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. 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. Let’s go over some general guidelines before we delve more deeply into the diagnostics of these engines. VCT systems only change valve timing events, they do not change valve lift or duration. 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.
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The foundational knowledge you need
There are three basic designs in use today. The first is the single independent system where either the intake or exhaust camshaft is moved. 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 third and most common today is the dual independent, where the intake and exhaust camshafts are moved independently from one another. 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.”)
Spline drive systems are being replaced by vane phasers, which offer greater range of movement and faster response times. 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. CTA phasers built by Borg Warner are used on some Ford engines and the Chrysler Pentastar 3.6 V6 engine. 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. Many oil pressure actuated phaser engines cannot move the cams at engine idle due to the low oil pressure present under idle conditions.
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. 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. This allows the powertrain engineer to remove the troublesome exhaust gas recirculation hardware from the engine. To increase valve overlap, you must either advance the intake camshaft or retard the exhaust camshaft. Several domestic engines, such as the GM 4200 in the Chevy\GMC Trailblazer and Envoy SUVs, use the exhaust cam to accomplish this task. 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. 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. The theory behind camshaft phasing can fill a decent sized textbook so we’ll wrap this up and move onto diagnosis.
VCT diagnosis should begin with an understanding of what the potential problem areas are. VCT problems can be grouped into three classifications: mechanical, electrical or hydraulic. Mechanical problems would be considered as stuck vane or spline actuators, stuck oil control solenoids, and jumped or stretched timing chains. Electrical problems include failed camshaft position sensors, failed oil control solenoids or any wiring problems to these items. Hydraulic problems can be low oil level or pressure, wrong oil viscosity or restricted oil supply passages. Each of these areas must be tested to determine the root cause of a failure and will require different tools to complete the testing.
The first tool used in almost every VCT diagnosis is the scan tool. This tool at the very least will provide any codes that have set if a problem develops and depending on the vehicle being serviced may deliver enough information to make a complete diagnosis of the problem. Some manufacturers (such as Ford) give so much data and bi-directional controls that you may need nothing more than a good scan tool. Other manufacturers provide very little VCT system information beyond codes and will require a technician to do more thorough testing with meters and scopes.
There are some need to know items for a technician before he or she begins a VCT diagnosis on any vehicle. You’ll need to know how the engine control computer displays cam timing data on the scan tool, the range of motion or phase angle of each phaser, cam timing specifications if performing in-cylinder analysis with a pressure transducer, oil pressure specifications, known good cam/crank synch waveform and how the oil control solenoid is controlled. Some manufacturers display cam timing data as a zero value when the camshaft is at its locked or home position and then the number of degrees of advance or retard are shown when the cam moves. A GM vehicle showing 18 degrees for the intake camshaft means the cam has advanced 18 degrees from its home position, pretty simple. Other manufacturers such as Chrysler, Hyundai or BMW may use the camshaft lobe centerline data to display cam timing. This means the cam timing PID’s can be different for the intake and exhaust cams and the numbers won’t start from zero. The BMW cam timing chart shows the intake cam centerline position with the cam in the home position is 120 degrees and the fully phased position is 50 degrees so the phaser range is 70 crankshaft degrees. If the lobe centerline is after TDC such as is the case with the intake cam the displayed numbers are positive and as the cam advances the lobe centerline moves closer to TDC so the number counts down. If the scan tool displays 100 degrees on this engine for the intake cam the cam has phased, or advanced, twenty degrees from its home position. The exhaust lobe centerline is before TDC so the scan tool numbers will be negative. As seen on this diagram the home position for the exhaust cam is -115 degrees and the cam can retard to -60 degrees for a range of 55 crankshaft degrees. Again, like the intake the numbers are counting down because the exhaust cam retards and the lobe centerline moves closer to TDC. Hyundai displays cam timing data in the same fashion.
|This Hyundai GDS scan tool display shows the current intake cam position as 117 degrees, (home position) and the current exhaust cam position as -112.8 (home position).|
When a scan tool doesn’t help
The next item to discuss is scope testing VCT systems and the issue of known good cam/crank synch relationships. This type of testing is critical in diagnosing problems such as stretched timing chains and will also be needed to test system operation if there is little cam timing data provided by the manufacturer such as some BMWs or early Toyota systems. Some, but not many, manufacturers provide known good scope patterns in their service or training materials. This problem of finding “known good” means there is an abundant amount of homework needed by techs in the field to scope vehicles before wear sets in and capture and save known good cam/crank synch waveforms and build their own database. This takes some considerable work but will pay big dividends later when confronted with making a decision of whether or not an engine is in synch without pulling off the timing cover to check camshaft alignment when diagnosing a camshaft timing correlation code.
I prefer to set up my scope the same way whenever I am testing VCT systems regardless of the vehicle I’m working on. If using a four-channel scope, I connect channel A to the #1-cylinder ignition signal so I can quickly identify the four-stroke cycle, channel B to the crankshaft position sensor signal, channel C to the intake camshaft sensor signal and channel D to the exhaust camshaft sensor signal. If you only have a four-channel scope and are working on a dual bank engine you must test one bank at a time. My eight-channel scope allows me to test both banks simultaneously, thus saving me some time moving scope leads. Once I connect to the vehicle, I start the car and capture a waveform.