Radiator Cap.docx

7/27/2019 Radiator Cap.docx

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Radiator Cap (Pressure Cap)

1.0 Function

1. Provides a way to close off the radiator (exactly like a cap on a jar), so the coolant

doesn’t just spill out as you start and stop the car. Just under the metal cap is a rubber

seat to seal against the top rim of the radiator fill opening.

2. It is designed to hold the coolant in the radiator under a predetermined amount of

pressure. If it was not kept under pressure, it would begin to boil, and soon all the

coolant would have boiled away.

3. the radiator (or pressure) cap prevents this from happening by exerting enough

pressure to keep the coolant from boiling. If your cooling system is under too much

pressure, it can “blow its top”! To prevent this, the radiator cap has a pressure relief

valve. The valve has a preset rating that allows it to take just up to a certain amount of

pressure.

4. some coolant has been expelled from the radiator, it is stored in the overflow tank.

But the coolant has to get back into the radiator, or the cooling system will be low on

coolant and more likely to overheat. This is the job of the vacuum relief valve - the

little metal tab on the bottom of the radiator cap. When the coolant in the radiator

cools, it creates a partial vacuum allowing the valve to open and the coolant to be pushed (by atmospheric pressure) from the overflow tank back into the radiator.



2.0 Radiator pressure cap component

Radiator caps are pretty simple in appearance, but looks can be deceiving. Each part

of the cap performs a special function:

• Under the top of the cap is a spring disc diaphragm (and upper sealing gasket on

closed system caps) that seals against the top of the radiator filler neck and provides

friction to keep the cap from working loose.

• The big spring is what maintains pressure on the coolant. The spring holds the

pressure valve and lower sealing gasket against the base of the radiator filler neck to

raise the coolant's boiling temperature and to keep it from escaping. When the

pressure rating of the spring is exceeded, the pressure valve opens venting coolant

into the overflow reservoir.

• The lower sealing gasket is what keeps the coolant where it belongs. This gasket

must be in excellent condition for the cap to seal tightly and do its job. If cracked,

age-hardened or damaged, pressure and coolant can escape, allowing the engine to

overheat.

• A vacuum valve in the bottom of the cap allows coolant to be siphoned back into

the radiator when the engine cools down. This keeps the radiator full, which helps

prevent overheating and the formation of steam pockets. On many domestic passenger

car applications with pressure vent-type caps, the vacuum valve also vents pressure

when the cooling system is under light load. This allows the cooling system to operate

at atmospheric pressure until the expansion of coolant and the creation of pressure in

the system closes the valve.



3.0 Types of Vacuum relief valves

To better understand the role of the vacuum relief valve there have two basic type

that is the "normally closed" or "spring pressed" type, and the "open' or "weighted'

type.

The normally closed type of vacuum relief valve is used in a "Constant pressure" type

cap. With this design, the vacuum valve is held shut by a very light spring, creating a

totally sealed system. When the engine is started and begins to warm up, coolant

pressure starts to rise immediately because the closed vacuum valve prevents pressure

from escaping as the coolant expands.

With caps that have the open variety of vacuum valve (called “pressure vent” caps),

there's no spring to hold the vacuum valve shut, only a small calibrated weight. When

the engine is first started and under light operating conditions, pressure can vent

through the vacuum valve. This allows the cooling system to operate at atmospheric

or minimum pressure, which reduces strain on the water pump seals, hoses, radiator,

and heater core. As the engine starts to heat up, the escaping steam or coolant pushes

the vacuum valve up and shut. This seals the system tight and makes the cap function

like a constant pressure type of cap.When the engine is turned off and cools down,

vacuum inside the radiator allows the weighted vacuum valve to reopen. This type of

vacuum valve design can also allow the water pump to pull additional coolant into the

radiator if the cap is located near the radiator hose that connects to the water pump

inlet.



4.0 Cap removal & inspection

When changing coolant, performing routine cooling system maintenance or repairs, or

when troubleshooting an overheating or loss of coolant problem, the radiator cap

should always be inspected.

1. Do not attempt to remove the cap if the radiator is hot or warm to the touch. Wait

until the radiator has cooled, or spray cool water on the radiator until the radiator is no

longer hot or warm to the touch .

2. Place a rag over the cap to protect your hand, and then turn the cap counterclockwise

(to the left) until it reaches the safety stop (about 1/4 turn). Then pause. Any residual

pressure within the radiator will be vented at that point.

3. Wait until the hissing stops, then press down on the cap so the locking ears will clear

the safety stop and continue turning until the cap comes off.

Visually inspect the condition of the upper and lower sealing gaskets. Replace the

cap if the gaskets are cracked, hardened or damaged.

Look up the pressure, rating specified for the vehicle, and compare it with that

printed on the cap. If incorrect, replace the cap.

Pressure test the cap and cooling system with the Stant 12270 pressure tester. If

the cap fails to hold the rated pressure for one minute, replace the cap.

Repair all cooling system leaks found by pressure testing.

Inspect the radiator filler neck sealing surfaces for nicks, dents or corrosion that

could adversely affect sealing.

Check the tube that runs from the radiator filler neck to the overflow reservoir to

see that it is not loose, cracked, pinched or obstructed.



4. Bad radiator cap can cause the following symptoms:

Leaking radiator

Overheating cooling system

Inspection:

Look for a "swollen" (enlarged) radiator cap gasket. See Figure 1a and 1b.

An enlarged radiator cap gasket can cause the cap to not vent properly. This can result

in coolant loss and overheating.

The Black Rubber Gasket should be smaller in diameter than the Metal Plunger

under it (see Figure 2). If the Metal Plunger can be seen around the edge of the Black

Rubber Gasket, and the gasket is clean without any deposits of a waxy residue or

foreign material, the gasket is OK.



If the Metal Plunger cannot be seen around the edge of the Black Rubber Gasket (see

Figure 3), the radiator cap must be replaced.

Thoroughly wipe out the radiator filler neck to remove any waxy residue or foreign material.

Install a new radiator cap.

If the Metal Plunger can be seen around Deposits the edge of the Black Rubber

Gasket, but there are deposits of waxy residue or foreign material on the gasket or the

metal retainer (see Figure 4), the radiator cap must be replaced.

Thoroughly wipe out the radiator filler neck to remove any waxy residue

or foreign material.

Install a new radiator cap.

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Radiator Cap.docx