Hi folks, I recently replaced a bunch of stuff on my Maxima (upper & lower intake gasket, injectors, knock sensor, clean EGR stuff, few other things) in an attempt to fix a stumbling idle when the engine was warm. All that work did clear my knock sensor code but did nothing to help the stumbling idle.

Trying to narrow things down, I went to the junkyard and bought a used IAC valve assembly, and throttle position sensor and swapped them out. The stumbling idle at warm is still there though less noticeable. However, now, I have a different issue. The car doesn't do it's usual high-idle during a cold start. If it's 32 degrees or below, I have to hold the gas pedal to a high idle (1500rpm or so) for a few minutes before it will idle on it's own, and then it drops to 500rpm or below. It wont start on its own unless I hold the throttle down a bit. I did the throttle position sensor adjustment with a multimeter (it's set correctly), is there anything else I need to do? I have to assume that the junkyard IAC is okay as it idles fine once warmed up, except for that same stumble I had before.

Can someone explain to me exactly how the computer / sensors / IAC controls the idle? That would sure help me in my troubleshooting.

 

For the low idle on startup, check your cold idle cam. A small plunger connected to the throttle body which holds the throttle plate open when cold, and as the engine warms up, the coolant passing through the plunger extends the arm closing the throttle.




To check if your IACV is working properly, after the engine has been sufficiently warmed up and then shut off, disconnect the TPS and start the car. You may have to turn the IACV screw counter clockwise a lot to help maintain the idle, and just slightly baby the accelerator until the car can hold its own idle. Then with no lights or any accessories running, adjust idle in Park for and Auto is 700 +- 50 and Manual is 650 +- 50 .

 

Go to the junkyard and get a few fuel injectors (super easy). I had the same stumbling idle and it was one of the fuel injectors closest to the radiator. The rear ones take a little more work. Be careful as its under some pressure. I would put back your original IACV after cleaning it really well.