New Twist on an Old Story: GMs Variable Valve Timing

December 11, 2009

By Dave Hobbs. Powertrain engineers continue to extract more power from and reduce emissions on conventional internal combustion engines. When both intake and exhaust camshafts get involved, GM’s variable valve timing system meets both goals. Remember the good old days? Chubby Checker was singing “The Twist” and we had cheap gas and fast cars. I was a young mechanic a decade later, in the ’70s, and I can still re- call hearing my first GM muscle car sporting a performance camshaft loping into my bay. It was the sound of power, and I knew that this engine wasn’t going to the run like the rest! 

With time comes change. Some 30 years later the stock engines of today can almost deliver the muscle car feel of yesterday, with hydraulics making camshafts to do “the twist.” Japanese cars have been doing tricks with cams and valves for years, but GM vehicles joined the variable valve timing game only in the last few model years. Now they’re coming out of warranty and in- to your shop.

Twisting pretty much describes what GM is doing with its camshafts today. Valve timing improvements used to be accomplished via camshaft lift, duration and lobe separation. Those values were set in stone once the engine was assembled and were strictly the province of engine designers and performance enthusiasts. Not so with variable cam timing.

In 2002, GM released its DOHC in- line Six 4.2L engine, starting a new generation of engines with variable cam timing. Given the term cam phasing by Delphi, the company that developed and manufactures the system, the engine first appeared in S/T model SUVs such as the Chevy TrailBlazer.

Several recent Delphi innovations such as flat-response knock sensors, electronically controlled viscous clutch cooling fans and throttle-by-wire—also made their way into this new power- plant. But in my opinion, the most no- table was the cam phaser. Attached to the sprocket on the exhaust camshaft, this device helps increase overall pow- er, improve throttle response and at the same time reduce emissions. Those goals are tough to accomplish simultaneously, but in this case they were, as a result of changing the relationship between the camshaft and crankshaft to match engine power demands and conditions.

How Cam Phasers Work

The exhaust camshaft is bolted to a special timing chain sprocket that can run in lock step with the cam or retard up to 25° of cam angle (50° of crankshaft angle) when conditions warrant late exhaust valve closure (see Fig. 1 on page 36). The swivel action of the cam phaser, which is identified in the service manual as a camshaft position actuator, is accomplished via oil pressure applied by an oil control solenoid into the actuator’s piston located in the hub of the camshaft sprocket. The sprocket’s internal hub (the piston) and the end of the camshaft are helically splined together. A spring holds the piston forward (no retard), while applied oil pressure drives the piston rearward, retarding the exhaust camshaft position based on rpm, crank position and cam position, along with TPS, MAP and BARO input values to the PCM.

In 2005, GM switched to a newly de- signed actuator for the 4.2. It uses a four-vane actuator to control camshaft retard and advance. Inside the vane- style actuator assembly are a rotor and stator that are not mechanically linked together, as in the splined actuator (left photo at the bottom of page 36). In- stead, oil pressure is controlled on both sides of the vanes of the rotor, giving a hydraulic link to the stator. Varying the balance of oil pressure on each side of the vanes is how the cam is phased.

A return spring sits under the reluctor of the actuator to help keep it at a 0° (home) position. The actuator contains two cavities for oil to flow into— one for retard and the other for advance. The electric oil control valve (OCV) controls which cavity receives pressurized motor oil (right photo on page 36). When the engine shuts down, so does the feed to the solenoid, allowing the actuator to move to the fully advanced position, locking a pin and ready for the next engine start.

Both the older spline-style and newer vane-style actuators use remotely mounted, 128Hz, pulse width modulated (PWM), 10-ohm, four-way solenoids (usually mounted in the front of the cylinder head). Newer vane-style actuators are coupled with an electromagnet- ic coil situated on the oil control valve, which mounts directly on the front of the camshaft. On this style you’ll see the cam at the home position (0% PWM) or fully twisted to the opposite position (100% PWM) and averaging around 45% to hold the cam in place at the desired phase of retard or advance.

Why Cam Actuators Are Used

As important as understanding how these cam actuators work is understanding the reason behind using them. It’s all about valve overlap. Designers of performance engines often use valve overlap in a manner that allows the in- take valve to open while the piston moves upward on the exhaust cycle. A draft at the intake valve creates a flow of intake air to scavenge the cylinder for the following intake cycle.

