Variable Valve Timing (VVT) – What All Car Owners Should Know

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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.

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

And, that’s how, variable valve timing (VVT), provides a way of getting around, the limitations of fixed timing.

P001 To P0099

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

Variable Valve Timing (VVT) - What All Car Owners Should Know
Variable Valve Timing (VVT) – What All Car Owners Should Know

First, there is a huge advantage with, variable valve timing (VVT).

Because, 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.

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.

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, 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. Consequently, as a means to reduce fuel consumption and emission.

So, 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 air/fuel mix is injected.

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

Variable Valve Timing (VVT)

Variable Valve Timing (VVT), Common Problems

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


Some of the (VVT) types are:

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

So, on (VVT) systems that use oil pressure to actuate the cam phaser, other issues can affect its operation. Some of these issues include: oil quality, viscosity and contamination problems.

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:

When You Have, 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

When You Have, 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.

So, valve timing determines, when the intake and exhaust valves open. Also, how long they remain open. And, when they close. Consequently, this is where, variable valve timing (VVT) takes a front seat.

Variable Valve Timing (VVT)
Variable Valve Timing (VVT)