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.
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 diagnostics
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.
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| 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.
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| This service information illustration is from a 2012 Hyundai Elantra 1.8 engine, code P0014 diagnosis chart. It shows the correct exhaust cam sensor to crankshaft sensor relationship scope waveform, or synch. This is very useful for performing VCT testing and diagnosis. |
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.
It is crucial to point out at this point that if you are going to capture a “known good” synch waveform there must be no cam phasing occurring when the pattern is saved. There are vehicles that can phase their camshafts at idle and if you are not familiar with the cam phasing strategy of the vehicle you are working on you would need to verify with a scan tool that the cams are in the home position, or better yet just un-plug the cam timing oil control solenoids before you capture a synch waveform. It would be a serious mistake to decide to tear down a motor to replace a timing chain that you think is off because you compared a waveform you captured to an incorrectly captured waveform uploaded to the Internet by a tech not aware that the supposedly known good waveform he was posting had the cams moved 8 or 10 degrees from the base position. The best “known good” waveforms are the ones you capture yourself and you know exactly what the conditions were when you saved the waveform.
There are a few items you need to know when analyzing a VCT system waveform, the most important being the design of the crankshaft sensor trigger wheel. Scopes that have rotation rulers make it easy to determine how many degrees of engine rotation each tooth on a trigger wheel is valued at. For instance, many engines use a 60 minus 2 tooth trigger wheel for the CKP. This means there is room for 60 teeth but 2 are removed to create a synch for the PCM to identify. If you divide 60 into 360 you get 6, which means each trigger wheel tooth is displaying 6 degrees of engine rotation. You can now line up a CMP waveform signal edge to the CKP signal and determine very accurately how far off an engine is from correct valve timing. I simply pick a point in the waveform where the two signals have a transition and start counting teeth.
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| This known good synch waveform from a BMW 3.0 N52 engine will be zoomed into to look at CKP\CMP correlation. |
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| Here you can see the relationship between the CKP missing tooth region and the leading edges of the intake and exhaust CMP sensors. The oil control solenoids were not connected. |
Once you determine the correct CKP/CMP relationship you can continue testing the system by applying power or ground to the oil control solenoid with the engine speed raised so the engine does not stall and capture a waveform of the cam sensor with the cam phased to its maximum travel. Count the number of teeth the CMP sensor has moved from its synch position and multiply by the number of degrees per CKP tooth to see if the camshaft moved its full published range. Remember while viewing the signals on a scope, the exhaust cam signal will move to the right because it retards and the intake cam signal will move to the left because the cam advances. This testing routine can be employed on any engine and can be used when the vehicle does not supply scan data PIDs for cam timing.
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| Grounding the intake cam VANOS solenoid moved the cam to the fully phased position. The scope measures 69 degrees, service information states a 70-degree range for intake cam travel. |
A real-world example
Putting this testing technique to use in the shop is straightforward yet many techs are intimidated by scope diagnostics or are unwilling to spend the time to capture these waveforms. A shop called concerning a problem they were having with a 2006 Nissan Altima with a 2.5 engine. A code P0011 was setting and their scanner read the intake valve timing PID as -26 degrees and they were not sure what that meant. Some scan tools read Nissan cam timing data incorrectly and this was one of those cases. The shop had replaced the cam and crank sensors but the code returned quickly. I mentioned they should scope the cam and crank sensors along with the ignition trigger but they decided to send the vehicle to me for a diagnosis. Having seen these vehicles before I have a known good synch waveform but I decided to look at service information to see if the data was available to the other shop to make an accurate diagnosis. Upon looking at code repair information for a P0335 I found a hand drawing of the correct CKP\CMP relationship from Nissan training information.
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| Nissan training book depiction of correct CKP\CMP signal relationship. I have added the callouts shown. |
Armed with this data and my scope the rest was fairly easy. The code chart diagram shows the single pulse CMP signal rising edge lining up with the first CKP tooth before the missing tooth area on the CKP signal and the CMP trailing edge lining up directly in the center of the missing tooth gap. The waveform captured from the Nissan with the cam timing solenoid unplugged shows there is misalignment. The CMP pulse is shifted to the right a little over 1 tooth. My Nissan factory scan tool read -13 degrees for the intake cam position, exactly half of what the other shops scan tool read. This slightly retarded signal means the timing chain has stretched and will need replacement. This example clearly illustrates the need for known good waveforms when testing VCT systems.
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| The slightly retarded CMP single pulse can be seen in the middle of the captured waveform. |
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| A known good CKP/CMP signal from a 2004 2.5 Nissan saved with a Snap-on scope. |
This next car came to the shop with a MIL on and a rough idle complaint. This 2002 Toyota Echo has the 1.4 1NZ-FE motor and is setting a code P1394 which is defined as a VVT system malfunction, timing does not change. This vehicle does not display cam timing data on the scan tool and the code charts lists thefirst step in diagnosis as checking for correct cam timing alignment. I would prefer not to pull the timing cover off as my first step in diagnosis and there is certainly an easier way. I connect the scope to the #1 coil trigger signal, the CKP signal and the CMP signal. The CKP trigger wheel is a 36 minus 2 tooth wheel with each tooth representing 10 degrees of crankshaft rotation. I set the time base on the scope to 1 second per division and start the engine and capture 2 screens. I will zoom in on the waveform just after the engine begins to start and then again after about 9 seconds of run time.
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| Waveform capture of 2002 Toyota Echo testing VCT operation. This engine should not move the cam at idle. Advancing the cam at idle will cause rough running due to increased valve overlap. |
The first zoomed in capture shows the transition across the zero line of the CMP sensor to be about 8 teeth ahead of the CKP missing tooth region. Remember there is not enough oil pressure built up to move the camshaft yet.
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| Checking CKP/CMP relationship before the engine builds oil pressure. |
The next capture shows the waveform zoomed in after the engine has been running and the oil pressure has built up. As can be determined with the scope, the CMP signal has shifted to the left, advanced, about 3 teeth or 30 degrees of cam advance. The cam should not phase at idle because advancing the intake cam increases valve overlap and will cause rough running, like opening an EGR valve at idle. The same thing happens with the oil control valve unplugged so the problem is not an electrical issue like a shorted to power solenoid. The oil control solenoid is stuck open and replacing it cured the problem. These stuck open solenoids on older Toyota V6 engines is fairly common and if you see an entire bank of the engine begin to misfire after raising the engine RPM and returning to idle, you should inspect these solenoids first.
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| Waveform capture showing the intake cam advanced after the engine ran a short time from a stuck open oil control valve. |
Don’t forget thebasics
While the scope can be a very useful tool to aid in VCT diagnosis, do not forget the basics because they can really bite you when you ignore them. Simple items like the correct oil viscosity or ensuring the engine has correct oil pressure will cause many techs to waste time testing these systems and then doubling back to check something that should be verified early on. VCT systems are tested by the engine computer for two distinct concerns; did the camshaft reach its target position and how fast the camshaft responds. Most cam phasing systems can move the camshaft its full range in less than 300 milliseconds. Even if a cam moves to its target position it may still code for over or under advance if it moves too slowly. Correct oil pressure is a necessity when it comes to VCT response rates. Remember to inspect any filter screens that can be found on oil control solenoids or in hydraulic passages in the engine if VCT codes keep returning. Many spline drive cam phaser performance codes have been repaired simply by using chemical cleaners added to the oil and then performing an oil change. This simple step allows the gear to gear movement to free-up and allows the cam to move to the commanded position faster.
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| This non-return valve removed from the cylinder head of a BMW N52 engine shows the filter screen for the intake VANOS cam phase system. |