QuakerJ

I have a '96 Maxima GLE with 285k and I'm having trouble with the idle-- it seems to be all over the place. When I start the car with engine cold, I'll have to hold the gas a bit for the first 30 seconds or so, or it will immediately die. When warmed up it's a 50/50 chance whether it'll idle at 400rpm or 1100. If it decides to idle low, it will sometimes stall and I'll have to feather the gas a bit at stoplights to keep vibration in the car down, or shift into neutral. If it idles high, well it's annoying but otherwise just sucks more gas than necessary. I've tried replacing the idle air control valve, admittedly with a junkyard unit, but no change. I've also tried toying with the idle screw mounted on the IAC assembly, which cures the condition for a moment (I can dial in the correct RPM), but goes back to its randomness very quickly. I have a code stored in the ECU that's related to idle control.

I'm really curious how the idle is controlled, so I can diagnose this. Most fuel injected cars I'm familiar with have a very simple system-- an electric idle air control valve that changes how much air is let in based in ECU inputs (coolant temp, RPM, A/C operation, etc.) Very simple. Doesn't seem so simple on the VQ. Pretty sure I counted three or four gizmos mounted to the idle air control valve assembly. What gives? Can someone give me a rundown of how the idle system operates and/or things to check?

JvG

Idle is controlled by the Idle Air Control Valve.

It's located next to the throttle body.

You might as well start by cleaning your throttle body and throttle plate. I used CRC Throttle Body Spray.

If that does not solve your problem it's time to remove the thing next to it its the IACV. also known as the Idle Air Control Valve.

Take it apart. Clean everything with spray carburetor cleaner. Then reassemble.
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QuakerJ

I had the throttle body and IACV apart last weekend working on it in an effort to fix this issue. Both were cleaned thoroughly with carb cleaner. They really weren't all that dirty to begin with. Still having the same issues.

I count at least three electrical connectors on the assembly, so there must be more to it than just the valve itself, which is why I inquired about its operation. There's also a gizmo on the throttle body itself that looks to be controlled by coolant temperature. It looks to change the position of the throttle blade in response to coolant temperature. Not sure this would cause my issues though, which occur at all engine temperatures.

The Wizard

iTrader: (43)

You may have a vacuum leak. Check for cracked/split vacuum hoses and/or gaskets.

JSutter

iTrader: (46)

The bottom stuff on the TB is mechanical cold idle. When the coolant warms, the linkage moves the TB plate to compensate for a warm engine.

The IACV has purple and pink solenoids to adjust for air-conditioning or power steering loads.

You can't adjust the black plastic idle screw without following the steps outlined in the service manual.

The Wizard

iTrader: (43)

Gotta use the FSM, very detailed procedure.

DizzyEdge

Do you find any difference between starting the car in the morning vs starting it during the warmest part of the day?

QuakerJ

First thing in the morning is usually most troublesome, but even in the afternoon after the car has been sitting for some time, I still have to press on the gas for it to start. When the engine is fully warm as in going to the grocery store and back out, most times I can start it without any throttle input.

The idle seems to have a mind of its own; in similar conditions sometimes it will idle at 500rpm in gear, other times 1200, and other times so low that it will stall. I've checked for vacuum leaks, and everything seems to be sealed as far as the intake tract and manifold goes.

QuakerJ

Just for grins I tried unplugging the IAC valve itself (not the power steering / AC solenoids) and the idle is stable now. It runs about 900rpm in park, about 650rpm in gear. Drove it 70 miles today and it hasn't budged or gone up and down on a whim like it used to. I guess I'll try a new IAC valve, is there any relearn procedure or such that I need to perform after replacement?