Variable valve timing and variable displacement systems have been on the road and continuously evolving for years. They go by a myriad of different names including Toyota and Honda VTEC, GM Cam Phasing, GM DoD (Displacement on Demand a.k.a. Active Fuel Management), Chrysler MDS (Multiple Displacement System), Nissan VVEL (Variable Valve Event and Lift) and the Fiat Multi Air System. Each year we see advances in electronics allowing for the control of intake and exhaust valves with increased accuracy.
Reading scan data PIDs for VTEC operation is essential in any diagnosis. Camshaft variance is particularly useful.
In spite of all the advances in technology, they still are susceptible to failures. Years ago when you had a mechanical engine problem related to valve timing or valve opening, the cause was cut and dry. A worn cam lobe, collapsed lifter or slipped timing chain/belt caused drivability problems that were simple enough to diagnose. Not so simple with the advanced valve train systems used on about every vehicle on the road today!
Diagnostics techniques for today’s advanced valve control systems can range from simply watching and listening to the engine while you activate a solenoid with jumper wires to scrutinizing scan tool data Parameter Identifiers (PIDs) to interpreting pressure transducer patterns via a lab scope. Before we dive into each of those areas of diagnostics, let’s discuss a little theory behind how a few of these systems work.
Why Vary Valve Timing?
Anyone who has ever built a performance engine knows the benefits of changing valve timing from the stock settings. At higher rpms, the need to scavenge the cylinder of exhaust gasses to make more room for fresh air/fuel in the intake stroke is essential for performance. Contrary to the most basic traditions of teaching 4-stroke Otto cycle theory, each stroke does not exist for ¼ of the 720 degrees required to produce a complete engine 4-stroke cycle. Depending on the application/camshaft profile, the intake cam lobe may allow its valves to open as early as 48 degrees Before Top Dead Center (BTDC) on the exhaust stroke and close as late as 84 degrees BTDC into the compression stroke.
Hybrid Electric Vehicles (HEVs) using a modified Atkinson cycle action (via variable valve timing) can take advantage of the extra torque assist of the electric motor(s) to allow for a more fuel efficient concept of limiting the compression stroke by drastically delaying intake valve closure during low torque demand conditions.
Don’t pass up the opportunity to look at Mode $06 data when diagnosing intermittent variable cam timing problems.
On the flip side of that coin are the full race cams that allow for more volumetric efficiency (VE) by delaying intake valve closure during the early periods of the compression stroke to aid in filling the cylinder to take advantage of the inertia of air moving into the cylinder during the downward piston movement portion of the intake cycle. Racers also know that an early opening of the intake valve during the last portions of the piston’s upward movement of its exhaust stroke (called valve overlap) will result in cylinder scavenging (clearing out exhaust gases) which means more room for more power contributing fresh air/fuel. Cylinder scavenging requires early intake valve opening while the piston is moving up with the exhaust valve open. This reduces an engine’s ability to rum smooth at low rpms, and can require a high idle speed to stay running. A side effect is the familiar loping sound that indicates “that car has a performance cam.”
Thinking emissions instead of performance, engineers in recent years have established that exhaust gas recirculation (EGR) can be managed w/o the problematic external valve that has been the tradition since 1973. With external EGR, the distribution of exhaust into the intake to provide the quenching effect of inert gas to a hot cylinder typically results in uneven doses of exhaust gas to some cylinders. That means some cylinders receive the needed amount of EGR while others get a bit overdosed. Not the case with internal EGR where the exhaust valve is closed later into the intake stroke effectively pulling some exhaust gas (called reversion) back into the cylinder during the intake event.
All of the aforementioned goals of VE, cylinder scavenging, modified Atkinson cycle and internal EGR were always a watered down compromise with stock engines that had to idle smooth and meet emissions under various engine conditions until variable cam timing came into existence. Now we know why variable valve timing exists, let’s take a look at how a few of these systems work.
Toyota / Lexus
Toyota has been modifying camshaft timing on the fly for more than two decades now beginning in 1991 with their two position cam phaser Variable Valve Timing (VVT) system. That system evolved to the VVT-i system which included the “i” in the acronym for “Intelligence” in determining cam timing position as well as allowing for continuous cam position variations.
On the simpler side of diagnostics, feeling for a firm click will verify the operation of the oil control solenoid.
