What Is Variable Valve Timing?

Mia Bevacqua
October 12, 2018

Variable valve timing (VVT) allows for precise control of the opening and closing of internal engine valves while you drive. It’s used in modern engines to improve performance and fuel economy, while also reducing emissions.

Signs of a variable valve timing problem

A problem with the VVT system can cause several problems:

  • Illuminated check engine light: If the engine’s computer detects a problem with the VVT system, it turns on the check engine light.
  • Poor engine performance: Any problem with the VVT system can result in an array of engine performance issues, such as rough running, lack of power and hesitation.
  • Reduced fuel economy: Poor engine performance quickly leads to reduced fuel economy.
  • Increased emissions: On many vehicles, exhaust gas recirculation — a primary emissions control function — is handled by the VVT technology. As a result, an issue within the VVT system can lead to increased emissions.
  • Noise: In some cases, a stretched timing chain or faulty camshaft actuator can make a rattling or tapping sound.
Get it diagnosed by a professional

Valve timing in a traditional engine

Before VVT became common, all cars had permanently set valve timing. With this setup, the pistons are attached to a crankshaft. As the pistons move up and down in the engine cylinders, they force the crankshaft to rotate. 

A timing belt or chain connects the crankshaft to the camshaft, and the two turn together. As the camshaft rotates, it moves the intake and exhaust valves, making them open and close. It’s this action that allows air to enter and exit the engine.

But traditional valve timing has limitations: while it provides smooth engine operation, it also creates a lack of power when the engine has to work harder, such as going up hills. 

Valve timing in a VVT engine

Varying the valve timing with engine speed solves these problems. Each automaker has a unique name for its VVT technology. For example, Toyota calls its setup VVT-i (Variable Valve Timing with Intelligence), and Ford calls it VCT (Variable Camshaft Timing). 

Regardless, the systems all do the same thing: adjust the valve timing. But they may accomplish the task in different ways.

Several VVT designs exist, but the most common type is called “cam phasing.” With cam phasing, the position of the camshaft changes with engine speed, thereby changing the valve timing. 

The movement of the camshaft is controlled by the vehicle’s computer, the engine control module (ECM). The ECM determines the desired timing by monitoring the camshaft and crankshaft position sensors. 

In some engines with two camshafts, VVT is used only on one of them — either the intake or exhaust. Most modern designs, however, apply the technology to both camshafts, controlling both sets of valves.

Also, in many applications, VVT is used together with variable valve lift, which alters the amount the valves are opened. It may also be paired with variable valve duration, which controls how long the valves remain open.

» LEARN MORE: Get an estimate for your engine repair

How to fix a VVT problem

The VVT system is complex. A failure with any of its components can cause problems. If you suspect a VVT system issue, the best approach is to let a professional handle it. A qualified technician will diagnose the problem using equipment like a scan tool. Then, a proper repair can be performed.

One way to help prevent VVT-related problems is to stick to the manufacturer’s recommended maintenance plan, which you can find in your owner’s manual. Several of the system’s parts rely on proper oil pressure, so getting your car’s oil changed on a regular basis is key to keeping things working. Contaminated oil, or a low oil level, can wreak havoc on your engine.

Mia Bevacqua

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.

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