(VVT) – Variable Valve Timing – Function – Types – Failure Signs

(VVT) - Variable Valve Timing - Function - Types - Failure Signs
(VVT) - Variable Valve Timing - Function - Types - Failure Signs

So, the advantage of variable valve timing (VVT), is that all the factors traditionally associated; with a given camshaft grind; are no longer fixed.

As a result, making it possible to change valve timing in response to; changing engine speeds and operating conditions.

So, valve timing determines when, the intake and exhaust valves open. Also, how long they remain open. And, when they close.

As a result, variable valve timing (VVT), provides a way of getting around; the limitations of fixed timing.

Consequently, this affects:

  • Intake and exhaust; flow
  • Intake manifold; vacuum
  • Running; compression
  • Volumetric; efficiency
  • Throttle; response
  • How much, horsepower and torque the engine develops; at any given (RPM)

So, How Does Valve Overlap Affect Performance:

More Valve Overlap:

  • Reduces oxide of nitrogen (NOx) emissions; under load
  • Increases Exhaust Gas Recirculation (EGR) flow; lowering combustion temperatures
  • On some engines (VVT) can eliminate the need for; an (EGR) valve
Valve Overlap Triangle
Valve Overlap Triangle

Less Valve Overlap:

  • Smooth idle and more; slow speed torque
  • Poor; high speed performance
  • Better engine breathing; at high speeds
  • Poor performance; at low speeds
  • Rough idle
  • Higher; exhaust emissions

So, there are a variety of different (VVT) systems, in use today. But, different automakers, use different variable valve timing (VVT) strategies; for different purposes.

Some of the (VVT) – variable valve timing types are:

  • Cam Changing (VVT)
  • Camshaft Phasing (VVT)
  • Cam Changing + Cam Phasing (VVT)
  • (VVC) System ( Unique To Rover )
(VVT) - Variable Valve Timing
(VVT) – Variable Valve Timing

So, in theory, maximum overlap is needed; between intake and exhaust valves opening, at high speed. However, when the car is running at medium cruising speed; maximum overlapping may also be useful. Finally, as a means to reduce fuel consumption and emission.

Consequently, the exhaust valves do not close; until the intake valves have been open for a while. Therefore, the exhaust gases; are recirculated back into the cylinder; at the same time as the new fuel/air mix is injected.

So, as part of the fuel/air mix is replaced by exhaust gases; less fuel is needed. Because, the exhaust gas is comprised of mostly non combustible gas; such as (CO2), the engine runs properly at the; leaner fuel/air mixture.

(VVT) - Variable Valve Timing
(VVT) – Variable Valve Timing

Above All, Automakers, Need To Get Together

Because, it seems like they are all trying to set the bar higher; with different types of technology. I think the end goal is the same; but we need to have some type of common ground.

Variable Valve Timing (VVT) Common Problems

So, on (VVT) systems that use oil pressure to actuate the cam phaser; oil quality, viscosity and contamination problems; can affect the operation of the phaser.

Also, the phaser may not work properly if:

  • The phaser does not receive; adequate oil pressure
  • Oil is the; wrong viscosity
  • The oil is; dirty
Oil Quality, Viscosity
Oil Quality, Viscosity

So, this can hurt engine performance; fuel economy and emissions. Also, turning on the check engine light; setting a (VVT)-related fault code.

Consequently, any codes could be the result of; a bad cam phaser, oil flow control valve or wiring fault.

P001 To P0099

So, don’t jump to any conclusions; regarding the variable valve timing (VVT) system. Because, if an engine is idling rough, or not developing power; you should also consider other possible causes, such as:

  • A large, vacuum leak
  • Heavy carbon buildup, on the intake valves
  • Dirty, fuel injectors
  • Low, fuel pressure
  • Ignition, misfires
  • Exhaust, restrictions
  • Loss of, compression
  • Turbo, problems


So, as you might have already guessed; (VVT) diagnostics is very application specific. And, it also depends on the configuration of; the phaser and system electronics.

Above all, oil quality, viscosity and contamination problems; can all affect the operation of a; hydraulically actuated (VVT) cam phaser. Finally, this, in turn, will affect engine performance; fuel economy and emissions.

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