When the engine is warmed up enough to ensure
The exhaust manifold is usually a cast-iron passage
adequate vaporization of the fuel, the valve turns, as
through which the used gases are carried away from the
shown in figure 4-24. Now, the used gases are deflected
engine cylinders. It is attached to the side of the
downward into the exhaust pipe and no longer circulate
cylinder block on L-head engines and to the side of the
around the intake manifold. The space around the
cylinder head on overhead valve engines. On V-8
intake manifold that is heated by exhaust gases is
engines there are two exhaust manifolds, one for each
sometimes called the "hotspot."
bank of cylinders. The manifolds are usually connected
by a crossover pipe and exhaust through a common
The manifold heat control valve should be checked
muffler and tail pipe. Some vehicles with dual exhausts
regularly to make sure it operates freely. If the valve
have two mufflers and two tail pipes, which increase the
sticks in the warm-up position, the exhaust valves will
efficiency of the engine by reducing back pressure.
burn due to the increased temperature that results from
the restricted flow of exhaust gases. If you find a stuck
The gasket between the exhaust manifold and the
valve, add a few drops of penetrating oil mixed with
cylinder block or cylinder head must withstand the
graphite to the shaft, and manually work the valve until
intense heat of the exhaust gases. Although gaskets are
it becomes free to operate by spring tension.
made of metal-covered asbestos, they can burn, and
they will have to be replaced if the connections become
Before reaching the tail pipe, the used gases pass
loose. Exhaust gas leaks can be detected by smoke
down the exhaust pipe through the muffler (fig. 4-25).
discolorations at the connections. If the leak is bad
enough, it can be felt and heard when the engine is
exhaust system.
running.
The muffler quiets the noise of the exhaust by
A device called a manifold heat control valve is
reducing the pressure of the used gases. These gases
often built into the exhaust manifold to promote better
expand and cool in the separate chambers of the
vaporization of the fuel during engine warm-up. (See
muffler. Flames and sparks leaving the exhaust
fig. 4-24.) In cold weather, evaporation of the gasoline
manifold with the gases are "trapped" by the muffler,
in the fuel-air mixture entering the engine is poor, since
which also acts as a flame arrester.
gasoline evaporates more slowly when it is cold. To
The muffler is connected between the exhaust pipe
assist in the vaporization of the fuel, the heat control
and the tail pipe by slip joints and clamps and is
valve deflects the exhaust gases toward the intake
supported from the frame of the vehicle by straps. Rust
manifold (fig. 4-24) when the engine temperature is
on the outside of the muffler is caused by snow, rain,
low. As the hot exhaust gases pass around the intake
and humidity. Rust on the inside results from burning
manifold, they heat the manifold and the incoming
fuel. You have learned that the products of combustion
fuel-air mixture.
FLANGE CAPSCREW
MUFFLER CLIP
EXHAUST
PIPE FLANGE
CLAMP
CLAMP
EXHAUST PIPE
TA I L P I P E
ASf04025
MUFFLER
Figure 4-25.--Exhaust pipe, muffler, and tail pipe.
4-16
