobber" consists of unburned diesel fuel, water, and sometimes crankcase oil which has passed through the combustion chamber, but did not burn. These are materials that did not burn because of cylinder temperatures which are too low to promote complete burning. The lower temperatures occur at start-up until the engine comes up to operating temperature, or when the engine is idled for prolonged periods, which causes the cylinder temperature to lower. Small amounts of crankcase oil sometimes find their way past the piston rings and valve guides in a new engine or in a "cold" engine while idling, and the unburned crankcase oil mixes with the unburned fuel to add to the "slobber." Joint in manifold is designed to prevent manifold breakage due to expansion and contraction during normal cold to hot operating conditions.