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 Post subject: How the PCM works
PostPosted: Sun Mar 03, 2002 7:09 pm 
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Got this from f-body.com ... thought this might be interesting ... Tnx to Fred for the detailed info ...

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The perception that the PCM "adapts to your driving habits" or "learns your driving style" is totally incorrect. The computer uses a variety of preprogrammed tables to feed data to the logic programmed into the computer. Where necessary, the PCM "learns" from sensor measurement, as required to keep the vehicle primarily in compliance with emissions regulations, and to prevent potential damage to the engine.

There is very little data "learned" by the PCM. The two things that come to mind are the long term fuel corrections and the learned idle setting for the IAC motor. The latter is simple.. the PCM "learns" what position of the idle air motor is required to produce the corrrect idle pseed on startup.... not exactly a performance parameter.

The major part of the "learning" is long term fuel correction. In the LT1 PCM's, there is a grid of 16 "long term fuel correction" factors (cells) which allow the system to operate as close as possible to the 14.7:1 stoichiometric air/fuel ratio that the emission system is designed for. 14.7:1 is the design point for the catalytic convertor to operate most efficiently. It was chosen because it produces the best balance of minimal (not necessarily minimum) levels of unburned hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). It also happens to produce a satisfactory level of fuel miles/gallon, although even that could be better optimized at levels closer to 15.2:1.

Point is, all this "computer power" is aimed at produced low emissions and high MPG, not at producing "performance". But, the "learning" does affect performance driving under some conditions. The grid of 16 long term fuel correction cells is arranged with engine load on one axis (as defined by manifold absolute pressure, MAP) and engine speed (as measured in RPM) on the other. When you clear the long term corrections from volatile memory, by removing the power supply to the PCM, the corrections revert to what is in effect "0". This means when you restart your car, all the corrections have been lost, and the PCM has to use "short term" (unlearned) corrections, based on feedback from the O2 sensors to keep the A/F ratio at 14.7:1. It then uses the short term data to develop new long terms corrections... a process that might take a few minutes or in some cases a few days.

Start your engine after a reset and let it idle. Wait for it to enter closed loop mode (coolant temp high enough, O2 sensors warm enough, timer timed out) and then watch the long term fuel corrections as it continues to idle. Within seconds they will start to move away from "0" (actual "0" value is "128", but lets not complicate things). Within a couple of minutes the long term corrections will have stabilized, and in effect be "relearned". But, that means the PCM has "relearned" only the values for the grid "cell" 16.... the "idle" cell.

Now start to drive your car, and the engine will operate in other load/rpm "cells". Some of the cells represent fairly high loads (low MAP, a measure of how far open the throttle is) and rpm. Cell 15 is the highest load/rpm combo. As you drive, it is unusual to spend more than a few seconds in the higher load/rpm cells.... so these cells will not stabilize (be fully "relearned") for perhaps day, or weeks, until you have spent an appreciable amount of clock time operating in them. So....."drive it like you stole it" simply means you are spending more time in the upper load/rpm operating ranges, and the PCM is "relearning" that data faster. But it isn't "learning" anything about how you drive, its simply spending more time in the upper load/rpm cells and rebuilding the data faster.

Why is any of this important?.... well in "normal" driving, it makes little difference at all. Until the low and mid load cells are relearned, the PVM uses short term values.... immediate feedback from the O2 sensors to keep the engine running clean. It is only under WOT use that the slowness in relearning the long term fuel corrections can alter your cars performance. When you go to wide open throttle (WOT) the PCM no longer uses the O2 sensors for feedback. It enters "enrichment mode", where it uses a richer target A/F ratio to allow more power than a lean mixture like 14.7:1 would produce. But, the PCM plays it safe....

If the upper load/rpm cells were "adding" fuel in closed loop operation to maintain 14.7:1, the PCM assumes that to be "safe" and avoid leaning the engine out and putting it at a point where it might encounter destructive detonation, it must continue to "add" fuel at WOT. It uses the data stored in Cell 15 to accomplish this. So.... if you haven't driven in Cell 15 operating conditions long enough to stabilize the Cell 15 long term fuel corrections, the value used at WOT might not be the correct one. It would probably be too low, and would not provide the enriched A/F ratio the engine needs for max performance. But, and here is where it all falls apart, the LT1 PCM"s are programmed to run at an A/F target ratio of about 11.7:1 at WOT, and this is too rich for peak power or peak torque (usually in the range of 12.8 to 13.2:1). So the system is set up too rich anyway (hence the expression "our cars run to rich from the factory").

If the PCM has found that the high load/rpm cells ar actually "subtracting" fuel and leaing out the calculations to meet 14.7:1 in closed loop, it ignores the corrections, and they default to "0" and Cell 18. Plays it safe... makes sure it never leans the mixture out at WOT. Point of all this is.... your "driving style" will just govern _HOW FAST_ the PCM "learns", NOT _WHAT_ it "learns". So driving your car hard will not make it any more powerful than driving it easy.... it will just rebuild the long term fuel corrections faster, and in the end the results will be identical.... same corrections, just took longer to get there.

Timing, unlike fuel, does not appear to involve any "learning" at all. The only possible "learning" would be some sort of default timing retard "learned" over an extended period of low octane fuel usage and excessive retard being invoked by the PCM in response to high signals from the knock sensor, but I have never found any documentation of this "learning". And there is no reference in any of the literature I have found, or in the tables that are revealed by LT1_Edit to indicate that there is any sort of timing related "learning" involved with WOT operation. I have reasonable experince with more sophisticated aftermarket engine performance computers, and have not found any that invoke "learning" for spark management.

I think that the idea that the PCM somehow reflects your driving style arises out of a lack of understanding of how the PCM works. It certainly appears to have achieved the status of "urban legend".

Fred - 1994 Formula 381/N2O/TH400 - 11.162sec/127.67mph - Central Jersey Camaro/Firebird Owners (CJC/FO) / InjuneerZZ@aol.com

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Last edited by Garry on Sun Mar 03, 2002 7:10 pm, edited 1 time in total.

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 Post subject:
PostPosted: Sun Mar 03, 2002 7:48 pm 
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I already figured that out, because every time I reset the ecm, the light goes out, but then comes right back on! :)

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PostPosted: Thu Mar 08, 2007 1:22 am 
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That is awesome information!!!!

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