Directing a portion of the exhaust through a passage in the intake manifold (fig. 3-17). The heat from the exhaust transfers and heats the mixture. The amount of exhaust that is diverted into the intake manifold heat passage is controlled by the manifold heat control valve.

Directing the engine coolant, which is heated by the engine, through the intake manifold on its way to the radiator (fig. 3-18).

Gaskets

Gaskets (fig. 3-19), otherwise known as static seals, are used to form pressure-tight joints between stationary members. They are usually made of a deformable material in the shape of a sheet or ring, which conforms to the irregularities in mating surfaces when compressed. Steel, aluminum, copper, asbestos, cork, synthetic rubber, paper, and felt are just a few of the materials that are used singly or in combination to produce leakproof joints. The proper material used in gasket construction depends on the temperature, type of fluid to be contained, smoothness of mating surfaces, fastener tension, pressure of the substance to be confined, material used in construction of mating parts, and part clearance relationship. Some of the most common engine gaskets are as follows:

CYLINDER HEAD GASKET which is placed between the cylinder head and the cylinder block to maintain a gastight and coolant-tight seal. It is made in the form of two thin plates of soft metal with asbestos tilling between them.

Figure 3-17. - Exhaust-heated intake manifold.

INTAKE AND EXHAUST GASKETS are made from asbestos and formed to a desired shape. Some of them are metal-covered and similarin construction to a cylinder head gasket.

OIL PAN GASKET is generally made from pressed cork. It may be made in one piece but is often made as two pieces.

Gaskets also can be formed by using a silicone sealant. This type is formed by applying sealant from a squeeze tube to the mating surfaces and allowing it to dry, forming a sealed flexible joint. This type of seal is becoming more popular on modern vehicles.

Oil Seals

Oil seals used in vehicle assembly are designed to prevent leakage between rotating and non-rotating

Figure 3-18. - Water-heated intake manifold.