How so? If the idle count is 100, what does it mean? It tells me that something is wrong and the IAC needs to open up a lot to get the right amount of air. So something in the passages is clogged or something is not running right because the throttle blades are not open enough. If the IAC counts are 0, it tells me that somehow, we have too much air going in and the IAC is out of compensation and it has to be done via other means like fuel or timing. To me, that means there is a leak somewhere because your gasket is torn, IAC housing is leaking air or a vacuum leak is present.

I have set mine to 20 counts with all the load off the engine. AC, radio, etc, etc. That way, if there is load, the IAC has room to open up the pintle and let more air in. At 20 counts, there is room for the IAC to shut down some air but it is unlikely since you will run the car with some sort of load.

 

No need to split hairs but there's a range for a reason (wear and tear being the big one) - 20 works for your car - it may be different on another. As a stand alone, there's nothing in any diagnostic procedure which makes the data useful. Yeah, 100 counts might be unusal, but only if something else was out of whack. You need to look at the big picture - Targeted vs Actual, Sensor Values, etc and the Trouble Trees follow that logic.

 

Thanks for all of the responses..sincerely appreciate your help, I feel better informed for when the car shows up next week. It did need that throttle body cleaning anyway, it was really gummed up inside.

The owner called today and relayed that the car is running great...but idling fast. Apparently she needs to maintain significant pressure on the brake pedal at lights or it idles away, shifting from reverse to drive gives a distinct engagement.

My C3 has a snubber on the rear differential to cushion the forward/reverse gear changes.. do the C4 units have a similar snubber?

Thanks

 

That could force the ECM to compensate.

Did you scan it for the idle speed? What we need to see is what it is idling at vs what the ECM is demanding for an idle speed. If the chip is told to idle at 900, that is what it will attempt to achieve. My idle speed is not stock but I have a programmed chip.

I thought the rubber donut is for balance? Some cars like MBs have a rubber donut that has 2 halves of the driveshaft connected to it to cushion the shock.

 

If the IAC counts are too high (i.e., significantly above 15-20 counts at operating temp w/o A/C or fans on), then the motor isn't getting enough air thru the TB to achieve present, programmed idle. By opening the TB blades further (and reseting the TPS), your "dashpot" operation won't be exagerated with fast idle too high -- taking longer to drop.

If the IAC counts are too low, the TB is open too far and/or theirs a vacuum leak. I would expect to this condition to compromise the "dashpot" operation. The car is likely to die more often too.

Notice the use of the word significantly. There is, as mentioned, room for aging and minor flaws. But, the further off the IAC is, the worse idle performance will be. To give you an example, my new motor had an initial setting of about 70 steps for the IAC. It wasn't getting enough air at idle. The dashpot operation was too pronounced. I hated the way it idled, pulled away from idle, and dropped back to idle. It wasn't until I increased the idle set screw (to reposition the IAC back to factory settings) before the motor idled/performed correctly.

 

All I know is that the OEM's designed our cars to account for wear and different operating conditions as well as variances in engine build tolerances AND they didn't have much choice given Federal and State Mandates for Emissions (which was meet this and continue to meet it for a set number of Years without any service or we're going to make you take it off the road until you fix it) to say nothing of buyers that demand a smooth idle. The ECM was programmed to look at whatever is Targeted as an Idle number and that's based on Load (generally a/c and gear position - AUTO) and the Coolant Temp Signal. It then moves the IAC pintle to match that Target. How much that pintle needs to move is influenced by wear - an oil film is left in most TB's from the PCV system everytime it's shut down which can cause a reduction in air flow - and the density of the air which is influenced by humidity and air temp though the latter is one of the reasons there's coolant flowing through the throttle body - so it's not a fixed, programmed number. Using IAC counts as a means of setting idle or as a diagnostic tool, defeats what was designed into it; ie, a means of overcoming the many variables that can influence idle performance and what you find on one day, at one temperature, at one altitude, etc etc etc, might be wrong for the next. If there's any maintenance required - and the Dealers have been soaking customers for this for Years - it's to occassionally clean up the oil film that's accumulating in the throttle body, though the Law requires that the buildup, during the Emissions warranty period (which is 5 years/50K out here and all our cars have the same stuff), not influence Emissions performance, so nothing should really be necessary, certainly not on your dime, for a fairly significant time. If you've got idle issues, you consider what makes it work and then figure out why it's doing what it shouldn't be doing, but monkeying with that screw is a good way to screw it up. And I don't know why L98 guys fiddle with it so much, it never seems to be an issue with the LT owners whom seemingly know to leave it alone.