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The symptoms that you describe do indicate that the VTC sprockets are making the noise. This noise only occurs at low RPMs and can be exaggerated during cold starts. Contrary to popular belief, the noise can occur in 100% stock engines typically with higher mileage; I attribute this to the internal parts wearing over the years and reducing the internal friction which helped prevent the noise from occurring when they were new. Let's get to your diagnosis now -
If you unplugged the solenoids and the noise disappeared, it is not the VTC sprockets physically making the noise. You cannot verify VTC noise by unplugging the solenoids. The only way to verify VTC noise is to energize the solenoids via a ConZult or by grounding their green w/black stripe wires, mind you, while they are still plugged in (note that the ECU triggers the solenoids by grounding them). After energizing the solenoids and the noise still exists, it is not the VTC making your noise. Now for some misconceptions -
- First and foremost, the little springs that go under the front cover have absolutely no effect on controlling VTC noise. These little spring's only purpose is to preload the sprocket axially so it does not move when the VTC is activated and oil pressure builds within them.
The reason people think they affect VTC noise is because when they replace them, the noise does go away for a little while. But, that wasn't all that was change within the system, when the covers were removed most of the oil was drained from the sprockets and replaced with air. Once everything is sealed back up, the air trapped inside takes some time to bleed past the VTC mechanism and while it is trapped insided it provides a compressible cushion that prevents residual oil pressure (see 2nd misconception below) from unseating the internal piston and making noise. These springs do not wear out, they are not stressed anywhere near the limits of the material they are made from. Regardless of Nissan's 60K maintenance recommendations, it is a total waste of money to replace them, even moreso with aftermarket HD springs. - Second, VTC function - they operate off of oil pressure, during idle and high-RPM the solenoid is not energized and the valves in back of the cams simply bleed off oil pressure. Once energized, the pressure builds within the sprocket, forcing the internal piston to move and rotate the camshaft ahead 20° (crankshaft). The oil pressure to the VTC is restricted at the engine block's deck surface, this is to prevent too much volume from reaching the VTC, but, just like raising fuel pressure to flow more through the injectors, if the overall engine oil pressure is higher more volume will reach the VTC than the valves in back of the cams can bleed off and this residual pressure will help to unseat the VTC internal piston and cause noise. Ultimately, as far as the VTC are concerned, more oil pressure is not good.
- Third, the primary cause of VTC noise - This is hands down due to valve spring installed pressure, camshafts with increased lift play a very minor part in causing this noise. Stock installed valve spring pressure is approximately 55 lbs with the valves closed and aftermarket springs will install from 70 to 85 lbs. Most people (as well as engine shops) don't have a clue what the installed valve spring pressure is on the VGs they are assembling, my recommendation is to know the installed pressure of your valve springs.
- Fourth, and last - there are three different springs mentioned that affect the proper operation of the VTC, these are:
1) valve springs
2) front cover VTC spring
3) internal VTC springI described #s 1 and 2 at some length above, so I'll explain the internal VTC spring here. This spring is internal to the sprockets and it is what the oil pressure works against when activating the VTC. It is only marginally capable of controlling the VTC with stock valve spring installed pressure and grossly overloaded with any valve spring pressure higher than stock. There is no DIY way to replace this spring.
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