By the early 1990s, almost all import automotive
manufacturers had a successful variable valve timing (VVT) system in
production. These systems offered higher performance from smaller displacement
engines at higher rpm.
As these vehicles exceeded their new car warranties,
technicians learned the ins and outs of these systems and how regular oil
changes could increase the life of VVT components. Now, the VVT is playing a
direct role in vehicle emissions and the way gases are burned in the combustion
These systems are simple from a diagnostic perspective. Most
VVT components are non-serviceable and have integrated sensors. But, they are
part of a larger diagnostic picture that includes everything from the throttle
body to the oxygen sensor.
On a conventional engine, both the exhaust and intake
valves’ open or closed position depends on their fixed position relative to the
chain or belt that’s driven by the crankshaft. The pattern and timing cannot
be altered, so there’s no way to increase or decrease the amount of valve lap
when both valves are open at the same time.
As engines go, some have great low-end performance, while
others have better top-end performance. (If you’ve ever heard an old street
rod or dragster popping and rumbling at an idle, you’ve heard the lope from the
cam. Because these engines are designed to have maximum performance at the top
end, the cam is “cut” for better performance at the high end, so sacrifices are
made at the idle end.)
With VVT, the valve duration can be matched to the engine
speed, torque requirements and valve overlap. Now an engine can produce both
low- and high-end performance without any erratic idle condition or high-end
loss. This also enables an increase in miles per gallon throughout the
engine’s power band by controlling valve timing and making the engine more
One great advantage of the VVT system is the way it can take
the force needed to expel the burnt mixture out of the exhaust valve. Pushing
the exhaust gas out of a cylinder requires some of the force that is generated
during the combustion stroke. Opening the exhaust valve when there is
still some pressure left in the cylinder allows a small portion of the exhaust
gas to escape before the piston starts its upward travel. This reduces the
exertion from the crankshaft and piston and provides a smoother, more
even-running engine at every rpm level.
Leaving the intake valve partially open at the right point
also allows fresh air to enter the cylinder while the exhaust valve is doing
its job of removing the already-burnt gases. This slight intake valve opening
creates low pressure and aides the piston in pushing out the remaining gases
and getting ready for the next turn of the crankshaft. This is all the result
of the configuration and shape of the exhaust ports and manifolds, all of which
work together and make the whole process seamless.
EGR AND VVT
One item that’s going the way of the smog pump is the
Exhaust Gas Recirculation (EGR) valve. The elimination of the EGR valve is the
result of the VVT’s ability to control gases entering and exiting the
EGR systems are designed to reduce smog-causing nitrous
oxides (NOx) by recirculating a portion of the exhaust gases from each cylinder
of the engine back into the intake manifold. This process lowers the combustion
temperature to under 2,500° F, above which NOx gases are formed, hurting both
the environment and a vehicle’s performance.
EGR systems work, but they are not able to react fast enough
or precise enough for modern engines and emissions standards. Modern VVT
systems are doing the same job as the EGV valve, only better.
A VVT system is able to control the timing of the exhaust
valve so that the right amount of inert exhaust gases remain in the combustion
chamber for the next combustion cycle. This controls combustion temperatures
and the production of NOx.
If you encounter a vehicle that has higher than normal NOx
levels, or a burnt or damaged pre-catalyst, make sure the VVT solenoid and
exhaust camshaft position sensor are operating properly.
HONDA’S i-VTEC SYSTEM
Honda’s term for a variable valve system is i-VTEC. The
i-VTEC system uses an oil pressure solenoid activated electrically by the PCM
to allow oil to pass into the rocker arm between the two normal rockers. This,
in-turn, “locks” the normally used two intake valves together with a set of
pins that are pushed outward into the two intake rockers and transfers their
motion to a higher eccentric lobe (operated by the middle rocker arm). This
higher lobe gives the engine the needed boost in power at an rpm higher than
When the rpm level drops below 4,500, the oil pressure
solenoid shuts off, blocking off the oil pressure and returns the engine to the
normally operated two intake valves.
The 2008 Accord takes this to a whole new level of valve
control, allowing the engine to go from six cylinders, down to four, and even
down to three cylinders. It uses a solenoid to “unlock/lock” the cam
followers on one bank and allows the followers to float freely while the valve
spring keeps the valve in the closed position.