GM, however, phases exhaust cam- shafts for a different reason: internal exhaust gas recirculation (EGR). Internal EGR is accomplished by overlapping the exhaust cycle into the intake cycle. Think of the basic four cycles, then imagine if the exhaust valve delayed closing well into the downward stroke of the intake cycle. The opposite of cylinder scavenging would take place. The movement of fresh air and fuel being pulled down into the cylinder would create a draft to pull some of the exhaust gases back into the cylinder.

Exhaust gas recirculation, termed by powertrain engineers as charge dilution, has been doing the business of NOX reduction indeed for a long time. NOX, or oxides of nitrogen, is a compound created when nitrogen (N2) and oxygen (O2) combine at extremely high temperatures (2400°F and above). The primary factors that promote NOX formation include temperature, time at temperature and the concentration of O2. These conditions usually occur when the air/fuel ratio is leaner than 14.7:1 for most nondirect injection gas engines. Oddly enough, when the air/fuel ratio goes past 18:1, the temps head back down and NOX formation is reduced.

The problem with external EGR as we know it is the nature of how the exhaust goes back into the engine. EGR valves are in one location external to the cylinder, so whether they sit on the intake manifold or on a pipe that runs to the intake manifold, one or more of the cylinders are going to get too much EGR in order for the other cylinders to get enough. This unequal EGR distribution has plagued power- train engineers for years.

Internal EGR accomplished via exhaust cam phasing is more effective. If the camshaft is twisted to a retarded position, with a cam actuator delaying exhaust valve closure by a few degrees while the intake stroke occurs, each cylinder gets an equal amount of EGR. Conventional external EGR has also been associated with engine pumping loses, which plague volumetric efficiency. In addition, internal EGR performs NOX-reducing charge dilution without increasing hydrocarbons (HCs). In a nutshell, internal EGR improves performance while reducing emissions, and that’s something every- one can be happy with.

Would the Intake Cam Like to Twist?

Starting in 2005, on twin-cam 3.6L V6 engines used in such models as the Buick Allure, Chevy Malibu, Cadillac CTS and Pontiac G8, GM moved its attention to phasing the intake camshafts as well as the exhaust camshafts. Whereas the primary benefits of exhaust cam phasing are reduced emissions and greater fuel economy, intake cam phasing provides increased low- end torque and high-end power. In- stead of moving the intake cam to effect overlap in the exhaust stroke (cylinder scavenging), intake closure is delayed at the bottom of the intake stroke.


At lower speeds, an open intake valve during the first few degrees of compression often results in air being pushed back out the intake valve as the piston moves upward. At higher speeds this isn’t a problem, and the intake valve that lingers open into the compression stroke allows the air that’s been moving into the cylinder to keep coming in under the momentum the air charge has acquired. The result is a cylinder with greater volumetric efficiency.

Overhead-cam engines that phase both intake and exhaust cams use a vane-type actuator. Pushrod engines, as of the 2007 model year, are no longer confined to the set-in-stone mode of cam timing, either. Look for full-size trucks, SUVs with V8s, along with Corvettes and the all-new 2010 Chevy Camaro to start appearing in your bays soon sporting variable cam timing on their single-cam-in-block engines. Using a vane-style actuator, these engines differ from overhead-cam engines in that they push the oil control solenoid back into a hollow portion of the front of the camshaft. Four small oil holes are situated in the camshaft to line up with the oil control valve/solenoid. The electromagnetic portion is also similar to vane-style actuators.

When Cams Don’t Do the Twist

In addition to the typical OBD II camshaft position sensor (CMP) codes, there are a new set of trouble codes to determine if the camshaft(s) moved to the correct position when commanded.



Supplementing those are DTCs that advise us if the actuator/oil control solenoids are open or shorted. Testing these systems is quite easy if you have a scan tool with bidirectional capability for advanced GM engine output functions. Simply idle the engine, go into the out- put controls function of the scanner and command the actuator to move the cam from its current position (see Figs. 2 and 3 above). You’ll know it has moved by the way the engine runs. It may even stall. PIDs relating to engine load, such as MAP sensor values, will indicate an engine that’s idling poorly. Cancel the cam timing command and the engine should go back to a nice idle.


