I had some struggles with my 2002 Corvette, and so do lots of people who use these PCMs, and I figured I should document my solution in case it helps other people. All of the numbers below are what I have in my tune as a I write this. They won't be perfect for your car's setup. They're not even perfect for my car's setup - they're working well enough, but I'm still experimenting. But if you're struggling to get your car to drive well, consider trying this approach, and using these numbers as a starting point.
The stock tune works so well you'd think this was very simple, but the stock tune starts to suck when you change things. Especially these things:
My car has all of the above.
Getting the engine to idle was pretty easy. Getting it to return to idle, for example after approaching a stoplight and pressing the clutch pedal, was hard. Often, the engine speed would just drop from cruising RPM all the way down to zero. With the setup below, that problem is gone.
There are a lot of differnet features in the PCM software that interact to shape this behavior.
| Feature | What It Does | How I Tuned It |
|---|---|---|
| Adaptive Idle | This feature adjusts the throttle blade angle (or Idle Air Control valve, for cable-throttle vehicles) based on engine speed. More precisely, based on difference between the actual engine speed and the desired idle speed. | I'll come back to this later. I'm just mentioning this here to clarify what it does and why it's useful. |
| Throttle Cracker | This feature tells the PCM to hold the throttle blade a little bit further open when you lift your foot off the accelerator pedal. The amount of additional airflow depends on vehicle speed and RPM, and the decay rate depends on vehicle speed. | Unfortunately, when this is active (even if it is active with zero additional airflow) adaptive idle is disabled. With my car, that means the engine surges +/- 200 RPM, even though the throttle blade angle was fixed. The surging was annoying, so I disabled throttle cracker by raising the "enable" speed to 250mph. |
| Throttle Follower | This feature also tells the PCM to hold the throttle blade a little bit further open when you lift your foot off the accelerator pedal. Unlike Cracker, the amount of Follower airflow is based on accelerator pedal position (regardless of vehicle speed or engine speed). The decay rate is based on vehicle speed (regardless of engine speed). | This is useful to slow down the rate at which RPM falls when you push in the clutch pedal. Like cracker, this also disables adaptive idle until the additional airflow reaches zero, which is not ideal, but I configure this to taper off to zero after just a second or two, so the drawback is minor. I add 2 g/s of airflow from 13%-100% of pedal angle, ramping down gradaually to 0 airflow at 0% pedal. The decay rate in gear is zero, because otherwise it would decay to nothing while I was engine-braking, and the whole point is to get this cushioning effect when I put the clutch in. The decay rate with the clutch pressed is 0.015. |
| Adaptive Idle Early-Entry, or Hysteresis | If this is set to zero, adaptive idle will only engage when the engine speed drops to the target idle speed. If this is set to 500, adaptive idle will engage when the engine speed is 500 RPM above the target idle speed. This is useful, because one of the adaptive idle features slows the rate at which RPM falls. | I set this to 1000. |
| Maximum adaptive idle airflow correction | This limits how much airflow the adaptive-idle system can add to (or remove from) the base target airflow for idle. | Stock is +/- 3 grams per second. I bumped the maximum to +10 grams per second, and left the minimum at -3. |
| Deceleration Fuel Cut-Off (DFCO) | This feature disables the fuel injectors when you're engine-braking - in-gear with 0% accelerator pedal. | While this is not directly related to idle, it can interfere by keeping the injectors disabled too long. There's a "negative delta RPM" feature that fixed that problem. In the stock tune it was -6400 RPM, which effectively prevented this feature from ever disabling DFCO. I changed it to -75 (which is also the default setting for automatic transmissions) and that causes the fuel injectors to return to active duty as soon as I push the clutch down. |
| Idle Spark Feedback | This adjusts ignition timing based on the difference between the target RPM and the current RPM. When the engine speed is too low, it advances the spark, and when the engine speed is too high, it retards the spark. If this is tuned well (andn if your cam does not have absurd amounts of overlap), the spark corrections alone will keep the idle pretty close to the target RPM, with very little or no throttle blade adjustments. | Idle overspeed is -1 at +12.5 RPM, increasing to -8 at +100 RPM.
Idle underspeed is +0.6 at -12.5 RPM, increasing to +20 at -300 RPM. |
| Adaptive Idle Airflow - Proportional | This also adjusts the throttle blade based on the difference between the target RPM and current RPM. | Zero correction for +/- 60 RPM.
From +80 RPM to +400 RPM, the correction gradually increases to +0.25 At -80 RPM, the table adds 0.5 grams per second. From -100 RPM to -400 RPM, the table adds a flat 10 grams per second. That's a lot. It's roughly equivalent to stabbing the accelerator when you think the engine is about to die. I intend to try smaller corrections at -100 and -150, because this will overshoot the target intermittently. But, generally speaking, if RPM falls to 100 below target, something is wrong, and this ensures that the engine doesn't sputter and die. |
| Adaptive Idle Airflow - Integral | This also adjusts the throttle blade based on how much time there has been a difference between the target RPM and current RPM - periodically multipled by the amount of difference. The proportional correction alone can reduce the difference between the target RPM and actual RPM, but it can't eliminate that difference. The integral correction can eliminate the difference. | Integral overspeed correction is very mild. Zero at 0, 0.00098 at +20 RPM, ramping up to 0.00488 at +100 RPM, and still 0.00488 from +100 to +400 RPM.
Integral underspeed correction is zero at 0, 0.00098 at -20, ramping up to 0.00391 at -80. Then it jumps to 0.08 at -100, 0.15 at -150, ramping up to 0.44 at -400. The idea behind the steep underspeed correction is that if RPM drops below target, the integral feedback should add enough airflow to ensure that it doesn't drop below target again. It will over-rev slightly, and the mild overspeed correction will then gradually reduce the airflow back to the target. |
The idle control routine follows these steps:
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| ETC Scaler (or scalar?) | The PCM uses this to calculate the percentage of the total throttle body area that corresponds to a desired throttle opening area (in square millimeters). | There are plenty of threads on the internet about which scalar to use for what size throttle body. |
| The table that maps from "desired opening area" to throttle blade position. | The relationship between opening area and blade position is different for every throttle body. Surprisingly few people adjust this table. | The critical thing is the blade position at which air begins to flow through the throttle body. The Katech 103 that I'm using now* doesn't even begin to flow until the blade position has moved to about 10% - whereas the stock throttle body starts flowing air at about 4%. The rest of the table is hard to describe, I'll upload a .bin file shortly. |
* Katech's 103mm throttle body for the C5 is horribly calibrated. And I have a hunch that's why their website no longer lists them for sale for C5s. The problem with their calibration is that the TAC model imposes an 18% limit on the throttle position at idle, and since the damn thing doesn't even start flowing air until 10%, 18% is barely enough air for the engine to idle at a cold start. It's tolerable in the summer, just barely, but intolerable in the winter when the oil is thicker. The PCM calls for about 22% throttle position, the TAC says nope, 18% is all you get. And you cannot tune the TAC. We're just stuck with that 18% limit. With the 5.7L / 346ci engines that come in the C5, it'll work. With a dry-sumped 454, on a cold winter day, forget about it. Keep an LS2 TB handy for the winter months.