5ktq high 'idle' - another update - getting there!

[E. Roy Wendell IV]

>
> I've been following the tips and advice I've gotten on the list and  
> I am
> getting there.
>
> Looks like I have more than 1 problem - but at least 1 is now fixed.
> I swapped the Idle Volume Control relay (3 relays wide, under the  
> dash) and
> made an immediate and noticeable difference.
> Idle is now consistently at 1500 rpm or less. (as opposed to the 3k  
> rpm
> before)
> Idle does fluctuate somewhat.  I can really tell when it is being
> 'controlled' by the ISV system
>
> So idle is still a bit high and the engine is either running very  
> cold, or
> something is wrong with the temp gauge.  The MFTS is brand new 3  
> wire one
> but gauge barely moves upward.
> going to replace thermostat and check gauge next couple nights.  I'm
> guessing the high 'idle' is because the ECU thinks the engine is  
> colder than
> it really is and is therefore giving some fuel enrichment???   
> hopefully the
> idle fluctuating is related.
>
> Thanks again to everyone for your pointers and advice.

You're on the right track but your thinking is a little off. The idle  
controller neither knows nor has any effect on the fuel mixture. It  
will vary the amount of air getting into the engine in an attempt to  
maintain 800 rpm. If the mixture was off enough to cause the engine  
to misfire the idle controller would do it's best to compensate by  
changing the airflow. At idle speed the engine is just barely running  
and very small changes in airflow cause large changes in engine  
speed. That is to say that idle speed is primary determined by  
airflow. Engine temperature related changes in fuel mixture are  
controlled by the warm up regulator that is part of the mechanical  
fuel injection system. The one wire temperature sensor near the water  
inlet that sends a signal to the main ECU is used only to trigger the  
cold start injector.

The inputs to the idle speed controller are idle switch position,  
rpm, and temperature via the other one wire temp sender on the water  
inlet. It's only outputs are a variable duty cycle signal to the ISV  
and an on/off signal to the decel valve.

You are correct in thinking that a cold running engine may be causing  
your high idle. If the engine is running cold enough the idle speed  
controller will mistakenly think that the engine is still warming up  
and set the idle higher than normal.

You may also still have an incorrectly set idle speed screw. Because  
the idle speed control is closed loop the screw will seem to have a  
"dead" range where it doesn't seem to have any effect. This is  
because once you close the idle screw enough to where the valve can  
control the idle it will then compensate for further adjustment as it  
varies its airflow to offset what you are doing with the screw. It  
will do this until you close the screw to the point where the valve  
is running full open and can no longer compensate. At this point idle  
will drop below 800 rpm. The happy spot for the screw is somewhere in  
between. You have to set the idle screw such that the ISV is neither  
full closed nor full open all the time and can instead change its  
opening and vary airflow in order to maintain rpm. You can just drive  
the car and fiddle with the screw until you find the position that  
allows proper warmup engine speed as well as correct warm idle or you  
can do it correctly by hooking up a meter that shows duty cycle and  
adjusting per specs. Refer back to the SJM site for the info.

Roy Wendell
erwendell at mac.com
Too many type 44 tq
A pair of MR2s