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Idle valves

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I honestly can't remember if i already asked this... so im gonna potentially ask again.

1. Whats the operation strategy of oem IAC's (ISC's)?. As in... does the air valve fully open, fully close or stay where it was-just-before-rpm-rose-above-IAC-activtion-threshold, when you go WOT?. And what about on overrun too?

2. My corolla has developed an idle that is too low. How could the IAC/ISC allow this to happen?. When i put my parking lights on... it bumps the idle up just a tad. Closer to what it should be. As soon as i turn the lights off... the idle drops too low again. Why is this happening?

I can only imagine that the operating strategy for an OE idle control valve would likely vary from one manufacturer to the next. Some engines also have two idle control circuits as such. One is a conventional ECU controlled ISC while the other is an air bypass purely controlled by coolant temperature. These are often referred to as 'wax pellet' idle speed control.

A common issue with ISC operation, particularly on older engines, is the ISC becoming partially clogged up with carbon. This can jam or limit the movement of the valve.

Why do some have 2 separate circuits?. Why not just use the one and simply target a higher rpm/valve-opening at lower coolant temps?.

I think my 2003 Corolla might have the "wax pellet" type as well... because there are coolant lines running in and out of my throttle body. But i always thought this was to stop ice from forming and seizing up the throttle plate on really cold days.

People have suggested that its simply a caked up IAC. But that doesn't make sense because when i just turn the parkers on, the idle goes up a tad. The idle is clearly below what it should be... shouldn't it throw a fault code?. Idle control is closed loop isn't it?.

Will a scan tool tell me what the TARGET idle-rpm is supposed to be?

On Toyota's i usually see a throttle butterfly zero position adjustment nut. If you have it try to open a butterfly a little bit more while idling by rotating that nut. I had the same problem on 4EFTE.

Target idle RPM on stock 4 cylinder Toyota's is between 750-800 RPM

You're not supposed to touch the throttle stop screw. Its set from the factory.

Some IAC valves are simply stepper motors (GM used these quite extensively and you also find them in LPG installations). All they do is change the amount of steps they have to suit the conditions and to achieve the desired idle speed. Unless the engine is stopped the stepper motor is always open to a certain degree.

All IAC valves can get mucky but alot depends on the condition of the engine and where the crankcase is ventilated into the intake stream. Ford 6 cylinders have a service item on throttle bodies because the get muck in them and they stick (not a nice thing when the throttle is drive by wire) so they must be cleaned at regular intervals, Holden VK Commodores (1984-1985) with the EFI engine also had to have the TB taken off and cleaned at regular intervals because they developed problems, so I can't see IAC valves being immune to the same issues. If I were you I'd take it out give it a spray (or replace it) with carb clean and clean out the passages it is connected to as well.

If its stuck why wouldn't it throw a fault code?.... and also, how could the idle slightly rise when i turn the parking lights on?

Marek it depends on how smart the ECU is. On a current generation OE ECU then yes I'd expect a fault code if the idle circuit wasn't functioning as expected. On older models though a lot of faults weren't reported.

Is that two questions or one Marek? I'll treat is as 2.

Andre is spot on with the answer about fault codes. You don't tell us what model Corolla you have so without knowing more specific information the answers you 'll get will always be generic.

On many OEM vehicles anytime you put a load on the engine the idle is supposed to rise. Turning the parkers on creates a load through the electrical system. The valve could be sticky or there could be a restriction in the air passages. If it is sticky it may not be sticking throughout its travel it may only have the problem at the location where it sits at idle, I had a similar problem with the IAC on my Holden V8. If there is a restriction in the passages or the seat of the valve you will loose air flow and therefore have a lower idle (this is why Holden used to have the service item on the VKs), turn the lights on the ECU recognises load and it opens the valve to compensate so you get your slight increase in idle.

All this is just conjecture, until you pull it out and check it and the passages it is connected to we wont know what is going on.

I already said its 2003, Michael.

Andre, we don't know if its a circuitry problem... but we definitely know its not staying at *target* rpm. That fact its not reaching a target value in closed loop mode... it should throw a code. Or maybe its still within its "tolerance" range?. Whats the tolerance usually... like +/- 50rpm??

So you did, my bad.

Do you know, for sure, what type it is yet?

Ok... so lets say i clean it out real good... and it still does the same thing. What wold be the problem then?.

Is it groundhog day?

Without knowing what type it is, and also knowing how each type works (I myself only have experience with stepper motors), the only answers you will get are going to be generic. If a stepper motor is cleaned up "real good" and the passages are "cleaned out real good" and nothing is sticking then the motor is not getting the appropriate signal to suit the conditions. What could be causing that is another matter.

What exactly have you checked? If you haven't checked anything I think you're at a stage where you need to start. We are now in a long weekend (unless your in WA), if I were you I'd give your TB (and ISCV which is bolted to the TB) a service. Let us know how you go.

I'll get onto it post-haste!!

... or not.

From my experience with toyotas especially 4 cylinders of those year models if the IAC is dirty it will give you the exact symptoms of low idle or stumble. No it will not throw a code in that case. The older ecu is not smart enough. Say you have a vaccum leak it wouldnt throw a code for idle higher than expected as well. Newer ecu with DBW will throw a code for IDLE lower than expected or vice versa. Clean it up with some carb cleaner and you should be good to go.