prlj91

Like the title says, my GT's idle bounces back and forth when in park, and also while in gear stopped at a stop light. Here's a video I uploaded to help you see what I'm talking about (please ignore the oil pressure reading, as I have a malfunctioning sending unit)

It doesn't bounce much, as you can see in the video, but it's enough to be really annoying, and it's not supposed to happen. I don't notice any loss in performance or trouble driving, and it never dips low enough to die or make the engine shudder.

I've tried everything. I cleaned the MAF, IAC, sprayed almost a full can of brake cleaner around all the vacuum lines hoping to maybe uncover a vacuum leak. I've changed the fuel filter, run endless gallons of injector cleaner, but nothing. There are no codes or anything popping up. It does it regardless of outside temp, and engine temp, though it is a little worse when the engine is up to temp rather than cold. It does it regardless of the AC being on or off, and I'm at my wits end.

All that being said, anyone have a clue what it could be?

8111870a

Change the IAC motor. Cleaning doesnt always work, when it does its usually a temporary fix.
Use OEM part.

prlj91

I'll try that too, whenever I get the chance.

Stevo86

How old are the 02 sensors. As they wear out they begin to switch slowly which leads to bigger swings in fuel so you can get kind of a surging effect.

They also start to fail lean by virtue of how they work which means you may already be running a little fat if they're old.

Thatsweetstang

I'm leaning towards IAC, I've never successfully cleaned an iac and all was well. They crap out a lot. I would put a new one in, unplug the battery for a bit, then fire it up and see what happens. If you don't have any vacuum leaks I would start with a new IAC.

tadpole1969

get a new IAC and problem solved

prlj91

The O2 sensors are as old as the car, so maybe that's what's up. Lots of people say replace the IAC, so when I've got a spare Franklin sitting around, I'll go do that.

Stevo86

The only real function of the IAC is to hold the RPM after you let off the throttle and to allow extra air flow for the A/C and cold start. The base idle adjustment is still at the throttle plate which is why you sometimes need to adjust the aftermarket ones to get the car just right. You just use the IAC to tweak it.

If you want to check just unplug it, the car should stay running. If the idle smooths out you've found your culprit.

prlj91

Just did it twice. The car dies when I unplug the IAC.

cliffyk

The PCM's control of the IAC (and of ignition timing) have everything to do with the engine's idle speed, under all conditions--with proper adjustment of the throttle stop screw (not an idle adjustment in the classic sense)--being a critical part of the overall system.

Properly set the stop screw keeps the throttle cracked open just a bit to allow a small amount of air, insufficient to idle the engine, to bypass the throttle valve. The PCM then commands the IAC, by sending it a variable duty cycle square wave signal, to open enough to provide the additional air needed for the engine to idle. At a normal hot idle the IAC is held about 35% to 45% open by the PCM--here is what that signal looks like:



Because of this unplugging the IAC, causing it to close, with the engine idling and hot should cause the rpms drop or even cause the engine to stall.

If a higher/more powerful idle is needed the PCM opens the IAC a bit more, it can also close the IAC a bit if the idle is too high--it does this continuously and dynamically to keep the idle at the speed set in the tune. Adjusting the throttle stop screw will only affect idle speed if the butterfly is so open that engine speed cannot be controlled even with the IAC closed, or if the butterfly is closed such that opening the IAC as far as possible will not increase idle.

Here is how to properly set the stop screw, and a bit more about how the PCM uses the IAC to control idle.

Here is how to test the IAC...