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Diagnosing variable camshaft timing systems

Sunday, April 1, 2018 - 07:00
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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.
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 the first 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.

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.

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.

Waveform capture showing the intake cam advanced after the engine ran a short time from a stuck open oil control valve.

Don’t forget the basics 
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.

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.

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