Radiator Glossary of Terms
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Vacuum
The absence or reduction of air pressure. Vacuum is created in the intake manifold by the pumping action of the pistons. Air is pulled out of the manifold into the cylinders faster than it can be replenished by air bypassing the throttle plate. The thrott
Vacuum Advance
A device that advances the ignition timing in response to increased engine vacuum.
Vacuum Delay Valve
An orifice-controlled valve which delays a vacuum signal to a diaphragm, such as in the distributor vacuum advance unit. Used to improve derivability and emissions when the throttle suddenly changes position.
Vacuum Gauge
An instrument used to measure the presence of vacuum in a chamber.
Vacuum Motor
Same as "vacuum actuator" and "vacuum power unit." It is a device that opens valves (heater controls) or doors (air control doors in the HVAC plenum) using vacuum as a power source.
Valve
A device that controls the pressure, direction of flow or rate of flow of a liquid or gas.
Valve Clearance
The measured gap between the end of the valve stem and the rocker arm, cam lobe or follower that activates the valve.
Valve Job
This is when the engine valves are reconditioned. It requires removing the cylinder head, disassembling the head and checking it for cracks or warpage (a common problem on aluminum cylinder heads), regrinding the valve faces and seats, replacing or restor
Vapor Lines
Lines carrying refrigerant vapor. See "suction line" and "discharge line." May also refer to hoses in the evaporative emission control system that route fuel vapors to the charcoal canister.
Vapor Lock
When gasoline overheats and boils inside the carburetor bowl or fuel pump of a hot engine, it ceases to flow. This can cause stalling or hard starting. This is called vapor lock, and it usually happens during hot weather. If a hot engine won't start, all
Variable Assist Steering
A type of power steering system where electronics are used to vary the amount of power assist provided as vehicle speed changes. Most such systems provide maximum assist at low speed to make parking maneuvers easier, and reduce assist at higher speeds to
Variable Rate Springs
A type of spring that changes stiffness as it deflects. A variable rate spring uses coils of varying thickness or spacing to provide a soft ride when the vehicle is lightly loaded, but a firmer ride when the load increases. Only a few vehicles have variab
Variable Valve Timing
A method that advances or retards camshaft timing to improve engine performance. A hydraulic mechanism on the cam drive uses oil pressure to rotate the cam's position slightly as engine speed changes. This increases valve duration to produce more horsepow
Varnish
Deposits of hardened oil on the interior of the engine caused by extreme heat.
Ventilated Rotor
Disc rotor that has a series of fins (or cooling passages) between the two friction surfaces to aid in heat dissipation.
Venturi
The narrow part of the carburetor throat. When air passes this point, the restriction causes an increase in velocity and a drop in pressure that siphons fuel from the fuel bowl into the airstream.
VIN
Abbreviation for Vehicle Identification Number. This is a vehicle's serial number. You'll find it stamped on a small metal plate affixed to the dash at the base of the windshield. The number may also be stamped on various body parts, the engine and transm
Viscosimeter
Device used to determine the viscosity of an oil sample by heating it to a certain temperature and conducting its flow through an aperture. The time it takes for a set amount of oil to flow through the aperture determines the oil's viscosity.
Viscosity
The rating of a liquid's internal resistance to flow.
Viscosity Breakdown
Form of physical degradation that derives from the engine's shear forces and results in a lowering of motor oil viscosity.
Viscosity Index
The measure of an oil's ability to resist changes in viscosity when subjected to changes in temperature.
Viscosity Index Improvers
Viscous chemical compounds (polymers) which have the ability to decrease the rate at which oils change viscosity with temperature.
Volatility
The ability of a liquid to evaporate quickly and at relatively low temperatures. The lower a temperature a liquid will evaporate at, the higher it's volatility.
Volatility Burnoff
A measure of the amount of oil lost from evaporation when exposed to high temperatures.
Voltage Regulator
A part of the charging system that controls how much electricity the alternator puts out (See Alternator). The voltage regulator on today's cars is an electronic black box, which means you can't adjust it or repair it if anything goes wrong with it. On mo
Voltmeter
An instrument used for measuring electrical force in units called volts. Voltmeters are always connected parallel with the circuit being tested.

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