Turbojack

I have pulled this from FAST's web site on Idle speed. Hopefully this will help people understand a little more. With the FAST system you have the option to make adjustment to the programing. As you will read the orginall throttle position of the throttle body & the position of the TPS is showing will make a difference. I know on my throttle body I can make about a 5% change by adjusting the TPS verses the throttle. The fast system will do things differently when it thinks the throttle plate is not in the idle position.


Idle Speed Control

This section applies to all Bank to Bank ECUs and Sequential ECUs with the Idle Air Control option.

Throttle Follower
The throttle follower is a 2D graph used for controlling the position of the Idle Air Control motor as a function of throttle position. The horizontal axis of the graph represents throttle position, and the vertical axis of the graph represents the targeted IAC position. The throttle follower graph is typically set so that the IAC position is very low at an idle and opens up more as the throttle opens up. When throttle position decreases, the IAC motor will close slowly. This helps prevent the engine from stalling when you lift off of the throttle with a substantial load on the engine.

Idle Speed vs. Coolant Temp
This is a 2D graph used for controlling idle speed as a function of engine coolant temperature. The horizontal axis of the graph represents engine coolant temperature, and the vertical axis of the graph represents the targeted idle speed.

Starting IAC Position vs. Coolant Temp
This is a 2D graph used for controlling the targeted IAC position when the ignition switch is turned on as a function of engine coolant temperature. The horizontal axis of the graph represents engine coolant temperature, and the vertical axis of the graph represents the targeted IAC position when the ignition switch is first turned on. Opening the IAC motor before starting the engine allows bypass air to enter the engine during cranking, eliminating the need to depress the throttle to get the engine started.

Timing Trim
This is a 2D graph used to vary ignition timing to help regulate idle speed. The horizontal axis of the graph represents the difference between the actual idle speed and the target idle speed, and the vertical axis of the graph represents the number of degrees to alter the ignition timing. If the idle speed is too high, the ECU will retard the timing by the amount specified in the graph. If the idle speed is too low, the ECU will advance the timing by the amount specified in the graph. It is recommended to set this entire table to zero as a starting point. Add timing trim only if you are otherwise unable to achieve a smooth, stable idle.

Idle Parameters
This is a form used for setting gain controls on the idle air control motor and for setting a threshold on the TPS position where the ECU no longer considers the engine to be idling. The IAC "P" gain value can be set from 0 to 255. Raising this value will cause the IAC motor to open or close further in an attempt to correct the idle speed. The IAC "D" gain value can also be set from 0 to 255. Raising this value will cause the IAC motor to open or close faster in an attempt to correct the idle speed. The default values of 32 for the P gain and 4 for the D gain are generally close to correct for most applications.

An excellent way to better understand these settings is to liken them to a sensitivity control on a cruise control circuit. If the circuit was set for maximum sensitivity (gain values both set to 255) the accelerator pedal would go to the floor as soon as you dropped below the target speed, and the pedal would drop to an idle as soon as you were going too fast. If the circuit was set for minimum sensitivity (gain values set to 0) the accelerator pedal would hardly move if you were going too fast or too slow, and you would almost never be traveling at the desired speed. This same concept applies to how your IAC motor will regulate idle speed.

The "Maximum TPS for Idle" field is used to set the threshold where the ECU no longer considers the engine to be idling. It is recommended to observe the TPS reading at an idle and setting this field to that number plus one. In other words, if your TPS reads 10% at an idle, set this value to 11. When the TPS percentage exceeds the value specified in this field, the IAC motor will be "locked" into the position specified in the throttle follower graph, and the Timing Trim graph will not be used.

tcelano

This is a great thread!

I have been fighting a similar problem on my 540 EFI motor using a FAST controller. Checking your actual TPS while at idle and making sure that the idle threshold is above it is crucial, otherwise the IAC doesn't ever try to do its job. I finally got mine to idle fine under load, but my next problem is a stumbly spot right off idle. Once through it, I'm fine, but I am suspecting I'm getting bitten by the throttle follower position being set too low. It's causing a small "dead zone" right as soon as I leave the idle mapping and before I get the throttle blades open enough to make up for the IAC going shut on me. If you get in a situation where there is too much load on the motor for the IAC to hold, and you advance the throttle into the "dead zone" you not only can't get enough air without really opening the throttle, but also have lost your automatic idle control. Solution is to adjust the mechanical limit stop to slightly crack the throttle blades, and increase the idle TPS threshold setting to accomodate it.

articfriends

Tom,are you sure that the motor your working on doesen't have a vacuum leak?Maybe it isn't even a electronic problem,possible intake gasket misalignment or a valve that isn't adjusted right or something similar. Just to rule it out i'd compare vacuum readings on both motors free idling,Smitty

aTX427

Does the HP500EFI have an IAC and TPS adjustment. I did not see any. I remember having a special mount on my Whipple setup to adjust but did not see the capability on the stock engines. Can you provide pictures of the adjusters?

Turbojack

tcelano
In you CComWP software
Go to View-Fuel Tables - AE fuel Vs TPS position, find on the chart what % your throttle position is at idle & increase the enrichment %. That will eliminate your "Dead Spot"

Here is one I had on my office computer. I have tweeked this table since but it will give you an idea.

US1 Fountain

Stupid question, but with the Daicom software, You just plug your laptop to the diag. plug and read?

On that note, anyone have a used Rinda scan tool they want to sell? I think for my use it will be more than adeqate. Borrowed one before and liked it.

Dave_N

Yes, the software comes with the connection cable (USB or serial), and you just tell it what port your using. One of the cool things about the Diacom software is the ability to export recorded data to Excel for analysis. Below is a graph of IAC data that I created just for fun.

Dave

tomcat

Dave N: In your example, desired throttle position is based on the throttle folower, but the actual position doesn't quite reach the desired peak. Why is this? And how does the ECU know this?

Dave_N

tomcat,
In this graph the IAC position closely follows the Desired IAC position except for the point that I believe you are asking about where the IAC stops at it's limit of 150 counts. It's not easy to see that it was topped out on the 3D graph. Below is the same graph in 2D form. You can see the IAC tops out (pintle is fully retracted). I also added engine speed to this graph (divided by 10 so that it would work on the same scale as the rest of the data).

As far as how the ECM knows where the IAC actually is, I'm not 100% sure, but I think that at key-up the ECM moves the IAC to it's limits, sets the calibration, and then calculates the position from there.

Dave

Grog

blown formula -

I was reading one of your threads from back in 2002 where AZ Speed and Marine did a blower install for you. What was the final result of your fried motor? Did Jim take care of it?

Why do I ask?? I also paid $90/hr for $7/hr flunkie work. I could go on forever about examples really of poor workmanship but to make a long story short, I ended up paying for a complete rebuild. It took me 13 months to get my 271 Fastech back for a Whipple install. To date, I have about 5 hours on the break-in and zero confidence in their work.

Any insight you could give me would be greatly appreciated.