Idle Control: Theory of Operation

Posted by Doug99RS, May 15, 2003
Articles: Engine & Fuel -

  1. Doug99RS

    Doug99RS DSM N/T Wiseman

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    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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  2. Doug99RS

    Doug99RS DSM N/T Wiseman

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    OK, there's been a number of help needed threads lately so I thought I'd add some more tech information that no one will ever read.

    Some of the questions relate to the base idle set screw (BISS) and the throttle position sensor (TPS) so I grabbed a throttle body (TB), a multimeter (DVOM) and a camera and wasted my evening being a nerd.

    This picture shows a TB with the throttle plate shut and what the resistance reading through the TPS sensor is for that given position:
    <a href="http://s306.photobucket.com/albums/nn280/Doug95neon/multimeter/?action=view&current=100_1291.jpg" target="_blank"><img src="http://i306.photobucket.com/albums/nn280/Doug95neon/multimeter/100_1291.jpg" border="0" alt="Photobucket"></a>
    The resistance is 4.68K ohms.

    This next picture shows a partially open TB with a resistance of 4.31 K ohms:
    <a href="http://s306.photobucket.com/albums/nn280/Doug95neon/multimeter/?action=view&current=100_1294.jpg" target="_blank"><img src="http://i306.photobucket.com/albums/nn280/Doug95neon/multimeter/100_1294.jpg" border="0" alt="Photobucket"></a>
    Notice how the cell phone magically appears now. This is caused by an increase of airflow which is sucking more items in to the TB.


    The next picture shows the resistance to be even lower (3.475 K Ohms) as the throttle plate is around 50% +/- open.
    <a href="http://s306.photobucket.com/albums/nn280/Doug95neon/multimeter/?action=view&current=100_1295.jpg" target="_blank"><img src="http://i306.photobucket.com/albums/nn280/Doug95neon/multimeter/100_1295.jpg" border="0" alt="Photobucket"></a>


    The last picture shows the TB plate fully open. Resistance has dropped to it's lowest point of 1.583 K Ohms.

    <a href="http://s306.photobucket.com/albums/nn280/Doug95neon/multimeter/?action=view&current=100_1296.jpg" target="_blank"><img src="http://i306.photobucket.com/albums/nn280/Doug95neon/multimeter/100_1296.jpg" border="0" alt="Photobucket"></a>

    What does all this mean? Well, the computer sends to the TPS sensor a voltage. Usually 5-8 volts. The resistance inside the sensor dictates how much of that voltage flows through the sensor and returns to the ECU/PCM. So at rest resistance is greatest and the TPS signal is going to be low. As you open the throttle plate the resistance decreases and the signal voltage will increase.

    What does all this mean? Well, if you read many of the posts I had before this one, in various other threads, then you'll now be able to understand the value of a multimeter when you're trying to see if your TPS (or any sensor) is bad. Additionally, you'll be able to TUNE your BISS and TPS sensors with the information above and below.

    Understanding the adjustments on your Throttle body:

    Many people see something they can manipulate and they want to do so without understanding the effects. The BISS and TPS are two of those items and messing the them without knowing what you're doing can cause poor idle, poor part throttle performance, poor fuel economy, a hunting idle or other strange problems.

    On the turbo and 2.4L models (according to the 99FSM) the resistance between pins 1 and 4 should be between 3.5 to 6.5 K Ohms. To check the sensor's range of operation like the pictures I just posted above you move your test lead to pins 2 and 4. As you increase throttle position the resistance should change proportionally and smoothly. Open the plate very slowly as you will increase your chances of finding a glitch if there is one. "If the resistance is outside the standard value, or if it doesn't change smoothly, replace the throttle position sensor. NOTE: Always adjust the TPS after replacement."

    There is a second portion of the throttle position sensor called the "Closed throttle position switch". To check this put your leads on pin 3 and pin 4. With the throttle plate open you should have no continuity (Should show up a 0.L meaning open loop or infinite resistance). With the throttle plate shut you should show 0.00 ohms of resistance.

    It's important to check this circuit when you're adjusting the BISS and TPS. The TPS sensor has a wide range of resistance BUT the computer typically only has 4 different modes of throttle range each with their own purpose.

    Cruise/idle mode for little to no throttle input, acceleration mode for light to moderate acceleration, Deceleration which the ECU/PCM reduces or shuts off injectors, and WOT for heavy or all out acceleration.

    If you have an improperly adjusted TPS sensor then you may be allowing in enough air for cruising but the signal from the TPS puts the ECU just beyond cruise and in to the acceleration mode. Likewise if you're sitting at idle and the TPS is adjusted too far then it may think you're trying to accelerate when you're really not thereby wasting gas. Also, if it adds fuel for an acceleration but you're just idling it's going to keep fighting o2 sensor feedback and TPS input.

    There's several problems with having so many adjustments and they frequently show up when the car is in the hands of someone not fully versed in the nature of the beast. Things like vacuum leaks, improperly adjusted throttle cables and dirty throttle bodies typically get overlooked because they are never mentioned by the guy with a crystal ball helping you "Fix" your car.

    A dirty throttle body like this one:
    <a href="http://s306.photobucket.com/albums/nn280/Doug95neon/multimeter/?action=view&current=100_1304.jpg" target="_blank"><img src="http://i306.photobucket.com/albums/nn280/Doug95neon/multimeter/100_1304.jpg" border="0" alt="Photobucket"></a>


    Is basically starving your engine for air. The carbon, oil, and dirty build up is blocking airflow required to maintain your idle quality. This is a big reason why people start messing with the BISS in the first place. They don't clean the throttle plate, it starts lowering the idle as it gets dirtier and because some Ass Hat on the web said "Twist this cool screw" first people create a bigger problem when they finally start learning how to do things right.

    A cleaner throttle body like this one:
    <a href="http://s306.photobucket.com/albums/nn280/Doug95neon/multimeter/?action=view&current=100_1307.jpg" target="_blank"><img src="http://i306.photobucket.com/albums/nn280/Doug95neon/multimeter/100_1307.jpg" border="0" alt="Photobucket"></a>
    Will improve air flow thus improving idle quality.

    This is a Base Idle Set Screw on a 420A TB:
    <a href="http://s306.photobucket.com/albums/nn280/Doug95neon/multimeter/?action=view&current=100_1299.jpg" target="_blank"><img src="http://i306.photobucket.com/albums/nn280/Doug95neon/multimeter/100_1299.jpg" border="0" alt="Photobucket"></a>

    While the TPS on the 420A is fixed and non-adjustable it's still possible to create a problem with it. When you have a dirty throttle body and you twist on the BISS not only do you increase airflow to bring idle back up but you change the throttle position sensor signal too. Now, every time you crank the car, you're starting out at a higher TPS signal then you should.

    With an adjustable TPS sensor it's possible to increase airflow while changing the TPS sensor to show that it's even more closed than it was before you messed with the BISS. What this does is create a condition where the manifold pressure sensor sees more airflow than what the TPS is reporting it should have. It really gets crazy when you have so many adjustments and you don't know which is further out of adjustment.
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  3. Doug99RS

    Doug99RS DSM N/T Wiseman

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    If anyone would like to part with a 4g63 or 4g64 throttle body that has the BISS, IAC and TPS still in place and functional then I'll do a write up using that TB as well. If you've got a spare one laying around I'd appreciate it. PM me please.
    Thanks,
    Doug
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