My engine stumbles under full throttle at 3500 to 5000 rpm. Not terribly, but noticeable. I've recently changed plugs, had the throttle body and fuel injectors cleaned, and tried replacing the 3 coil packs on the front of the motor (I have yet to try the back 3 yet). Any idea what may be causing the stumble?

Thanks,

Bill
2000 SE

 

you said you tried to replace the ones on the front bank, but i am guessing it was unsuccessful? if so you might not have secured the original coils. did you get a code indicating misfires in multiple cylinders or did you just want to replace the coils? also what kind of plugs were used when they were replaced?

 

Could be a problem with the VIAS system--2k maxes have a TSB out about one of the vent hoses to the VIAS control solenoid being kinked and causing the VIAS to stay in high-RPM operation (open) all the time. Do a search (use http://test.maxima.org) for "VIAS TSB" in the 5th Gen forums.

Typically if the VIAS is stuck open, you'll notice a little "stumble" around 4K RPM-ish as the ECU expects the VIAS to go from closed to open, and enrichens the fuel mixture to anticipate the extra airflow. If the extra airflow isn't there (due to the VIAS being open all the time), then a short stumble may be experienced.

 

where is the vias, i think im experiencing this problem also

 

the control box is right under the right side of the intake manifold. i believe it has 2 or 3 vacuum lines coming out of it on the top. in order to see if it is functioning you can run a car of similar equitment and see if you get pulled during higher rpms or you can disconnect the vacuum line and if it feels the same in high rpms then something is wrong.

 

I received no codes. I read somewhere here in a post that the "coil packs" may cause that problem. Instead of spending almost $400 for 6 of them, I bought one and swapped out one at a time to see if there was any difference. (There wasn't).
The plugs I used were the stock Nissans.

Bill

 

By the way, what does VAIS stand for? I'm trying to locate info about it in my repair manual.

Thanks,

Bill

 

stands for variable air intake system according to nissans site. i thought it was VIAS for variable intake air system but it doesnt really matter.

 

Variable Intake (Actuator?) System.
It's basically a setup where the intake manifold has 2 runners down near the intake ports--one that goes straight through, and one that kinda curves out of the way (a "long-way" passage)
The VIAS system involves a set of butterfly valves which rotate one way or the other, to switch between the long runner passage (<4-5K RPM) and the straight-through short passage (>4-5K RPM).
This set of butterfly valves (which are all on one rod, built/cast inside the plastic intake plenum) are actuated by the Power Valve, a vacuum-diaphragm device that moves this "rod" between one position (closed, i.e. enabling the long-runners) and the other (open, allowing airflow straight through). The Power Valve's vacuum source is switched by the VIAS control solenoid, located in a small box bolted to the intake plenum towards the driver's side.

I believe the vacuum source for that valve/solenoid comes internally through the Vacuum Tank which it is bolted to. The "hoses" you see coming out the front are actually fresh-air vent hoses, which if you trace back, lead to the intake tubing. They supply the fresh air which fills the Power Valve to allow it to spring back to the "closed" position at lower RPMs.
This VIAS control solenoid (which, BTW, has a wiring harness plugged in underneath) is controlled entirely by the ECU

I took a pic of my engine bay and added some captions to it--


I suppose as an experiment, you could disconnect the VIAS control solenoid fresh air hose, give it a run, and see if you get the stumbling still. That will rule out the Power Valve staying stuck open; it won't necessarily rule out the Power Valve not opening at all... to do that, you'll need to peek at the end of a rod underneath that box, which has an L-shaped piece that moves when the Power Valve opens... (you can rev the engine up by turning the throttle manually and watch for that L-shaped piece to rotate when it hits 4-5K RPM). And if that L-shaped rod does indeed move (and return to its original location below 4K RPM), then that proves the Power Valve/VIAS control solenoid is working properly. HOWEVER, this STILL does not guarantee proper operation of the VIAS system, as the butterfly valves embedded inside the intake plenum could be internally broken, rendering the action of the Power Valve useless. The only way to truly prove this is to run yours against another (similar spec) Maxima which is known to work properly.

 

Thanks a bunch Spirilis for your in-depth response to my orginal question! I'll be trying your tests tomorrow.

Bill

 

I think this may have more to do with the swirl control valve rather than the VIAS. The swirl control valve is present on cali-spec 2000 and all 2001 maximas. It is closed below 3200 rpm and when idling. It makes for efficient fuel consumption and produces a swirl in the combustion chamber. At 3600 rpm, this valve opens to provide less air intake resistance and more power.

From the sounds of it, the valve may not be opening smoothly once you hit 3500 rpm. I would check all vacuum hoses and the check valve located near the driver side strut tower. The control solenoid is actually right next to the VIAS control solenoid as shown in the pic... it is the brown connector. If you have no MIL, then the actuator system should be working and the valves themselves may be a little sticky.

I actually noticed the same problem as you recently. This may sound bad, but I actually revved the engine from 3000-4000 rpm a few times in 2nd and 3rd gear and noticed that the slight bog that I had went away. Maybe the valves had some build up on them If the valves were stuck shut, I think you would notice it as the power loss you would experience would be significant.