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This is the write-up for how to fix your VIAS. If you buy your upper manifold and find that it is broken, fix it ASAP. If it is not broken, it is still a good idea to do this fix so that it doesn't in the future. This write-up assumes the manifold is already off the car and you're using it for a 4th gen. To see the full write-up intended for 5th gens, go here.
VIAS stands for Variable Intake Air System. It is made up of a power rod going through your intake manifold that holds a series of butterfly valves. A solenoid turns the rod at 5000 rpm to open the valves to allow the engine to breathe better. Nissan VIAS video link.
The VIAS system is the main reason 5th gen maximas have more horsepower than 4th gens.
However, the VIAS has a design problem - an extremely weak connection in the internal linkage that is destined to fail. You lose a great deal of power on the top end as a result. The VIAS is very easy to fix and only requires a few simple tools and some JB weld. New bolts are reccomended, but optional.
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This is the VIAS solenoid unit. It is on the drivers' side of the intake manifold. Even if the solenoid works properly when you hit 5000 RPM, your internal linkage is still either broken or soon will be. The solenoid mechanism is held on with four very soft yet quite stuck screws. The heads strip easily on these, and if they do, smaller vise grips will do the job easily. It is reccommended that you run to your local hardware store and replace the 4 screws with hex-head bolts for easier and "safer" removal in the future.
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The cup should be attached here, but it is broken off due to crappy design. Some owners find their cup is still attached, but loose. The cup should NOT be able to rotate independently of shaft. Clean the whole area shown with brake cleaner as well as inside and outside of cup.
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Put a little JB Weld or JB Kwik on bottom of cup where it mates with solenoid shaft, then a good coating on inside bottom of cup. Try to keep sides clean. Pay attention to cup "handle" orientation. Double-check to be sure it's facing the right way. If you're not sure, take a look at how the power rod in your manifold faces. Make sure everything rotates freely when you push lever on solenoid.
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Pressing the spring in while JB is tacky prevents a clearance issue with end of rod. Otherwise your pool of JB Weld may make cup too shallow for spring and rod. Doing this, you don't need to cut spring. Also, if you want to play with removing power rod later, you don't need to dissasemble solenoid end. Let JB get good and hard before re-assembly. With Kwik, I'd wait at least 2 hours; regular JB should be left overnight at least.
CREDIT: .org member mhadford