For nearly as many years as internal combustion engines have been in operation, automakers have been trying to maximize fuel atomization by manipulating engine timing. Early cars, trucks, and motorcycles used mechanical manipulation of ignition timing; later ported engine vacuum was used to actuate a vacuum diaphragm and advance ignition timing upon acceleration. Today’s computerized automobiles control ignition timing by using coil over systems that deliver spark precisely and consistently according to engine RPM and engine load.
With ignition timing squared away, engineers have turned their attention towards valve timing (as well as valve lift and valve duration) to further enhance engine performance and optimize fuel efficiency. Virtually every automaker has their own dedicated variable valve timing (VVT) system, and no two are exactly the same. As numerous and diverse as these VVT systems may be, the names chosen for them can be even more confusing.
Despite the numerous terms used to describe VVT systems for different automakers, they can be slotted into one of these two categories: stepped and continuous. Stepped systems were used primarily in older automobiles. These VVT systems were limited to only a few positions. Typically there was a setting for low engine RPM and high RPM; with another for mid-level RPM in certain cases. Most of today’s VVT systems could be classified as continuous. By design, the degree to which intake and exhaust valves may be opened (lift) and the duration to which they may be held open are limited only by the shape and size of the camshaft lobes and/or rocker arms (valve actuator) in this type of system.
The amount of air and fuel that are allowed to enter/exit the combustion chamber directly affects the degree of horsepower that each cylinder is capable of producing. Intake valves are responsible for opening to allow air to enter the compression chamber and exhaust valves open in order to allow air to escape. When engine RPM levels are lower, less air is required for optimum engine performance and fuel efficiency. As engine RPM levels are elevated, more air is required for maximum engine performance. The difference between the amount of intake air required at low engine RPM and high engine RPM is what makes the VVT system practical in automobile applications.
Understand that any stored VVT system code may result in deactivation of the entire VVT system. If this is the case, an immediate and noticeable decrease in engine performance and fuel efficiency will likely take place.
Most modern VVT systems utilize engine oil (hydraulic) pressure and some type of an electronic solenoid to initiate changes in valve timing, lift, and/or duration. One of the most common problems found when diagnosing a VVT system malfunction is low oil pressure or low engine oil level. Without proper engine oil pressure, the VVT system cannot operate as designed. Obviously, low engine oil pressure can cause catastrophic engine damage in addition to adversely affecting the VVT system. Make sure that the engine is filled to the correct level with the appropriate oil. If oil pressure is questionable, a manual oil pressure test may be in order.
Some VVT systems operate using individual cylinder actuators; some adjust cylinders in groups. Others adjust all the cylinders on a particular engine bank (at once) with an oscillating camshaft. If the vehicle in question has a single cylinder of the VVT system disabled, fuel efficiency and engine performance may diminish minimally. If an entire bank of the engine has the VVT system disabled, a serious decrease in fuel efficiency and engine performance will be experienced. So, in summary, the exact type of VVT system with which the vehicle in question is equipped will be a major factor in the degree to which fuel efficiency and engine performance are diminished during VVT failure.
As automotive technicians, we are well acquainted with the dangers of allowing an interference engine to lose timing. When valves and pistons occupy the same space at the same time, it can become very ugly – very quickly. Interference is often associated with failed timing belts, tensioners, and water pumps, but VVT actuators may also fail with catastrophic consequences.
When a VVT system code is exhibited, it is critical that the problem is addressed and rectified as quickly as possible.