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Idle Control: Theory of Operation
Since there's always someone with an idle problem I figured it best to post some info on how the engine is supposed to control the idle to aid in diagnosing an actual problem.
First thing's first. The "Gas" pedal should be called the "Air" pedal now because depressing it only lets in air. The computer now controls all the fuel.
The throttle body is the single point in which air is meant to enter the engine. The throttle body has typically two points where it enters. The throttle body plate opening and the Idle Air Control bypass opening. The Idle Air Control motor has numerous names depending on manufacturer. IAC, AIS, Idle motor, and many more. They all work the same way. They open and close a passage which allows air to bypass the throttle plate.
When operating properly, the IAC motor will open all the way at Wide Open Throttle and partially at idle, part throttle, and when an extra engine load is detected such as air conditioning clutch cycling on, power steering pump input or when an electrical load is detected (on vehicles that have clutchs on altenators/generators).
When the IAC motor opens at WOT, it is not really trying to get more air in. It's object is to stay open when the throttle plate is suddenly shut. This will still allow sufficient air flow to keep the engine running. Were it closed as quickly as the throttle shut then the car would fall flat on it's face and stall out.
When the car is at idle, the motor controls engine RPM by making small movements to increase and decrease airflow bypassing the throttle plate. The added engine load of an a/c compressor cycling on without the assistance of an IAC movement would cause RPM to drop and possibly stall the car as well. Likewise at part throttle, the IAC motor opens enough to maintain airflow when the TB plate is shut again.
The throttle plate is designed to allow a small amount of air to slip between the plate and the housing. This maintains a specific amount of airflow that is not designed to change unlike the IAC path.
The throttle body and intake often get very dirty. This is caused by a number of things which include small dirt particals that the air filter cannot filter out, oil residue and crank-case fumes from the PCV system, carbon from the EGR system and many other contaminants. As the car is driven more and more the intake and throttle body begin to build up deposits on their surfaces. In the intake the problem is not noticeable accept on highly tuned vehicles and flow bench readings. However, the build-up in the throttle body can be very obvious. Basically, this build-up will coat the walls of the TB where the plate meets the housing and also in the IAC bypass and on the IAC motor and tip itself.
This build-up will eventually start to strangle the engine. It's not noticeable at first for many reasons. The build up is only minimal. Air flow is restricted at the throttle plate opening and at the IAC path. The PCM/ECU sees that there's less air flow and so it commands the IAC motor to open a bit more. Everything's fine now for many more miles. Then again, the build up is too much and the IAC motor is commanded to open further. This continues on and on until the IAC motor can no longer open far enough to maintain enough air flow to the engine and it basically is strangled until it stalls.
Symptom: stalling out when letting off accelerator
Possible cause: dirty throttle body
Possible solution: removed TB and clean with carb cleaner
Also during the course of an IAC motor's life, it is opened and closed against the throttle body housing so much that it eventually developes spot where it unevenly seats. The Idle motor is basically a cylinder with a piston that moves in and out in a straight line. At the end of it is a pointed tip. This tip pushes in to a hole to close off air flow. If the tip does not seat properly, then the computer will continue to open and close the motor trying to find a position that will maintain the idle it thinks is necessary for a closed throttle plate. This is called "Hunting" when the idle surges up and down trying to find the right RPM.
Symptom: Idle surge
Possible cause: AIC motor not seating properly
Possible solution: Remove AIC motor and check for uneven wear marks,
replace as necessary
One of my biggest complaints with many suggestions made about an idle problem is that it normally involves adjusting the Base Idle Set Screw (BISS). This frequently will make the symptoms dissappear for a short time but problem still remains and will soon show it's face again. If adjusting the BISS makes your problem go away and the BISS has NEVER been adjusted before then chances are high that your problem is either a dirty throttle body or a mis-adjusted throttle position sensor (420A TPS is not adjustable). Adjusting the BISS should only be done when the TPS is KNOWN to be set correctly and the throttle body is KNOWN to be perfectly clean. It can also be adjusted to compensate for idle lope due to after-market cams with an overlap that may cause stalling out at idle.
An air pocket in the coolant system may also cause an idle surge. On some vehicles, there is a coolant passage in the throttle body. The TB is frequently the highest point in the coolant system and an air pocket may form there. This air pocket may also cause an idle surge so make sure you've properly bled the air from the coolant system before addressing any idle concerns.
Doug
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