84 Auto Engine Performance and Driveability
The cooling system is pressurized to raise the boiling
point of the coolant. The boiling point of any liquid goes
up as the pressure is increased. For every 1 psi (6.9 kPa)
increase in pressure, the coolant boiling point increases by
3–F (1.7–C). Therefore, a 15 psi (105 kPa) pressure cap will
raise the boiling point of the coolant by 45–F (25.5–C).
Coolant-Recovery System. The coolant-recovery
system is designed to keep the cooling system as full as
possible at all times. The system consists of a tank con-
nected by a hose to the radiator overflow neck, as shown
in Figure 5-20. The plastic tank is called the recovery tank.
Coolant that is pushed out of the radiator as the engine
heats up enters the recovery tank. When the cooling system
loses heat, a lower pressure is created in the radiator. The
difference between this pressure and atmospheric pressure
draws coolant from the recovery tank into the radiator.
Warning: Some coolant-recovery systems
are pressurized. The cap on the recovery
tank is a pressure cap like the one found on
the radiator. Never open a pressurized recovery tank
if the cooling system is under pressure.
Thermostat. The thermostat helps the engine to quickly
warm up by keeping coolant from circulating through the
radiator when the engine is cold. The thermostat is located
at the engine outlet gooseneck. This is where coolant leaves
the engine on its way to the radiator.
The thermostat consists of a heat-sensitive material,
such as wax, sealed in a chamber with a piston at one end.
The piston is attached to a valve that opens or closes the
thermostat to control coolant flow, Figure 5-21.
When the coolant is cold, the wax is contracted and
holds the piston in the closed position. As the coolant warms
up and circulates under the thermostat, the wax expands
and pushes on the piston. This cracks the valve open and
allows coolant to begin circulating through the radiator.
As the coolant continues to warm up, the valve continues
to open until the wide-open temperature is reached. The
thermostat has no effect on engine temperature once the
coolant temperature is at or above the thermostat’s wide-
open temperature.
Most engines use a thermostat with a wide-open
temperature of between 190–195–F (87.8–90.6–C). In very
cold weather, the thermostat may not fully open. Especially
when the vehicle is only operated for a short period of time,
the coolant may never reach the wide-open temperature.
Hoses and Tubing. The radiator is fixed to the chassis
and stationary. However, the engine moves on its mounts
as torque increases and decreases. Therefore, the radiator
is usually connected to the engine by flexible hoses. These
hoses are made of rubber or neoprene molded around a
fiber mesh. Clamps are used to secure the hoses to the
radiator and engine.
Some engines have a bypass hose that allows the cool-
ant to circulate inside the engine until the thermostat opens.
On other engines, the bypass is built into the engine cast-
ing or the bypass system is the heater core and hoses. The
bypass system prevents damage to rapidly heating parts,
such as exhaust valves or cylinder walls, by allowing cool-
ant to circulate past the parts before the thermostat opens.
Some cooling systems have fixed metal tubing to route
coolant along straight runs. The tubing is either directly
connected to components or connected via a flexible hose.
This tubing is made of soft steel or aluminum.
Many cooling systems also have one or more small
bleeder valves. If a portion of the cooling system is higher
than the radiator fill cap, the system will have at least one
bleeder valve. Bleeder valves are used to remove air from
Figure 5-20. The coolant reservoir tank is usually mounted in
a conspicuous location and can be used as a quick diagnostic
tool. Bubbles in the reservoir bottle while the engine is running
is a good indication of severe engine problems. Most tanks are
not pressurized. Never open a pressurized tank when it is under
pressure.
Recovery tank
To radiator overflow neck
Fill levels
Add
coolant
here
Figure 5-21. Parts of a typical thermostat. Most engines use
a thermostat with an opening range between 190–195–F
(87.8–90.6–C).
Flange
Flange seal
Piston
Nut
Valve seat
Valve
Frame Teflon seal
Wax pellet
Coil spring
Rubber diaphragm