These systems typically have used an oil spool valve moved against a spring by an electromagnet. As the spool valve moves it controls the flow of oil to a vane style cam phaser, which physically moves the camshaft to a more advanced or retarded position in relationship to the to the sprocket driving it. By the early 2000s, Toyota used a VVT-i system to accomplish an Atkinson cycle on their hybrid engines. Gen I Prius engines (01-03) will vary intake cam positions by as much as 43° while the Gen II models (04-09) vary cam angle positions by as much as 33° .
Keep in mind that these cam phasers reduce the effective compression ratio of the engine when doing a compression test. On that topic, when performing a compression test on any hybrid that starts the engine with a high voltage motor-generator, keep in mind that the engine is being cranked at over 1,000 rpms, which is much faster than the traditional 12-volt cranking motor. This higher cranking speed is almost a running (idle) compression test which normally means about ½ the normal pressure on your compression tester.
That being said, you will need to enter into the reduced cranking speed compression test mode that most scan tools allow you to initiate with HEVs. Expect a good compression reading to be between 90 and 120 PSI on these hybrid engines. Next in evolution for Toyota was their Lexus division’s introduction of their VVT-iE system. The last letter “E” stands for electric which in this case means an electric motor changes cam timing instead of an electrically actuated hydraulic valve. Some models used an electric motor between the camshaft and sprocket for both intake and exhaust cams while a newer design Lexus system utilized the electric VVT-iE electric motor system on the intake cams only and the conventional oil controlled VVT-i system for the exhaust cams.
Improper maintenance and a torn screen in this MDS oil control valve resulted in an intermittent hard misfire on a Dodge Hemi.
Negating the need for oil pressure to change cam timing means the engine can be started with modified cam timing. The electric motor spins the cam faster than the crank driven sprocket and then locks into an advanced position. Conversely, if no intake valve advance timing (increased valve overlap) is desired, the electric motor runs at the same speed as the driving cam sprocket. More traditional systems only allow cam timing changes to be made after the engine is running and oil pressure established.
GM – Same Idea, Different Hardware
GM’s earliest variable cam timing systems came out in the 2002 Trailblazer/Envoy models sporting an inline 6 cylinder DOHC engine. This engine applied a cam phaser of a splined gear design. The single phaser located between the exhaust camshaft and sprocket can vary cam timing by as much as 25 degrees (50 degrees crank angle). A spring applied between the two splined gears to hold the phaser to 0 degrees where a lock pin holds it in place after engine shut down.
The electrical connections for achieving variable valve timing are located on the solenoid used for controlling pressurization of oil that controls the movement of the splined phaser. Later models utilized a vane style phaser design similar to what most other OEMs use including the previously discussed Toyota VVT-i systems. For push rod engine applications, GM took the route of advancing the single cam in block with a vane style phaser controlled by a solenoid actuated oil control valve. The electrical portion of the control valve system is located on the front of the timing chain cover and electro-magnetically pulls the oil control valve that has the double duty of being a cam sprocket retaining bolt.
When replacing a timing chain/gear set, camshaft or the cam phaser actuator, special care must be exercised. On the front of the cam sprocket/actuator assembly is a reluctor wheel held in place with three roll pins. Don’t pull on the reluctor wheel. Remove the sprocket by pulling on the sides of the sprocket while holding the reluctor wheel up to it and in place. When removed, hold these components together by inserting a plastic wire tie into the hole in the center to tie everything together. The actuator assembly contains springs that could hurt you or at least make a useless puzzle of the actuator should you disassemble the assembly accidently.
GM and Chrysler Collapsed Lifters?
One might compare that age-old problem to what some OEMs are doing to save fuel with variable displacement engines such as GM’s Active Fuel Management system, a.k.a. Displacement on Demand which debuted on select 2005 trucks and SUVs equipped with the 5.3 liter V-8 engine. A special set of valve lifters for cylinders 1, 4, 6 and 7 are key components in this system. Cylinder deactivation allows for 4 cylinders to no longer pump air while the non-deactivated cylinders continue to function thereby giving the engine the fuel economy closer to that of a 4 cylinder.
Customers who ignore their engine’s oil maintenance may be saving pennies but setting themselves up for expensive repairs.
