Because ethylene glycol and propylene glycol do
not have the same specific gravities, the use of a
hydrometer will be inaccurate. Therefore, a refracto-
meter (Tool 8286) (Refer to 7 - COOLING - SPECIAL
TOOLS) is recommended when testing either ethyl-
ene or propylene glycol coolants.
STANDARD PROCEDURE - COOLANT SERVICE
For engine coolant recommended service schedule,
(Refer to LUBRICATION & MAINTENANCE/MAIN-
TENANCE SCHEDULES - DESCRIPTION).
PRESSURE CAP
DESCRIPTION
The cooling system pressure cap attaches to the
coolant bottle. The cap construction includes, a plas-
tic swivel top, rubber seals and retainer, a main
spring, and a spring loaded vent valve (Fig. 10).
OPERATION
The pressure cap allows the cooling system to oper-
ate at higher than atmospheric pressure which raises
the coolant boiling point. This pressure allows for
increased radiator cooling capacity. The pressure cap
will release cooling system pressure at some point
within a range of 117 kPa ±7 kPa (17 psi ±1 psi).
A spring-loaded vent valve in the center of the cap
allows the system to pressurize and depressurize
without creating a vacuum. If the valve is stuck
open, coolant will escape to the overflow hose. A gas-
ket seals the cap to the top of the filler neck.
DIAGNOSIS AND TESTING - COOLING SYSTEM
PRESSURE CAP TESTING
WARNING: THE WARNING WORDS “DO NOT OPEN
HOT” ON THE PRESSURE BOTTLE CAP IS A
SAFETY PRECAUTION. WHEN HOT, PRESSURE
BUILDS UP IN COOLING SYSTEM. TO PREVENT
SCALDING OR INJURY, THE PRESSURE BOTTLE
CAP SHOULD NOT BE REMOVED WHILE THE SYS-
TEM IS HOT AND/OR UNDER PRESSURE.
Dip the pressure cap in water. Clean any deposits
off the vent valve or its seat and apply cap to end of
the Pressure Cap Test Adaptor that is included with
the
Cooling
System
Tester
7700.
Working
the
plunger, bring the pressure to 104 kPa (15 psi) on the
gauge. If the pressure cap fails to hold pressure of at
least 97 kPa (14 psi), replace the pressure cap. Con-
tinue adding pressure to cap. The pressure cap
should relieve at 124 kPa (18 psi) and hold pressure
at 110 kPa (16 psi) minimum.
CAUTION: The Cooling System Tester Tool is very
sensitive to small air leaks that will not cause cool-
ing system problems. A pressure cap that does not
have a history of coolant loss should not be
replaced just because it leaks slowly when tested
with this tool. Add water to the tool. Turn tool
upside down and recheck pressure cap to confirm
that cap is bad.
If the pressure cap tests properly while positioned
on Cooling System Tester (Fig. 9), but will not hold
pressure or vacuum when positioned on the radiator.
Inspect the coolant bottle filler neck and cap top gas-
ket for irregularities that may prevent the cap from
sealing properly.
There is no need to remove the pressure cap at any
time except for the following purposes:
• Checking and adjust antifreeze freeze point.
• Refilling system with new coolant.
• Conducting service procedures.
• Checking for vacuum leaks.
INSPECTION
Hold the cap in your hand, right side up. The
vent valve at the bottom of the cap should not open
(Fig. 10). If the rubber gasket has swollen and pre-
vents the valve from closing, replace the cap. The
vent valve should open with a slight pull with your
finger nail.
Hold the cap in your hand upside down. If any
light can be seen between vent valve and rubber gas-
ket, replace cap.
Fig. 9 Pressure Testing Radiator Cap
1 - PRESSURE CAP
2 - PRESSURE TESTER
7 - 16
ENGINE
ZB
COOLANT (Continued)