A vacuum delay valve is a valve with a small orifice, which delays a vacuum signal. These are commonly used in automobiles to alter the behavior of vacuum switch

Vacuum switch

Prior to effective engine control unit computers, engine vacuum was used for many functions in an automobile. Vacuum switches were employed to regulate this flow, and were commonly controlled by temperature, solenoids, mechanically, or directly...

es, vacuum motors, and other vacuum devices.

The engine in a common automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 produces almost 20 inches of mercury (67.7 kPa) of vacuum, and this pressure differential has been utilized for everything from automatic locks and windshield wipers to operating emissions control

Emissions control

Emissions control may refer to:* EMCON, military state of readiness.* Technology involved in vehicle emissions control* Regulation of air pollution via emission standards...

 items.

Engine vacuum is also the best direct source of information on the engine's load.

Most delay valves have a one-way function, where there is either no restriction or no movement in one direction.

Delay valves are usually color-coded to their function.

Color Code		Time (seconds)	Threshold (+/- seconds)	Direction
Orange	Black	2	0.5	Forward
Purple	Black	4	0.8	Forward
Gray	Black	10	2	Forward
Brown	Black	20	4	Forward
White	Black	63.5	13.5	Forward
Yellow	Black	100	20	Forward
Green	Black	200	40	Forward
Orange	White	2	0.5	Reverse
Purple	White	4	0.8	Reverse
Gray	White	10	2	Reverse
Gold	White	15	3	Reverse
Brown	White	20	4	Reverse
Yellow	White	100	20	Reverse
Red	White	375	75	Reverse