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Unlocking the mysteries of variable valve timing

Friday, February 26, 2016 - 09:00
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Vehicles that roll into your service bay today have different engine configurations and operating characteristics and are significantly different than the vehicles that we took care of even 10 years ago. I’m sure you are aware that, in this industry, technology changes in the blink of an eye. The challenges faced by the OEMs in meeting ever-demanding CAFE standards means increasingly efficient (and sometimes complex) new engine designs.  

In this article, we will cover some of the mechanical characteristics of how the Variable Valve Timing (VVT) engine works, what to inspect and look for and how to make your diagnosis of these engines a little easier.

In an engine, timing is everything. All the elements (compression, ignition, fuel) have to occur in the right amount and at the right time in order for the engine to run even close to right. The problem is that the timing of these events is affected by the speed of the engine. What is right at idle won't work at all at cruise speeds. For years, the moment of ignition was the only factor that engineers could alter — first, by mechanical means and then later by computer. That's the idea of "timing" that many of us grew up on.

The computer was a neat addition to engine management. Not only did it allow engineers to  more precisely control ignition, it also opened the door to conrolling the fuel event's timing. Still, there were gains to be made.

To truly gain ground in engine efficiency, the timing of the valve events needed to be altered as well. 

Variable valve timing is the process of altering the timing of a valve lift event, and is used to improve performance, fuel economy and emissions. Common designs use a variable cam timing system that adjusts the camshaft to crankshaft timing, which is advanced at lower RPMs and retarded at higher RPMs, and effectively alters when the valves open and close. The timing is controlled by oil entering a passage and into a component called a camshaft actuator. A valve located on the block controls the actuator. When engine RPM increases, so does oil pressure. As oil pressure increases, the oil enters the camshaft actuator and forces the actuator to move off its position just a little bit. When that happens, the camshaft timing is altered.

Here is a scantool shot of the fuel trims. Notice the difference in the bank 1 to bank 2 LTFT. Bank 1 LTFT is showing a rich condition.

On this scope capture you can clearly see that the timing is off. Channel 1 is the camshaft sensor and channel 2 is the crankshaft sensor.

Here is the camshaft gear on bank 2. The timing marks are correctly aligned.

Here is a scantool screen capture of the multiple cylinder misfire history. This is why the PCM flagged the code P0300.

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