By Matt Dixon, assistant professor, Southern Illinois University
Vehicles with variable valve timing (VVT) have become commonplace over the last decade. Even more commonplace are engines that use a phaser to manipulate camshaft position and, hence, valve timing. (See Figure 1.)
The phaser style of VVT is the focus of this article.
Figure 2: The oil control valve is the traffic control device of oil pressure. In this hold position, neither chamber receives pressure nor is drained.
Phasers commonly can be found on just the exhaust cam or on both the intake and exhaust cams.
Alteration of camshaft position changes the cam centerline and the lobe separation angle between intake and exhaust cams.
This gives engineers flexibility in improving fuel economy and power while continuing to meet emissions standards.
VVT presents additional diagnostic challenges and repair opportunities to the service industry including new trouble codes.
If you are not familiar with these units, it’s time to advance your diagnostic readiness by examining the VVT system, its controls and operation.
Figure 3: The oil control spool valve moves by PCM control of the solenoid. One set of chambers is pressurized while the opposite chambers are drained to create phaser movement.
Mechanical, hydraulic and electrical controls have been added to VVT engines.
Motor oil is the hydraulic medium that makes VVT work.
That means it is imperative that engines are filled to the correct level with clean motor oil of the proper viscosity.
Low oil level or the wrong viscosity can result in system slow response codes such as P000A or P000B and possible drive complaints including an illuminated MIL.
Oil pressure is critical, and as bearings wear and develop clearance, pressure will be affected.
Figure 4: The spool valve directs oil pressure and oil drain to move the phaser in the opposite direction as Figure 3.
Engines are machined with additional oil galleys for VVT and are equipped with one or more fine mesh screens to prevent debris from entering components.
Replacing these screens often requires major engine disassembly.
Sensors that monitor oil pressure and oil temperature are common on VVT engines and are a part of system control strategy.
The major control component in camshaft phasing is the oil control valve (OCV). The OCV is a spool valve much like those found in automatic transmissions. The PCM (powertrain control module) duty-cycles a solenoid that alters valve position.
The OCV is an oil traffic control device of sorts. It determines which ports receive pressurized oil and which are vented.
Figure 5: These are the oil drain ports that allow oil directed by the OCV to drain into the front timing cover. (See Figures 2, 3, 4 and 5.)
Pressurized oil travels through the OCV to one of the camshaft bearing journals. Oil flows though passageways inside and toward the front of the camshaft. (See Figure 6.)
At the nose of the camshaft, oil enters ports of the camshaft phaser. The phaser is a mechanism with two major pieces, the rotor and the phaser body.
Figure 6: The oil control valve feeds or vents these camshaft bearing ports. Oil flows into and out of passageways inside the camshaft.
The phaser body is physically bolted to the camshaft sprocket. The rotor is connected to the camshaft using a dowel pin. (See Figure 7.)
The two pieces are able to move about 20° (40 crankshaft degrees) independently of each other. Ports inside the phaser direct oil in or out of eight chambers.
Four chambers are considered side “A” and the other four are side “B.”
As one group of chambers receives pressurized oil, the others are vented to provide the force necessary to move or hold the rotor relative to the phaser body.
Figure 7: Internals of a cam phaser. The larger hole near the center at the 3:30 position is where the dowel of the camshaft locks to the phaser. The other four holes direct oil to and from the camshaft passageways. The outer body attaches to the cam sprocket. Also notice the oil seals that separate the oil chambers.
Oil seals fit into machined grooves of the rotor to provide a tight seal between the chambers.
Vented oil from the phaser ports travels back through the camshaft, the cam bearing ports, through the oil control valve and then drains into the front timing cover.
There is a mechanical device inside the phaser known as a lock pin.
The spring-loaded lock pin on the rotor engages into the phaser body to lock the two pieces together. (See Figure 8.)
The lock pin prevents noise and potential wear upon engine start. Oil pressure is required to disengage the lock pin.
The 2.4L Chrysler engine that I disassembled also featured a spring on the exhaust camshaft.
Figure 8: The lock pin is on the rotor at right at about the 9 o’clock position. It locks into the phaser body at left. During low oil pressure events such as cranking, the pin locks the two components together to prevent noise and wear.
The locked phaser positions on this engine are full retard on the intake and full advance on the exhaust.
Because of the clockwise rotation when viewed from the front of the engine, the exhaust rotor requires additional assistance in reaching the full advance position.
In the default position, there is no valve overlap. It should be noted that service information does not recommend phaser disassembly and individual parts are unavailable.
As for service parts, phasers are sold as an assembly.
Electrically, the OCV solenoid has two terminals. I measured the resistance of several solenoids from various manufacturers.
They ranged between 7 and 12 ohms of resistance.
Both circuits connect to the PCM, which provides duty-cycle control either on ground or the insulated (power) side.
I found versions of both on our laboratory vehicles. OCV solenoids are typically cycled upon ignition run mode as part of a cleaning and diagnostic strategy.
Regardless of control specifics, the PCM monitors solenoid circuits for faults including opens, shorts to ground or shorts to voltage.