![]() The first tool used in almost every VCT diagnosis is the scan tool. Each of these areas must be tested to determine the root cause of a failure and will require different tools to complete the testing. Hydraulic problems can be low oil level or pressure, wrong oil viscosity or restricted oil supply passages. Electrical problems include failed camshaft position sensors, failed oil control solenoids or any wiring problems to these items. Mechanical problems would be considered as stuck vane or spline actuators, stuck oil control solenoids, and jumped or stretched timing chains. VCT problems can be grouped into three classifications: mechanical, electrical or hydraulic. VCT diagnosis should begin with an understanding of what the potential problem areas are. The theory behind camshaft phasing can fill a decent sized textbook so we’ll wrap this up and move onto diagnosis. Of course, phasing both cams allows more benefits to be realized such as improving torque output by advancing the intake cam or reducing pumping losses by moving both cams and lowering engine vacuum. Many Asian-produced vehicles like to phase the intake camshaft to accomplish this task, so you will see many Nissan and Toyota engines that phase the intake cam. Several domestic engines, such as the GM 4200 in the Chevy\GMC Trailblazer and Envoy SUVs, use the exhaust cam to accomplish this task. To increase valve overlap, you must either advance the intake camshaft or retard the exhaust camshaft. This allows the powertrain engineer to remove the troublesome exhaust gas recirculation hardware from the engine. One of the main benefits of variable cam timing is the reduction of oxides of nitrogen through in-cylinder exhaust gas recirculation resulting from increasing valve overlap when the camshaft is phased. While knowing that camshafts are adjusted is important, it is more important to understand why camshafts are “phased” or moved in relation to the crankshaft. Many oil pressure actuated phaser engines cannot move the cams at engine idle due to the low oil pressure present under idle conditions. The most unique part of these cam torque actuated phasers is their ability to move the cam without the need for engine oil pressure so they can move the cam its full range during cranking! While this is not a strategy employed by the manufacturer, it is important to know this capability. ![]() CTA phasers built by Borg Warner are used on some Ford engines and the Chrysler Pentastar 3.6 V6 engine. The two most common types of vane phasers in use today are oil pressure actuated phasers and cam torque actuated (CTA) phasers, which use the force of the valve springs to move the camshaft and not direct oil pressure. ![]() Spline drive systems are being replaced by vane phasers, which offer greater range of movement and faster response times. Within these general layouts are vehicle specific systems that actually do the work of moving the camshafts with the two most common being spline drive cam adjusters or vane-style cam adjusters (or as I commonly refer to them as “phasers.”) The third and most common today is the dual independent, where the intake and exhaust camshafts are moved independently from one another. Second is the dual equal where both the intake and exhaust are moved the same (think single camshaft designs like cam in-block camshafts on a V8). The first is the single independent system where either the intake or exhaust camshaft is moved. There are three basic designs in use today. There are variable lift and duration systems on the market such as Chrysler\Fiat Multi-air or BMW Valvetronic, but we will be discussing camshaft phasing systems only in this article. VCT systems only change valve timing events, they do not change valve lift or duration. Let’s go over some general guidelines before we delve more deeply into the diagnostics of these engines. VCT, or Variable Camshaft Timing, has been around for quite some time now so every working technician has dealt with these systems in one form or another. Today almost every engine produced has some type of variable valve timing system installed to take advantage of the improvements in power and efficiency that adjusting valve timing affords.
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