Today,my 96 started to idle really,really low,when it warmed up,it did not stall,but when driving and slowing down at a stop light,it idles way down low and the I could feel the engine acting as if it was starving for air or something..the vibration etc.It was almost at stall but did not.There is no check engine light.

The car has been flawless the last few weeks,driving perfect,plenty of power and etc.It was idling perfect until today.

My past experience with GM TPI and MPFI,the idle air control can sometimes be the culprit.If the TB or IAC is dirty,it can cause this.Im assuming alot of pfi engines are similar.

I did not find a vacuum leak and I dont think there is one,since most of the time,a Vac leaks causes a erratic idle,which usually is very high or goes up and down,hunting.This car idles perfect then gets lower and lower...

Since I dont have a scanner,and cannot see the IAC counts at idle,what would you suggest I start with?Cleaning?

What usually causes a very low idle to happen on these engines?

The engine does not miss or sputter,not stall like an egr is hanging up...its idling very low and almost stalling,but isnt.

Thanks for any ideas.

 

Make sure the throttle cable goes slack at least a little bit.
Make sure the throttle plate closes. The mechanical stop should only just prevent it from wedging stuck closed.

Then try setting the base idle.

1 Warm up the engine completely & turn off ALL electric & accessory loads.

2 Unplug the IACV - nurse the throttle to keep it running.

3 Use the idle air bleed screw (needle valve above throttle bore) to set 550 rpm.

4 Turn it off, cool it down, plug IACV back in, reset ECU (pull battery cable).

5 Start it without touching the gas pedal.

6 Let it warm up without touching the gas pedal.

 

Thanks Jim!

I will take a closer look this weekend at it all.Dont have the time right now...that said,could a cracked exhaust manifold cause this?

In my experience,it usually allows false air into the exhaust,and the 02 down stream reads more air then whats really in there,makes the engine ruch a bit richer,but I am not sure if the ecm is lowering idle speed due or injector pulse to that,sensing more air than is normal...dang I need to get a new scanner soon!

The reason I ask,Is I took the shield off in the engine bay and there is a 2 inch long crack in the exhaust manifold.It was making a little noise so I had to take a closer look.

That said,the car is running fine today...wasnt dropping down to low rpms like the other night,so im not going to try and fix it unless it returns again...it prob will tho.I do not want to adust the minimum air speed screw and etc if a possible crack can cause the idle problems.

Thanks for any info.

 

Could be... but it helps to understand the mechanism.
O2 sensors really measure oxygen DIFFERENCE from inside to outside. So exhaust blowing onto the outside of the sensor will throw it off.