An assembly under the upper intake manifold plenum referred to as a LOMA (Lifter Oil Manifold Assembly) contains an oil control solenoid for each lifter for the deactivated cylinders. When low torque and road speed cruising conditions merit the V-4 cylinder deactivation mode the solenoids turn on one cylinder at a time during each cylinder’s compression stroke. The compression stroke is chosen so that one last power stroke occurs to allow for combustion gasses to maintain some pressure on the piston as it moves up and down basically doing nothing.
This trapping of gasses aids in reducing piston rattle during the deactivation process. Ignition is continued to keep the spark plugs from fouling while fuel injection is halted for obvious reasons. To prevent the driver from feeling a sudden lack or surge of power when the cylinders are deactivated or reactivated (remember those old Cadillac 4-6-8s?) a throttle-by-wire algorithm feathers throttle angle to make the process feel seamless.
The electro-mechanical process for deactivating cylinders begins with the LOMA solenoids pressurizing oil to special passages in the lifter (separate from normal lifter oiling) to press locking pins on each side of the lifter body. When these pins are pressed together against spring pressure inside the lifter the lifter’s inner body drops down while the outer body continues to follow the cam lobe movement. A spring at the top of the lifter then compresses to keep pressure on the push rod as it goes along for the ride w/o lifting up on the rocker arm.
The use of a pressure transducer in the cylinder is the best way to inspect variable valve timing operation.
Reactivating each lifter to regain valve operation is as simple as turning off the deactivation solenoid in the LOMA. This is performed for each cylinder in its proper firing order and allows each lifter’s locking pins to move outward and catch so the lifter is once again rocking to its rocker arm. Chrysler systems work much in the same manner as GM. Its system, called the Multi Displacement System (MDS) also debuted in 2005 and was improved in 2009 with increased camshaft lift and variable valve timing in the 5.7 liter Hemi engines. A reflash was released to enhance operation. The new lifters can be used in previous model engines. The Chrysler systems are quite similar to GM although their special deactivation lifters utilize springs that are internal as opposed to external.
The Common Ailments
All of the variable cam timing and cylinder deactivation systems mentioned regardless of design variations seem to suffer from many of the same ailments. By far the most common problems revolve around lubrication issues. Owners who neglect regular oil changes or engage in continuous heavy-duty/high-performance engine operation are the most likely candidates for drivability problems and DTCs.
Not every variable valve timing problem that creates a drivability symptom results in a DTC. Honda is one prime example. Unlike other variable valve timing systems, Honda’s VTEC system utilizes rocker arms that can connect overhead cam lobes with greater lift and duration when rpms rise upward of 5,800. This operation is accomplished with locking pins actuated by oil pressure control solenoids. The cam lobe with greater lift is situated between two lobes with lesser lift and is fixed into motion with the locking pins. The pins can become stuck when the customer reaches VTEC application rpms and not move back to the lower profile cam lobe valve actuation mode at lower speeds. The result is an unstable idle.
Though there are plenty of scan tool PIDs for rocker arm solenoid oil control status and rocker arm oil pressure (switch status) the no code problem occurs when Honda’s PGM-FI computer doesn’t see a change in rocker arm oil control pressure switch status after commanding the higher lift VTEC mode. The PGM-FI assumes there is a problem with oil pressure in the VTEC oil passages and commands a fuel cut off condition. Sludge build up from neglected engine oil maintenance can delay but eventually allows some oil pressure to change the switch status. The driver, however, feels the fuel cut off which causes a severe hesitation. DTC P1259 will eventually set if the problem occurs long enough or frequently enough but a non DTC idle complaint after running at high RPMs is not uncommon.
This GM system can be tested much like you would a transmission valve body.
Again, getting your customer to change oil more frequently and use the correct type and weight is paramount to preventing VTEC and Multi-Displacement system failures. GM released a calibration update last year for close to 800,000 2010-2012 Equinox, Terrain, Lacrosse and Regal models using the 2.4 inline 4 cylinder engines. The software update reduces the oil change intervals that are automatically calculated via an oil life monitor.
Regardless of oil type the newer intervals can alert the driver to a needed oil change on the average of 5,000 to 7,000 miles based on driving characteristics. The original interval was longer and was suspected of a causing premature wear on timing chain components and other internal engine parts. Nissan’s complex VVEL system requires engine’s sporting that technology to run 5W30 API SM Ester Oil. That certainly is a unique motor oil you won’t find in many stores frequented by the DIY oil changer!