If the engine is idling poorly and/or stalling when it comes to your shop and the cam timing control function of your scan tool has no effect, then you know the actuator is stuck or is not being sup- plied with oil due to a clogged oil passageway or bad solenoid. The stories of poor oil change maintenance and its effects on GM variable valve timing are common with both dealers and independent shops, so if your customer has a vehicle with one of these systems, you really should encourage him (more passionately than usual) to adhere to the scheduled oil change intervals appropriate for his style of driving.

Besides the oil cleanliness issue, one pattern failure seems to be with the oil control solenoids on the earlier spline- style systems used on the 4200 L6. The two-wire electrical connector has a seal that leaks oil from within the solenoid. Pop off the connector and look for leak- age (see the photo and inset at left). If it leaks you’ll have to replace the solenoid, and that means pulling the power steering pump out of the way to clear the cylinder head.

A word on R&R procedures is merit- ed here. To replace the actuator on the 4200 L6, you have to pull the intake, alternator and few other choice components just to get the cam cover off. gm-variable-valve-fig5About the time you get all of that done, it’s easy to get anxious and just remove the actuator from the front of the camshaft. Don’t! If you do, the timing chain tensioner will ratchet the chain down as you slide the actuator off. If you thought you had to do a lot of work to get this far, just wait until you start pulling the front cover off and find out you have to remove the oil pan. Removing the oil pan on 4WD models means pulling the halfshafts out of the passthroughs in the oil pan. Better take your time and purchase or borrow a Kent- Moore J-4417 or equivalent (see the photo), a hooklike tool that holds tension on the chain, allowing you to re- move and replace the actuator without pulling the front cover.

gm-variable-valve-fig6Another tool I see some dealer techs using is a simple pitchfork-looking de- vice (such as the one shown in the bot- tom photo below) that goes through the gap between the chain and the front cover all the way down to the tensioner to keep it from pulling up when the actuator is removed. V6 models require a pair of long rods (such as the Kent-Moore EN-4813) with spreaders on the bottom to prevent the tensioner from flopping back as the actuator is removed.

As with any OHC timing chain service procedure, consult the service manual to determine how to properly line up the sprockets before taking anything off. When removing or installing the actuator on a cam-in-block V8, make sure you don’t push on the actuator’s reluctor wheel. It’s held together with three roll pins that could become dislodged, causing an internal spring to push the assembly apart in your hands. This will create some extra grief at the least and an injury to your hands at worst. Do your pushing and pulling by the sprocket and once the actuator is off the engine, it’s a good idea to run a tie wrap through the hole where the camshaft goes around the entire sprocket/actuator assembly to hold things in place prior to laying the actuator down on the bench.

On the subject of proper care for these actuators, GM has special TSBs called Preliminary Information bulletins (PIs) that are very helpful. PIs are typically not available via aftermarket ser- vice information sources; you must have a subscription to ACDelco’s SI 2000 on- line information system to see them. Subscriptions are now available on a short-term basis for a modest fee, similar to other OEM websites.

PI 00771 gives techs a good heads- up on proper cam actuator handling on inline 4200 6-cylinder engines. Prior to replacing the actuator on these engines, you’re supposed to turn the crankshaft several times until the name “Delphi” on the actuator’s face is on top and horizontal. When tightening up the bolt for the new actuator, many techs hold the actuator with slip-joint pliers, which is a recipe for a DTC and a comeback. Use a 1-in. wrench on the hex surface to hold the actuator and you’ll be fine. PI AIP3334 makes a slight correction to TSB 08-06-01-011 along similar lines on how to install an actuator on V6 models.

While these engines aren’t quite the muscle car powerplants from the good old days, variable cam timing, whether in a 2002 TrailBlazer L6 or a 2010 Camaro V8, brings GM a little closer to that mark while still maintaining good idle, reduced emissions and better fuel economy. For technicians in the field, a little knowledge, up-to-date service info and remembering to stress to customers the need for proper oil change maintenance will allow engines with variable timed camshafts to keep “doing the twist” for years to come. Chubby Checker fans should be proud!