Understanding Idle Control Circuit

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Re 1999 Avalon V6

Many articles and DIY videos say you can eliminate low and/or rough idle by cleaning the IAC valve and throttle body. Why does this work? The ECU is always comparing sensed rpms to target values in the look-up table. If rpm is too low, the ECU changes IAC duty cycle to get the correct target speed. If the IAC passages are restricted, wouldn’t the ECU simply open the IAC to compensate? I am not talking about a sticking IAC valve which causes abrupt rpm changes or a no cold start because it can not open at all. The ECU can not compensate for this condition.
 
Dirt in the IAC makes it sticky.

If the passages are restricted / clogged, the ECU will command full open but it still wouldn't flow enough air.

The ECU also uses spark timing to control idle speed. This has a faster response than the air valve but limited range.
 
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Originally Posted By: mk378
Dirt in the IAC makes it sticky.

If the passages are restricted / clogged, the ECU will command full open but it still wouldn't flow enough air.

The ECU also uses spark timing to control idle speed. This has a faster response than the air valve but limited range.


Right. As I posted, I am not talking about a sticky valve or totally clogged port. This is a failure state. I am talking about minor fouling which might accumulate over a few months or even a year. I am interested in this case because several articles talk about routine cleaning to improve idle. It made me wonder why on earth you need to clean your IAC valve and throttle body every month?

Re spark timing, would a failing crank position failure cause a low/rough idle?
 
Last edited:
Originally Posted By: artbuc
Originally Posted By: mk378
Dirt in the IAC makes it sticky.

If the passages are restricted / clogged, the ECU will command full open but it still wouldn't flow enough air.

The ECU also uses spark timing to control idle speed. This has a faster response than the air valve but limited range.


Right. As I posted, I am not talking about a sticky valve or totally clogged port. This is a failure state. I am talking about minor fouling which might accumulate over a few months or even a year. I am interested in this case because several articles talk about routine cleaning to improve idle. It made me wonder why on earth you need to clean your IAC valve and throttle body every month?

Re spark timing, would a failing crank position failure cause a low/rough idle?



You don't need to clean your IAC every month.Who told you this? eWhat kind of car do you have?
I find replacing it is better than cleaning it, replaced mine about 5 yr's ago and it still working fine.
Bad crank sensor can make it run rough,but will make your check engine light come on.
 
Quite a few articles suggest routine TB & IAC valve cleaning as good PM. It strikes me as being totally unnecessary unless you enter a failure mode such as cold no start.

I have read that a failing crank sensor can cause poor idle before it gets bad enough to throw a code. Don’t know if that is true or not.
 
Current cars typically are throttle-by-wire with idle airflow integrated into the regular throttle control.

In the 1990s it was common, even with the early throttle-by-wire, to use a separate idle airflow valve. At idle these gate most of the airflow, including the crankcase ventilation gases. The small cross section results in lots of gunk condensing/settling on the valve. Oil mist is not a problem, but combustion blow-by is quite unpredictable. Sometimes it's like tar, and gums up the valve.

For the Bosch ones I'm familiar with, solvent cleaning is just as effective as buying a new one. Mechanical cleaning risks damaging the soft aluminum which leads to further jamming.
 
IAC cleaning is helpful IF it's gunked up, but is not needed often at all. if the idle is rough and nothing else seems to work, consider a stretched timing belt. I replaced one years ago that cleaned up a rough idle which threw an occassional misfire code as it progressed.
 
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