Chrysler’s required 5W-20 weight oil is also often ignored by the DIY and even some professionals. The Chrysler PCM’s unique software will calculate incorrect oil weight by using data from oil pressure and temperature inputs to set DTC P1521 to prevent a mechanical problem with their cam phasers or MDS systems.
I encountered a 2005 Dodge Magnum with the 5.7 Hemi that was having severe misfires intermittently at highway speeds. It was determined that the MDS had a lifter sticking in the cylinder deactivation mode. An examination of the oil control solenoid that affected that MDS lifter showed a broken screen that allowed sludge to lodge in the solenoid. Intermittently when that cylinder went into deactivation mode the sludged up MDS oil control solenoid would keep the oil flowing and maintain deactivation of the lifter long after the MDS system commanded the cylinder to reactivate.
This GM system can be tested much like you would a transmission valve body.
The engine showed obvious signs of neglected oil changes. Keep in mind that when sludge and other contaminants in an engine’s motor oil reaches these types of components, they may not circulate in and out. They may simply reach their destination and lodge in that location resulting in a cam timing or variable displacement problems.
Diagnostics – As Simple or Complex as You Desire
My first advice obviously is to retrieve trouble codes and follow the published trouble trees. Sometimes, however, the book doesn’t lead to a clear solution so let’s go over some common diagnostic methods for VTEC and AFM / MDS problems. After pulling DTCs, always first and foremost in your diagnostic process is to determine the correct type and amount of clean motor oil and proper engine oil pressure. After establishing that baseline keep in mind the importance of watching the camshaft variance PID in powertrain data. Some OEMs allow for no more than 4 degrees for variance from where the cam is being commanded to move to (positive or negative degrees) and where the camshaft position sensor (CMP) actually says it is in reference to crank angle.
Also watch for duty cycles commanded to the solenoids being excessive compared to what you consider normal for a particular engine. This takes some experience in VTEC PID observation. If the powertrain control module (PCM) can’t get the cam to advance / retard to a certain number of degrees desired angle it may keep increasing the pulse width modulated (PWM) duty cycle to try to get it there. Actuating solenoids via your scan tool or with jumper wires is a simple way to determine system operation. Simply watch cam angle with your scan tool or scope CMP to see if a change occurs. The engine should run poorly (or at least differently) when performing this test as well.
The potential uses of an in-cylinder pressure test for diagnostics includes verifying VVT operation.
If there is no change, remove the oil control solenoid and activate it again – watching for physical movement and/or listening for a click. Some OEMs only allow mode 8 scan tool bi-directional commands for VTEC solenoids to operate at key on-engine off (KOEO) so simply listening or feeling for a solid click is of value as is removing the actuator / solenoid to watch for a physical movement when activated. Shop air can be used to blow into cam phaser or variable displacement solenoid oil circuits to see if the expected mechanical result occurs.
These simple steps help to determine whether the problem is strictly hydraulic-mechanical or electro-mechanical. Before you brush off of these more routine diagnostic processes and grab a lab cope and pressure transducer remember to see if the vehicle has a variable cam timing OBD II monitor. Most that do are non-continuous so you’ll find valuable info in Mode $06 data.
The most common design for a VVT actuator is the vane-style. Note the locking pins that can become stuck due to lack of proper oil maintenance.
Pressure Transducers
The use of a lab scope and pressure transducer is by far the most accurate way of determining what the valves are really doing in an engine. The downside of this type of testing is that it is one of the most complicated procedures you can perform with a lab scope. Having made that disclaimer the process of diagnosing VTEC problems as well as slipped timing chains/belts is quite practical for most lab scope savvy techs.
This process begins with selecting a pressure transducer capable of measuring pressures encountered in the combustion chamber. There are numerous models on the market that will do the job. To measure valve opening and closing angles the transducer gets connected to a hose used for compression or leak down testing (minus any Schrader valve) and then electrically connected to a lab scope.
The test works whether or not the engine runs or only cranks which is helpful because some VTEC problems can be severe enough to cause no starts! Obviously there is a quite a bit more to explaining this advanced method of diagnosing engine problems than my space here allows but this gives you a general overview to prepare you for more study on the subject of pressure transducers and you can learn it right here at Motor Age.
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