Vehicles equipped with Honda’s VCM (Variable Cylinder
Management) systems also include Active Noise Cancellation (ANC) and Honda’s
Active Control Engine Mount (ACM) system. The ANC and ACM systems work in
cooperation to cancel both noise and vibration that could occur in relation to
the cylinder deactivation process.
The ANC system uses the audio speakers to cancel out noise
by incorporating an opposite phase sound. This whole process is controlled by
the computer system and becomes imperceptible to the driver. These systems
use a mechanical/electrical solenoid that operates with oil pressure to
accomplish the range of variable valve timing.
The VTECE system is slightly different in its configuration
than the VTEC. The emission qualities have been increased but, at the same
time, it performs the same functions as the VTEC system.
TOYOTA’S VALVEMATIC SYSTEM
Toyota came out with the Valvematic system in
2008. This system uses an intermediate shaft to achieve a continuous
variable valve lift. The intermediate shaft consists of followers on
either side of a roller bearing. These followers rotate with respect to the
roller member and “finger followers” (small followers) by means of an
internal gear and electric motor attached to the shaft. As the shaft moves, the
roller member and followers will move in opposite directions (either closer or
farther apart). As the angle increases, so does the valve lift. This
system can vary the valve timing to any angle needed.
In 2007, the VVT-ie system was introduced on the Lexus
LS460. This system is both electrical and hydraulic. The exhaust
valve is still controlled by way of an oil pressure solenoid, while the intake
is controlled by an electric motor on the front of the cam. This allows valve
timing to be adjusted with no regard to engine temperature or oil pressure.
COMMON VVT PROBLEMS
The two most common codes I’ve run across are P0011 and
P0021 (Camshaft position sensor “Bank 1” and Camshaft position sensor “Bank 2,”
respectively). These codes (like any code) don’t entirely mean the sensor is
faulty, however the diagnostic charts will tell you to look at the VVT system
for a fault and check the sensor as well. Some of the common areas to look
into are: valve timing, oil control valve, oil control valve filter screen,
camshaft timing/gears, and, of course, the electrical side of the operation as
well as the PCM.
The very first thing I do before turning nuts and bolts is
to check the oil. Oil is an essential part of most VVT systems. Dirty oil and
the lack of regular oil changes can leave a buildup of sludge or debris in the
passages that lead to the pressure control valve that operates the variable
valve timing. If the oil is dirty and too much sludge accumulates at the valve
ports, the sludge can be passed on through the cam and the valve
Then, the oil passages in the cam can be compromised and
could result in a cam failure due to scored journals. Keep in mind that the VVT
system is not operated at a normal driving condition rpm. For example, the
Honda VTEC system doesn’t operate below 4,500 rpm. So, if you have someone who
never gets the car out on the highway and never changes the oil, you can have a
potential problem waiting to happen, if and when the car is revved up above
4,500 rpm the next time it heads onto an on-ramp of the local interstate
Code P0521 (Oil pressure sensor/switch range/performance)
could be an indication of the quality of the engine oil. It might not be
the best diagnostic answer, but when I’ve seen this code on several vehicles,
the oil was black and neglected. In some cases, the code can also indicate that the wrong type of oil has been used. I
wouldn’t use this as the final solution to the problem with variable valve
timing, but rather an indication of things to come.
Lack of regular maintenance seems to be the big factor in
most of these systems. Unlike vehicles from years gone by where certain
maintenance issues could be neglected, these newer engines and newer systems
require the utmost in care. Stressing this point to your customers and
performing the required basic maintenance per the manufacturer’s schedule will
safeguard their vehicle and increase your profits.
In my opinion, VVT will be as common as a spark plug in the
near future. Reducing emissions and reducing the need for an EGR valve,
improving fuel economy and getting more performance out of smaller engines
tells me that the VVT systems are here to stay.
The next generation of VVT systems are on the drawing boards
now, and it won’t be long before they’ll be in the marketplace. With the latest
requirements in fuel economy emerging, engines with variable valve timing will
become the norm, so it’s time to get ahead of the curve now.
About the Author
Scott “Gonzo” Weaver
Scott “Gonzo” Weaver is the owner of Superior Auto Electric in Tulsa, Okla. and has owned the shop for 27 years. He was given his trademark nickname “Gonzo” while serving in the USMC. He is the author of the book “Hey Look! I Found the Loose Nut”, that can be purchased online at Amazon.com or at www.gonzostoolbox.com.
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