Controlling the VCT System
On this system, when the PCM sends the PWM signal of 0%, or simply off, the spool valve is moved so that the oil is forced to fully advance the timing. Fully advanced is simply normal or rather no change to the timing in this application. As the PCM sends a PWM signal of more than 0%, the spool valve moves and starts to allow the pressure to equalize between the advance and retard sides of the phaser. As the PWM signal increases, oil is allowed to pressurize the retard side of the vanes, forcing the phaser to turn counter clockwise, meaning cam timing retards. When the solenoid is commanded at 0%, the locking pin should remain engaged. As the solenoid commands more duty cycle the locking pin will release from oil pressure. Simply unplugging the VCT solenoid will default the spool valve in the fully advanced position, or OFF. If there are any detected faults in the system, the PCM will keep the solenoid off so the phaser stays in the default locked position.
VCT Common Failure Points
The locking pins are prone to failure and unintentionally allow the camshaft sprocket to rotate separately from the chain when not currently in active use. As the camshaft turns, it fights against the valve spring force causing the phaser to jump back and forth at lower RPMs if oil pressure is insufficient enough to hold it. This causes a rattle. These excessive forces on the chain could also lead to timing tensioner, chain, or guide breakage. (These failures is my personal explanation of what I have noticed, I could be wrong.) As the phaser jumps back and forth, it forces oil backwards into the camshaft and front camshaft bearing and cap. Debris can get stuck in this process and can lead to repeat phaser failure as well as fatal camshaft damage.
Another type is failure is the roll pins holding the cam position pickups to the phaser. Broken roll pins can cause the cam sensor reluctance pickups to become out of place and allowing the spring to push the end off into the cam sensor. Other failures would be from oil contamination, VCT solenoid failures, and main oil system problems. These engines have a tendency to leak pressurized oil out of the center of the oil pump which leads to lower system pressure. A VCT system failure could be the result of the oil pump being unable to hold a minimum of 26psi at a very hot idle or a possibly clogged pump sump screen.
Diagnosis of VCT System
IDS, or Integrated Diagnostic Software or equivilent, is required to monitor the PIDs or control output states necessary to properly diagnose this system. Failure or disabling of this system can result in noticeable power loss, efficiency loss (less MPG!!) and possible engine damage. Locking the VCT system in full advance will certainly result in higher emissions, lower fuel efficiency, higher combustion temperatures, and NO EXTRA POWER since the engine already advances the timing when it is needed BUT usually a good idea for high mileage vehicles or to just eliminate future issues. The PIDs you will want to monitor are VCTFAULT, VCTERROR1, VCTERROR2, VCTDC1, VCTDC2. Lockers or limiters may need to be installed if aftermarket aggressive camshafts are to be used otherwise piston to valve damage will occur. Base oil pressure should be checked as well.
The ERROR pids should read near zero at all times, but they will jump around a little bit. As long as they return to near zero fairly quickly during throttle changes or commands you generally are good to go. The VCTDC pids show the commanded duty cycle of the VCT solenoids. The VCT ADVANCE pids show the current command cam position.
Those who have a rough idle, rattling noise under light load at 1500 rpm, or diesel sounding running rough condition after returning to a hot idle, you should replace the VCT solenoids first and read about the Ford Technical Service bulletin related to VCT Solenoids. Replacing these components can be a tedious task on certain model years, just because removing the valve cover is a pain. If your issue is not resolved, the issue is most likely going to be oil pressure related. I would suggest replacing the timing chains, timing components, and a oil pump - and possibly installing a VCT lockout kit. I wrote an article that provides step by step instructions to replace the timing chain, tensioners, guides, and phasers on the 5.4L 3valve engine. The article lists all the parts you will need, where to get them, and any specialty tools required for Changing 5.4L Phasers and Timing Chain
For those others interested in learning about other common problems with this engine can refer to another article I wrote Common Problems with the 5.4L Triton